CIS 5930-04 – Spring 2001

[email protected] 1

CIS 5930-04 – Spring 2001

Part 3: Introduction to the Java Language:

Object-oriented Concepts

http://aspen.csit.fsu.edu/it1spring01/

Instructors: Geoffrey Fox , Bryan CarpenterComputational Science and Information Technology

Florida State University

Acknowledgements: Nancy McCrackenSyracuse University

[email protected] 2

Java Language Basics

[email protected] 3

Obvious similarities to C, C++

Java syntax has many similarities to C, C++.

All variables must be declared

Syntax of expressions and control structures almost identical to C, C++

C or C++ style comments allowed.

[email protected] 4

Obvious differences from C, C++

No low-level pointers or pointer arithmetic.

– Instead have variables and expressions of reference type.

No malloc() or free()—instead have a “new” operator for creating objects, plus automatic garbage collection.

Can declare variables almost anywhere (like C++).

No struct, union, enum, typedef—classes and objects are used uniformly instead.

[email protected] 5

Primitive types rationalized

Java characters use 16-bit Unicode Worldwide Character Encoding instead of 8-bit ASCII. Supports all alphabets and languages.

Primitive types for integers and floats have machine independent semantics.

Boolean expressions in Java have value “true” or “false” (not 0, 1, . . .)

[email protected] 6

Three kinds of comments in Java

/* ignore all between stars */– As for C

// ignore all till the end of this line– As for C++

/** this is a documentation comment */– Should appear immediately before, eg, class or method

definition, and describe intended use.

[email protected] 7

Documentation Comments Used by documentation-generating tools like

javadoc to produce documentation, typically in HTML form.

Optionally include formatting tags like @param, which flags a description of a method parameter:

/** This method does what it feels like. @param bar This is a pointless

argument. */

void foo (int bar) {. . .}

Other formatting tags include @returns which flags a description of a method result value, or @see name, which creates a hypertext link to name.

[email protected] 8

Java Keywords

Java reserves the following keywords:

abstractbooleanbreakbytecasecatchcharclassconst

continuedefaultdodoubleelseextendsfinalfinallyfloat

forgotoifimplementsimportinstanceofintinterfacelong

nativenewpackageprivateprotectedpublicreturnshortthrow

throwstransienttryvoidvolatilewhile

goto is not allowed in Java, but it’s still reserved!

null, true, and false are literals with special meaning.

[email protected] 9

Java Language—Program Structure Source code of a Java program consists of one or

more compilation units, each implemented as a file with extension “.java”.

Each compilation unit can contain:– a package statement – import statements – class declarations and/or interface declarations.

In typical Java development environments, exactly one of the class (or interface) declarations in each compilation should be marked public.

The file should be named after the public class. e.g. if the public class is Foo, the file name should be “Foo.java”.

[email protected] 10

Java Types

Each Java variable or expression has a definite type, given by a declaration such as int i; double x, y, z; Color c;

There are two sorts of type:– Primitive types like ints or booleans are built

into the language. – Reference types. These include class types

like Color, and array types (and also interface types).


Primitive Types

There are 4 integer types: byte short int long Sizes are 8, 16, 32 and 64 bits, respectively.

float is 32 bits, double is 64 bits. Floating point arithmetic and data formats are defined by IEEE 754 standard.

char format is defined by 16 bit Unicode character set.

boolean is either true or false.

One can use casts for arithmetic conversion, as in: int i ; float x ; i = (int) x ;


Reference Types

These are the types associated with composite entities like objects and arrays.

They are called reference types because a variable or expression in a Java program with reference type represents a reference (or pointer) to a composite entity.– Any variable of reference type may take the value null.

Reference types can be divided into:– Class types– Interface types (discussed later)– Array types


Strings—an Example of a Class Type

Java environments provide predefined classes for common data types. Every Java environment provides a String class.

Declaration of a String variable looks like: String s ; // variable declaration

The variable declaration itself doesn’t create any objects. We can create a new String object by, e.g.: s = new String(“This is the text”) ; // object

creation

These may be combined on one line: String s = new String (“This is the text.”) ;


A Constructor Function In the object creation expression:

new String (“This is the text.”)

the term String (“This is the text.”)

is a constructor invocation. All classes have special “functions” called

constructors. These functions have the same name as the class. They initialize the fields of the object.

Constructor functions are only used in object creation operations—nearly always directly after a new operator.

In this example the constructor has one argument: a string literal.– We will see later that in general constructors can have

arbitrary argument lists.


Some features of Strings.

Strings are Java objects, but Java provides some syntax peculiar to strings.

In fact literal string in double quotes itself refers to a pre-existing String object—so in practice we may drop new operation for string constants: String s = “This is the text.” ;

After creation, characters of a string object never change.– In other words: string objects are immutable.


Operations on Strings. Although a String object is immutable, String-valued

variables can be reassigned to refer to new string objects:

String str = “Chicken soup with rice” ; int n = str.indexOf( ‘w’ ) ; str = str.substring(0,n) + “is n” + str.substring(n+6) ; // Result: “Chicken soup is nice”.

The operator + is used for concatenation (special syntax for strings).

indexOf() and substring() are methods of the String class—not special syntax!– They illustrate the general syntax of method invocation on an object.


Array Types

As for objects, declaring an array variable is distinct from creating on the array: int states[] ; // variable declaration

and: states = new int[128] ; // array creation

Again, these can be combined: int states[] = new int[128] ;

Alternative (better?) syntax for declaration: int[] states ;


Subscripts

With states is declared as above: int states[] = new int[128] ;

it can be subscripted by integers from 0 to 127.

Subscripts are checked at runtime: states[-1] or states[128] will immediately generate exceptions.

Array length is given by the length instance variable: int len = states.length ; // assigns len = 128.


Arrays of Objects

Arrays of arbitrary objects can be constructed, e.g.: Color manycolors[] = new Color[1024];

This creates an array of object references. It does not create actual objects for individual elements.

Before you use the array elements, you may need to use object constructors to allocate each object, e.g.: for (int i = 0 ; i < 1024 ; i++) manycolors [i] = new Color() ;


Multidimensional Arrays

Multidimensional arrays are arrays of arrays. In general these arrays may be “ragged”:

int graph[][] = new int[2][];

graph[0] = new int[4]; // Row 0 has length 4 graph[1] = new int[7]; // Row 1 has length 7 . . . graph[1][1] = 9;

Shorthand syntax for creating a rectangular array: char icon[][] = new char [16][16]; // 16 by 16

array

– Note icon is still logically an arrays of arrays, and nothing in Java forces it to stay rectangular. E.g. later someone might do:

icon [8] = new char [17] ; // Now ragged!


Java Language—Expressions

Most Java expressions are similar to C. Here are some examples:

– arithmetic:2 + 3(2 + 3) * i

– auto-increment and decrement:i++ // equivalent to i = i +1

– Boolean:((i > 0) && (j > 0)) || (state == –1)

– bit operations:i << 1 // Shift bit pattern 1 place left

– conditional expression:(i > 0) ? expression1 : expression2


Java Language—More Expressions

Java has some expressions of its own:

– string concatenation:“fred” + “jim” // Value is “fredjim”

– object “instance of” test:(a instanceof B) // true iff object a has type

(class) B


Java Control Flow. I: if Statements

Conditional execution of statements: if (some Boolean expression) { statements to be executed if true }

Optional else clause: if (some Boolean expression) { statements to be executed if true } else { statements to be executed if false }

Nested example: if (some Boolean expression) { . . . } else if (another Boolean expression)

{ . . . } else { . . . }


Control Flow II: while Loop Constructs

Normal while loop: while (any Boolean) { Stuff to do }

Example: int i = 0 ; while(i < a.length) { a [i] = i * i ; i++ ; }

while loop with test at end: do { What to do } while (another Boolean) ;


Control Flow III: The for Loop Construct In Java, most often use the C++-like variant:

for (declaration1 ; booleanExpression ; expressionList2) { Statements to do }

The declaration declaration1 is effected at start of loop, comma-separated expressionList2 is evaluated after every iteration, and the loop terminates when booleanExpression is false.

Typical example: for (int i = 0 ; i < a.length ; i++) a [i] = i * i ;

The original C-like form (no declaration) also available: for (expressionList1 ; booleanExpression ; expressionList2)

{ Statements to do }


Control Flow IV: The switch Construct

Identical to C:

switch (expression) { case Constant1: // Do following if

expression==Constant1 Bunch of Stuff break; case Constant2: // Do following if

expression==Constant2 Bunch of Stuff break; default: // Do the following otherwise Bunch of Stuff break; }


Control Flow V: break and continue

Unlabeled break statement immediately exits the enclosing switch, while, do or for construct: while (true) if (++i == a.length || a[i] == v) break ;

Labeled break statement allows to exit an arbitrary enclosing statement, provided it is labeled: assign: { if (i >= a.length) break assign ; a[i] = v ; }

(This is not the best way to do this!)

The continue statement skips to the end of the current iteration of the enclosing while, do or for.


The Java Object Model: Classes, Instances and Methods


The Java Object Model Overview

Programs are composed of a set of modules called classes. Each class is a template specifying a set of behaviors involving the data of the class.

Each class has variables, or fields, to hold the data, and methods—akin to functions or procedures in other languages—to define the behaviors.

Each object in a program is created as an instance of a class. Each class instance has its own copy of the instance variables defined for the class.

Classes can be used for data encapsulation, hiding the details of the data representation from the user of the class (e.g., by marking variables as private).

InstanceVariables

Methods


Defining a Class

A class declaration consists of: – a header giving the class name, modifiers, and possible

superclass and interface structure.

and a class body usually containing:– declarations of fields (possibly with initializations)—

class variables and instance variables.– declarations of methods.– declarations of constructors. These “functions” look

like methods, but have the same name as the class. They do initialization when objects—class instances—are created.

– nested class and interface definitions.– class or (rarely) instance initialization statements.


Example: a Predefined Class A (small) part of the Java Date class:

public class Date implements Serializable, Cloneable { public Date( ) {. . .} //

Constructor public Date(long msSinceEpoch) {. . .} //

Constructor

public int getTime( ) {. . .} // Accessor public void setTime(long msSinceEpoch) {. . .} // Mutator

public boolean after(Date when) {. . .} // Comparision

public boolean equals(Object obj) {. . .} // Comparision

. . . }

Note: all variables, methods and constructors visible from “outside” the class—parts of Date that programmers writing code in other classes are allowed to use—have the public modifier in their declaration.


Creating a Class Instance The Date class represents a particular date and

time, with a resolution of milliseconds.

The first of the two Date constructors (“no-argument constructor”) constructs an instance of the Date class and sets its value to the current moment: new Date()

Constructors (like methods) can be overloaded. Constructors of same name are distinct if they have distinct argument types. If ms is a long, the object: new Date(ms)

represents a moment ms milliseconds after January 1, 1970, 00:00:00 UTC (Coordinated Universal Time).

Java will become obsolete (2^63 – 1) / 1000 seconds after that (approximately 292 million years AD, UTC). . .


Using a Class

An example application using a method of the Date class:

import java.util.Date;

public class DateTest {

public static void main (String[ ] args) { Date early = new Date(1000) ; // very

early seventies! Date today = new Date() ; // Now! if (today.after(early)) System.out.println( "Today is not

early!") ; } }


Instance Variables

A very simple class: public class Complex { public double real ; public double imaginary ; }

Essentially like a C struct. Every instance of Complex has its own real and imaginary variables. These fields are therefore called instance variables.

Use: Complex z = new Complex() ; // Default

constructor

z.real = 0.0 ; z.imaginary = 1.0 ;


Class Variables Besides instance variables, a class may contain “global

variables” that are not associated with any instance.

A class variable (also called a static variable) is flagged by the static modifier in its declaration:

class Potato { public String name;

static public int num = 0 ; // Class variable—number of

potatoes. }

Potato p = new Potato(), q = new Potato() ;

p.name = “one potato” ; q.name = “two potato” ;

Potato.num += 2 ; // static field prefix is class name.


Method Definitions

Subprograms in Java are called methods. In the abstract, the declaration format is: methodModifiers returnType methodName (parameter

list) { declarations and statements }

The parameter list contains the types and names of all the parameters.

The declarations and statements are the body of the method. Parameter names, and variables declared in the body, are local to it.

Control returns from a method when the body finishes execution or a return statement is executed. return statements may return a result value.

Parameters are passed by value.


Local variables

Formal parameters of methods, and variables declared inside the bodies methods, are local variables.

These are a third kind of variable in Java: they are neither instance variables or class variables.


Static and Non-static Methods

Like fields, methods come in two varieties, which are properly called instance methods and class methods.

The terms non-static methods and static methods are also commonly used.

In all Java applications illustrated so far, the main() method had the modifier static—the main method of an application is required to be a static method.

All other examples of methods illustrated so far were instance methods.


Instance Methods

Instance methods operate in the context of a particular class instance (i.e. a particular object).

The instance variables of the current object can be accessed without any prefix:

public class Complex {

// Adds z to the current object

public void add(Complex z) { real += z.real ; imaginary += z.imaginary ; }

public double real ; public double imaginary ; }


Invoking an Instance method

This example initializes a and b, then increments the value of a by amount b:

Complex a = new Complex(), b = new Complex() ;

a.real = 0.707 ; a.imaginary = -0.707 ; b.real = -1.0 ; b.imaginary = 0.0 ;

a.add(b) ; // Method invocation


this

Within an instance method or constructor the keyword this refers to the current instance.– i.e. the object on which the method was invoked, or

which the constructor is initializing.

Appropriate usage—passing self-reference to some other method: public class Complex {

. . . Definition of add(), etc.

public void addTo(Complex accumulator) {

accumulator.add(this) ; } }

– The invocation a.addTo(b) adds the value of a to b, i.e. it is equivalent to b.add(a).


this as a prefix

Some programmers will write the this prefix explicitly on every access to an instance variable, e.g.:

public void negate() { this.real = – this.real ; this.imaginary = –

this.imaginary ; }

This is legal, but ugly! One time you must use this as a prefix to an

instance variable is when the field is hidden by declaration of a local variable with the same name.– The only common example is in constructor declarations.

A constructor parameter whose value is used to initialize a field is conventionally given the same name as the field it initializes. See examples later.


Static Methods

A static method does not operate in the context of a particular instance.

Instance variables of the class cannot be accessed inside the body of a static method unless an explicit object prefix is given.

The keyword this cannot be used in the body of a static method.

To invoke a static method it should be prefixed by the name of the class (similar rule to accessing class variables).– This prefix can be omitted if the method is invoked

from another method, etc, defined in the same class.


Constructors

Constructors are “functions” (not, strictly speaking, methods) that have the same name as the class they belong to.

Any number of constructors can be defined for a class, provided they can be distinguished by the number and type of their parameters (overloading).

If no constructors are explicitly defined, the compiler generates a single default constructor with no arguments.

– Note: the default constructor disappears once any explicitly-defined constructor is given!


A Better Potatoclass Potato {

public Potato(String name) { this.name = name ; // Idiomatic use of this

num++ ; }

public static int getNum() { // A static method return num ; }

private String name ; // Note: now private

private static int num = 0 ; // Also private}

Potato p = new Potato(“one potato”), q = new Potato(“two potato”) ;

System.out.println(“There are ” + Potato.getNum() + “ potatoes”) ;


Remarks

In the constructor, the unqualified symbol name refers to the local variable declared in the parameter list.– Because this declaration hides the declaration of name

as an instance variable, we must prefix with this to access the latter.

The data fields are now private. This means they can be accessed only from methods within the class, not from other classes.

The method getNum() returns a “global” property of the class—the total number of Potato objects that have been created.– Hence it is natural to declare it as a static method—it

is not associated with any individual instance.


Type Conversions Java allows implicit type conversions in some

contexts. Generally speaking the conversions allowed implicitly

(without a cast) are what are called widening conversions.

For primitive types, the widening conversions are from any integer type to any wider integer type, (int to long, etc) or from a float to a double.

Narrowing conversions, by contrast, would include conversion from long to int, or from a floating point type to an integer type.

Narrowing conversions usually have to be specified explicitly with a cast, e.g.

float x ; int i = (int) x ;


Overloading

A class can declare several methods with the same name, providing each declaration has a different number of arguments, or different argument types.– We refer to the combination of the method name and its

list of argument types as the signature of the method.

Example: class Shape { setColor(Color c) { . . .} setColor(int rgb) { . . .} setColor(int r, int g, int b) { . . .} . . . }

– The method setColor() is overloaded with three different signatures.


Calling an Overloaded Method

If the types of the argument expressions in a method invocation exactly match the types of the parameters in one particular declaration of the method, the compiler naturally chooses to call that particular method implementation.

There is a complication, though: the Java language allows implicit type conversion of method arguments.– The allowed conversions are the widening conversions.

In general overload resolution chooses the most specific method signature matching the actual arguments.– If there are several applicable signatures, and no single

one is more specific than all the others, a compile time error is flagged.


Examples of overload resolutionvoid foo(long p) {. . .} // Signature Ivoid foo(int p) {. . .} // Signature IIvoid foo(long p, int q) {. . .} // Signature IIIvoid foo(int p, long q) {. . .} // Signature IV

long l ;short s ;int i ;

foo(l) ; // Exact match—use Signature I.

foo(s) ; // Do widening conversion of s to int, and use // Signature II—unique “most specific” case.

foo(l, s) ; // Uses Signature III—only case applicable by // widening conversions.

foo(i, i) ; // Compile time error! Signatures III and IV // are both applicable but neither is more

specific // than the other!


Header of Class Definition—Details

In the abstract, the definition format is: classModifiers class className [ extends

superclass ] [ implements

interfaceList ] { body of class }

The optional extends and implements clauses will be discussed in detail in later lectures.


Modifiers of Classes

Possible classModifiers are:– public—the class may be used freely by code outside

the package.– abstract—the class contains abstract methods without

implementation (abstract classes will have subclasses that define implementation of methods—see later).

– final—this class cannot have a subclass: see later.– strictfp—all intermediate results in all float or double

expressions appearing in the class have strict IEEE 754 exponents.

– private—only allowed for a nested class. Meaning as for other members.

– protected—only allowed for a nested class. Meaning as for other members.

– static—only allowed for a nested class. Meaning analogous to other members.


Modifiers of Fields In the abstract, the declaration format is:

fieldModifiers type variableDeclaratorList ;

where a variableDeclarator has the format: fieldName [ dimensionStuff ] [ = expression ]

Possible fieldModifiers are:– public—this field is accessible from any code.– protected—accessible from code in a subclass (or the

same package—default accessibility).– private—only accessible from code in the same class.– static—this is a class variable: see earlier.– final—this field cannot be modified after it is

initialized.– transient—the value of this field will not be included

in a serialized representation of an instance.– volatile—any cached copy of the field maintained by

an individual thread will be reconciled with the master copy every time the field is accessed.


Modifiers of Methods In the abstract, recall, the declaration format is:

methodModifiers returnType methodName (parameter list) [throws exceptionList ] {

declarations and statements}

Possible methodModifiers are:– public—this method is accessible from any code.– protected—accessible from code in the same package, or a subclass.– private—only accessible from code in the same class.– abstract—the method has no implementation here—declaration has

a semicolon in place of a body.– static—this is a class method: see earlier.– final—this method cannot be overriden: see later.– synchronized—other synchronized methods are locked out while

this method is executing: see later.– native—the implementation of this method is given in a platform-

dependent language. Declaration has a semicolon in place of a body.– strictfp—intermediate results in all float or double expressions

appearing in the body have strict IEEE 754 exponents.


The Java Object Model: Inheritance and the Class

Hierarchy


Some Dependencies between Classes

Use– A uses B: the most informal and general relation. A

might, for example, call a method from class B, or have a method with argument type B or return type B.

Containment– A has a B: an important special case of use—class A

has a field of type B.

Inheritance– B is an A: class B has all the properties of class A.

The compiler treats B as a special case of A, and allows an instance of B to be used in any place where an instance of A could appear. In general the class B will extend A with some extra properties of its own.


Inheritance The inheritance relation is (unexpectedly?) powerful;

it is built into all fully object-oriented languages. In Java, if some class A has been defined, we can

subsequently declare a new class, B, and specify that it extends A.

Class A is called the superclass of B. Class B is a subclass of A.

The class B is automatically given (inherits) all the fields and method definitions of A. Further fields and methods can be added that are specific to B.

In particular, for every method signature in class A, class B will have a method with identical signature.

Crucially, though, the class B may define a different the implementation for some of those methods.


Trivial use of Inheritance

class Shape { void setColor(Color color) {this.color =

color ; } Color color ; int x, y ; // position of center, say }

class Circle extends Shape { void drawCircle() {. . .} double radius ; }

class Rectangle extends Shape { void drawRectangle() {. . .} double height, width ; }

Subclasses automatically inherit color, x, y fields of Shape, and setColor() method.


A Limited Kind of Polymorphism

void setAllColors(Shape [] shapes, Color color) { for(int i = 0 ; i < shapes.length ; i++) shapes [i].setColor(color) ; }

Shape [] bag = new Shape [N] ; bag [0] = new Circle() ; bag [1] = new Rectangle() ; . . . setAllColors(bag, Color.red) ; . . .

The function setAllColors works on a collection of shapes, and works correctly independently of whether each shape is actually a Circle or a Rectangle.


Class Hierarchies Class hierarchy diagrams represent inheritance

relations between classes:

Class: Shape

Class:Circle

Class:Rectangle

These diagrams become more complex as subclasses are further extended. But they are always trees, because in Java each subclass has a single superclass.


Inheritance with Overriding class Shape { void draw() {} Color color ; int x, y ; }

class Circle extends Shape { void draw() {. . .} double radius ; }

class Rectangle extends Shape { void draw() {. . .} double height, width ; }

Subclasses override the definition of draw() in the superclass.

Bodies of methods contain the actual code for drawing a circle or rectangle, respectively.


True Polymorphism void drawAll(Shape [] shapes) { for(int i = 0 ; i < shapes.length ; i++) shapes [i].draw() ; }

Shape [] bag = new Shape [N] ; bag [0] = new Circle() ; bag [1] = new Rectangle() ; . . . drawAll(bag) ;

The draw() method invoked is the method defined in the class of the referenced object (Circle or Rectangle).– not the implementation defined in the compile-time type of

the variable, namely Shape. drawAll() correctly draws a mixed bag of shapes

whose details may be unknown when this method is written.


Runtime Lookup of Methods

class Shape { void draw() {. . .} }

Square s = new Square() ;s.draw() ;

class Square extends Rectangle { // No declaration of draw()}

class Rectangle extends Shape { void draw() {. . .}}

class Circle extends Shape { void draw() {. . .}}

Search up the inheritancetree until find firstclass that defines method.


Inherited Methods and Overriding The method associated with the actual class of the

instance is called, even if it invoked from code in the superclass.

Suppose we add a drawInColor() method to Shape: class Shape { void draw() {}

void drawInColor(Color color) { this.color = color ; draw() ; }

Color color ; int x, y ; }

The implementation of drawInColor() is inherited by the subclasses. But when it is invoked on one, their own draw() methods are called! More polymorphism.


Abstract Methods and Classes

In our example, the draw() method in the Shape class did nothing. It may not be necessary to give a implementation of this method in the base class at all, because it may be that it is only ever invoked on instances of subclasses representing concrete shapes (as here).

In this situation, the superclass and unimplemented methods can be declared abstract:

abstract class Shape { //abstract class abstract void draw() ; // abstract

method

Color color ; int x, y ; }

– It is not possible to create instances of abstract classes. One must create a subclass that overrides all abstract methods of the base class, giving implementations.


Final Methods and Classes If a method is declared final, it may not be

overridden in subclasses (opposite extreme to abstract, which must be overridden!)

If we declared draw() in Rectangle to be final, we could never give a more specialized draw() in a subclass:

class Rectangle extends Shape { final void draw() {. . .} // final method double height, width ; }

class Square extends Rectangle { void draw() {. . .} // Compile-

time error!! }

– In places where the compiler can tell that a final method will be called, it can produce optimized code to avoid overheads of “late binding”.

A final class cannot be extended.


Protected Access

By default a field or method of a class can be accessed by any code appearing in the same package.– Packages are discussed later.

The access modifier protected on a field or method means that this member can also be accessed by any subclass of the class in which it is declared.

Note this modifier increases accessibility from the default. . .– . . . because a subclass may be declared outside the

package that contains the superclass.– Least accessible members are private (visible in

declaring class only), followed by default (declaring package only), followed by protected (package and subclasses), followed by public (visible everywhere).


The Universal Superclass—Object

The Java language provides a superclass for all other classes. If no extends clause is given in a class definition, the class implicitly extends Object.

Array types are also considered to extend Object.

A variable of type Object can hold a reference to any object or array.

Strictly speaking, Object is the root of every inheritance diagram.


Methods on the Object class

Public class Object {

public final Class getClass() { . . . } // Basis for reflection.

public String toString() { . . . } //A String representation

public boolean equals(Object obj) { . . . } // Equality test

public int hashcode() { . . . } // For use by hash tables

protected Object clone() throws . . . { . . . } // Bit by bit copy

public final void wait() throws . . . { . . . } // Deschedule this thread public final void wait(long millis) throws . . . { . . . } public final void wait(long millis, int nanos) throws . . . { . . . }

public final void notify() throws . . . { . . . } // Reschedule any . . . public final void notifyAll() throws . . . { . . . } // . . . or all threads.

protected void finalize() throws . . . { . . . } // invoked by GC.}


Reference Conversions Conceptually, we saw, an instance of a subclass “is

an” instance of the superclass. Hence one can assign a reference to a subclass

object to a variable of a superclass type. Concretely, this implies a conversion from a subclass

type to a superclass type is regarded as a kind of widening conversion.– Recall widening conversions are allowed implicitly in various

contexts.

Narrowing conversions on reference types go the other way—from a superclass down to some subclass.– Narrowing conversions require an explicit cast.

Good programming practice minimizes use of narrowing conversions, but sometimes they are necessary.


Simple Collections

The package java.util contains a family of collection classes.

Here we will only mention two of the most widely used:– Vector, and– Hashmap.

Note Vector is supposed eventually to be superceded by ArrayList.– Consider using ArrayList in your future programs, but

Vector is so widespread we describe it here.


A Vector is Like an Array

A Vector can be used essentially like an ordinary array.

It has a well-defined current size, returned by the size() inquiry.

This can be set with setSize(), but usually a Vector is grown dynamically using methods on next slide.

Vector stores all elements as if the have type Object

If 0 < idx < size(), the methods: void set(int idx, Object obj) Object get(int idx)

respectively assign and retrieve value of element idx.


A Vector can Grow and Shrink

Typically one grows a vector by adding a new element at the end with: void addElement(Object obj)

Causes size() to be incremented by 1. An arbitrary element can be removed by

Object remove(int idx)

This method causes higher elements to be shifted down one place, and size() to be decremented by 1.

An element can be inserted in an arbitrary place by insertElementAt(Object obj, int idx)

Element at idx and higher are shifted up one place, and size() is incremented by 1.


Using Vector void drawAll(Vector shapes) { for(int i = 0 ; i < shapes.size() ; i++) ( (Shape) shapes.get(i) ).draw() ; // Narrowing

conversion }

Vector bag = new Vector() ; bag.addElement(new Circle()) ; // Widening

conversion bag.addElement(new Rectangle()) ; . . . drawAll(bag) ;

For polymorphism, Vector stores items in Object references. Hence, get() returns an Object, which usually needs to be cast back to a more specific type.

If the referenced object is not an instance of the type in the cast, a run-time ClassCastException occurs.


A HashMap is an Associative Array For future reference, we also discuss HashMap

here A HashMap is similar to a vector, but the “index” is

an arbitrary object—very commonly a string. This index is now called a “key”. In simple cases you create a HashMap with the no-

argument constructor, then put key-value pairs in it using Object put(Object key, Object obj)

(returns old value if key was already in the table). Retrieve the element currently indexed by key by:

Object get(Object key) Remove the element currently indexed by key by:

Object remove(Object key)


Using HashMap

HashMap table = new HashMap() ;

table.put(“red”, “stop”) ; table.put(“green”, “go”) ;

String s = (String) table.get(“red”) ; // returns “stop”

String t = (String) table.remove(“green”) ; // returns “go”

String u = (String) table.get(“green”) ; // returns null


Widening Conversions on Arrays There is a widening conversion between two array types if

there is a widening reference conversion between their component types.

This is useful, but can lead to anomalies if used carelessly:Circle [] bag = new Circle [N] ;setAll(bag) ; // Widening: Circle [] to

Shape []. // OK at compile-time.

void setAll(Shape [] shapes) { shapes [0] = new Circle() ; shapes [1] = new Rectangle() ; // Widening: Rectangle to

Shape. // But throws

ArrayStoreException . . . // if invoked as above!}

Effect would be to assign Rectangle to array of Circles. Requires the compiler to add a new kind of run-time check.


Overloading with Inheritancevoid foo(Object p) {. . .} // Signature Ivoid foo(Shape p) {. . .} // Signature IIvoid foo(Object p, Shape q) {. . .} // Signature IIIvoid foo(Shape p, Object q) {. . .} // Signature IV

Object o ;Shape s ;Circle c ;

foo(o) ; // Exact match—use Signature I.

foo(c) ; // Do widening conversion of c to Shape, and use

// Signature II—unique “most specific” case.

foo(o, c) ; // Uses Signature III—only case applicable by // widening conversions.

foo(s, s) ; // Compile time error! Signatures III and IV // are both applicable but neither is more

specific // than the other!


Overload Resolution across Classes

class Shape { void foo(Circle q) {. . .} // Signature I } class Circle extends Shape { void foo(Shape q) {. . .} // Signature II }

Shape s ; Circle c ;

s.foo(c) ; // Uses Signature I—exact match.

c.foo(c) ; // Compile time error! Signatures I and II

// are both applicable but neither is more specific

// than the other! In compile-time overload resolution (choice of

signature), the prefix object expression is treated on the same footing as an extra argument.


Summary: Overloading vs. Overriding Resolution of overloading occurs at compile

time. The compiler chooses a unique method signature out of several different signatures available (or flags a compile time error if it cannot).

Overriding occurs in the context of a single signature. In general, if the class hierarchy contains several definitions with identical method signatures, the appropriate definition is chosen at run time.

Within the body of a class that overrides a method, the method from the superclass can be invoked instead by using the super prefix.


Constructors and Inheritance

Constructors of subclasses must invoke a constructor of their superclass, to initialize the fields there.

If a superclass constructor is not explicitly invoked, the no-argument constructor of the superclass is called, implicitly, by the compiler.– A compile-time error is flagged if no such constructor

exists.

If any superclass constructor other than the no-argument constructor is required, it must be invoked explicitly.

In this case the first statement of a subclass constructor is an explicit constructor invocation using the name super.


Superclass Constructor Invocation

class Shape {

public Shape(Color color, int x, int y) { this.color = color this.x = x ; this.y = y ; } Color color ; int x, y ; }

class Circle extends Shape {

public Circle(Color color, int x, int y, double radius) {

super(color, x, y) ; // superclass constructor invocation

this.radius = radius ; } double radius ;}


Exceptions


Exceptions are Pervasive

Java has a concept of exceptions similar to C++. Unlike C++, Java exceptions are strictly

checked. Most classes in the standard Java library throw

some exceptions. We will see, these must be caught or thrown.

This means that it is almost impossible to write useful Java code without some knowledge of the exception mechanism!


Exception Objects, and throw

Any kind of exception that can be thrown by Java code is described by an exception object. It’s class must be a subclass of Throwable.

If e is a Throwable object, the statement throw e ;

behaves something like a break statement; it causes the enclosing block of code to end abruptly.

If the throw statement appears inside a try statement who’s catch clause matches the class of e, control is passed to the catch clause.

Otherwise the whole method (or constructor) ends abruptly. The exception e is thrown again at the point of invocation (in the calling code).


throw compared with break

try { . . . throw new

MyException() ; . . .} catch (MyException e)

{ . . .}. . .

Control jumps to start of matching catch clause

myBlock : { . . . break myBlock ; . . . } . . .

Control jumps to end of matching block


Methods that throw exceptions

In general, any exception that might be thrown in the body of a method or constructor, in a place where it is not enclosed by a matching try-catch construct, must be declared in a throws clause in the header of the method:

void foo() throws MyException { // throws clause . . . throw new MyException() ; // No enclosing // try-

catch(MyException . . .) . . . }

The compiler will insist invocations of foo() are treated with the same care as actual throw statements—either enclosed in matching try-catch constructs, or declared in turn in the header of the calling method.


Exception Handling in Nested Calls

void method1() { try { method2() ; } catch (Exception3 e) { doErrorProcessing(e); }}

void method2() throws Exception3 { method3() ; // method2 just passes exception

through}

void method3 throws Exception3 { throw new Exception3() ; // create exception}


Example using java.ioimport java.io.* ;

PrintWriter out ; try { out = new PrintWriter(new

FileOutputString(“filename”)) ; // create

and open file out.write(“stuff put out”) ; . . . out.close() ;

} catch (IOException e) { // Catches all I/O errors, including read and write stuff, say

System.err.println(“IO error: ” + e.getMessage()) ; System.exit(1) ;}


How (not) to Ignore an Exception Sometimes you can’t think of a good way to recover

from an exception—e.g. an exception thrown by a library method. But the compiler forces you to do something.

Probably the worst thing you can do is to wrap the method invocation in a try-catch with an empty catch clause—– the useless try-catch constructs make the code unreadable, and

– meanwhile, ignoring an error condition and silently carrying on the program may produce code even less reliable than, say, a typical C program, where the library error probably at least aborts the whole program!

Usually it is safer to have your methods throw the exceptions—all the way up to the main method, if necessary. Then at least the program will stop.

If you are really lazy you can just declare every method you ever write with throws Exception. . .


Part of the Exception Hierarchy

catch(FileNotFoundException e) { . . . } would catch specific exception whereas

catch(IOException e) { . . . } would catch all IOexceptions

Throwable

...

Error Exception

RuntimeException IOException

EOFException

FileNotFoundException

InterruptedIOException


Unchecked Exceptions

There are two exceptions (!) to the rule that all exceptions must be explicitly caught or thrown.

Error classes usually represent problems that might occur unpredictably in the JVM. For example OutOfMemoryError (although unusual in practice) might occur at almost any time.

RuntimeException classes usually represent errors “built into” the language—not thrown by a throw statement. There are about 20, including:– ArithmeticException,

ArrayIndexOutOfBoundsException, NullPointerException, ClassCastException, etc.

Note that exceptions that are thrown but not caught appear as error message on stderr. For applets they appear in the “Java console” of the browser.


Defining you own Exceptions The Exception class has fields and methods to give

information how the exception occurred. There are two constructors; one includes a message in the instance.

Can throw an exception of type Exception with a unique message, or create a subclass:class MyException extends Exception { public MyException () { super ("This is my exception message.") ; }}public static void MyMethod() throws MyException { . . . throw new MyException() ; . . . }

Methods e.getMessage() and e.printStackTrace() can be used on exceptions.


Interfaces


Abstract Classes Revisited

Recall an abstract class is a class that contains some abstract method declarations, with no implementation.

An abstract class can only be instantiated indirectly, as a superclass of a class that overrides all the abstract methods, and gives them an implementation. You cannot directly create an instance of an abstract class.– Constructors, static methods, private methods cannot be

abstract.– A subclass that does not override all abstract methods is still

abstract.– A method that overrides a superclass method cannot be

abstract

But an abstract class will generally also contain “non-abstract” members—method implementations, instance variables, etc—and constructors.


Interfaces

An interface is something like an abstract class where every method is required to be abstract.

An interface specifies a collection of instance methods (behaviors) without giving the implementation of their bodies—akin to giving an API: public interface Storable { public abstract void store(Stream s) ; public abstract void retrieve(Stream s) ; }

Interfaces cannot include instance variables, constructors, or static methods.

They can include class variables, but only if they are declared final—essentially constant definitions.


Implementing an interface As for an abstract class, one cannot directly

create an instance of an interface. Unlike an abstract class, one cannot even extend

an interface to create a class. An interface is not a class, and it cannot have subclasses.

Instead, a class must implement an interface:

public class Picture implements Storable { public void store(Stream s) { // JPEG compress image before storing . . . } public void retrieve(Stream s) { // JPEG decompress image after retrieving . . . } }


An Interface is a Contract

Any class that implements an interface is guaranteeing a set of behaviors. The body of the class will give concrete bodies to the methods in the interface.– If any methods in the interface are not implemented, the

class must be declared abstract.

Example: a class that defines the behaviour of a new thread must implement the Runnable interface: public interface Runnable { public void run() ; }

Any interface defines a type, similar to a class type. An instance of any class that implements a particular interface can be assigned to a variable with the associated interface type.


An Interface Defines a Type Assume the classes Picture and StudentRecord

both implement the Storable interface:

public class StudentBody { Stream s; . . . public void register(Picture id_photo, StudentRecord

id_card) { save(id_photo); save(id_card); } public void save(Storable o) { // o has type Storable o.store(s); } }


Classes can Implement Several Interfaces

Interfaces address some of the same requirements as multiple inheritance in C++ (for example), but avoid various complexities and ambiguities that come from inheriting implementations and instance variables from multiple superclasses.

A class can extend its superclass and implement several interfaces: class Picture implements Storable, Paintable {

// Body must now include any methods in Paintable,

// as well as store() and retrieve().

. . . }

Instances of the class acquire all the implemented interface types, in addition to inheriting their superclass type.


Interfaces can Extend other Interfaces An interface can extend one or more other interfaces:

interface Material extends Storable, Paintable { // Additional methods if necessary. . . . . . }

If non-trivial “lattices” of types are really needed, eg:

NoColor

GreenBlueRed

AnyColor

CyanYellowMagenta

they can be implemented using interface types.


Interfaces can hold Constant Definitions

Interfaces can hold fields, provided are static and final.

An interface can be a natural place to define a collection of related constants, perhaps simulating a C-like enumeration type: public interface Direction {

public final static int NORTH = 0 ; public final static int EAST = 1 ; public final static int SOUTH = 2 ; public final static int WEST = 4 ; }

Use constants by, eg, Direction.NORTH. Sometimes a class will implement such an

interface, just so it can access the included constants without using the Direction prefix.


Interfaces can be used as Markers

The Java environment includes several examples of empty interfaces that are used only as markers.

By implementing such an interface, the programmer is typically telling the compiler or runtime system to treat the class in some special way:– Cloneable—the Object.clone() method will throw an

exception if invoked on an object from a subclass that does not implement the empty Cloneable interface.

– Serializable—the ObjectOutputStream.writeObject() method will not write an object that does not implement the empty Serializable interface.

– Remote—any class whose methods may be invoked remotely using the RMI mechanism, must implement the empty Remote interface.


Summary Interfaces play a crucial role in structuring

programs that need to declare multiple sets of behaviors such as applets and threads.


Packages


Packages

One file can contain several related classes, but only one of them can be public. If the public class is called Wheat, then the file must be called Wheat.java.

A set of classes in different files can be grouped together in a package. Each file must start with a package declaration, eg:

package mill;


Packages and Directory Structure (JDK)

In JDK, each of the files in one package must be in the same directory (which may be in an jar archive file).

For simple package names, the name of the directory should be the same as the package:

Directory name: mill

File: wheat.java: Stone.java:Package mill ;

Public class Wheat { …}…

Package mill ;Public class Stone { …}…


Hierarchical Package Names Packages can be grouped hierarchically. For

example, the mill package could be nested in a package called agriculture. Then the name of the package would be changed to agriculture.mill (full name required).

In JDK, the classes of agriculture.mill should appear in a directory called: agriculture/mill (UNIX) agriculture\mill (Windows)

(relative to some directory, which must appear on the user’s CLASSPATH).

Standard Java libraries are in packages with names like java.lang, java.util, java.io, etc.

If you need to construct a globally unique name, can use your Internet domain name, inverted, as a prefix, eg: edu.fsu.csit.mpiJava


Fully Qualified Class Names

A class can always be referred to in Java code by its fully qualified name which includes the package name as a prefix, eg: public class VectorTest { public static void main (String [] args) { java.util.Vector bag = new

java.util.Vector() ; bag.addElement(new

java.lang.String(“item”)) ; }

Using fully qualified names is tedious in general.


Import statements The import declaration allows you to avoid giving

fully qualified names, eg: import java.util.Vector ; // import declaration

public class VectorTest { public static void main (String [] args) { Vector bag = new Vector() ; bag.addElement(new String(“item”)) ; }

Can also import all classes in, eg, java.util by import java.util.* ;

(but note wildcard can only appear in last position).

Note classes (like String) in java.lang are automatically imported.


CLASSPATH The import declaration only controls

conventions on naming within a source file. It doesn’t address basic accessibility of the class files. You can use a class without importing it.

In JDK (except for classes provided with the Java language) jar files or root directories of any package used (or class files for any classes not in any package) must be in the current directory, or in a directory in the CLASSPATH environment variable.

This variable is used by both the compiler javac and the JVM command, java.


Java System Packages, I

java.lang contains essential Java classes and is by default imported into every Java file. So import java.lang.* is unnecessary. For example Thread, Math, Object and wrapper classes are here.

java.io contains classes to do I/O. java.util contains various utility classes that didn't

make it to java.lang. Date is here as are Vector, hashtables, etc.

java.net contains classes to do network applications. Sockets, Internet addresses, URLs etc.

java.applet has the classes needed to support applets java.awt has the original classes to support

windowing—The Abstract Windows Toolkit. java.awt.image has image processing classes.


Java System Packages, II

java.awt.datatransfer contains classes to transfer data from a Java program to the system clipboard (drag-and-drop).

java.beans contains classes to write reusable software components.

java.lang.reflect enables a program to discover the accessible variables and methods of a class at run-time.

java.rmi—classes for Remote Method Invocation. java.security enables a Java program to encrypt data and

control the access privileges provided. java.sql—Java Database Connectivity (JDBC) enables Java

programs to interact with a database using the SQL language. java.text are classes that provide internationalization

capabilities for numbers, dates, characters and strings. java.util.jar combines java .class files and other files into one

compressed file called a Java archive (JAR) file.


Additional Java 1.2 System Packages

javax.accessibility—contracts between user interface components and assistive technology.

javax.swing—additional user interface components as well as providing standard “look and feel” for old ones.– border, colorchooser, event, filechooser, plaf, table,

text, tree, undo org.omg.CORBA—Provides the mapping of the

Object Management Group CORBA APIs to the Java programming language, including the class ORB, which is implemented so that a programmer can use it as a fully-functional Object Request Broker (ORB).


Further information

The Java 2 API specification: http://java.sun.com/products/jdk/1.2/docs/api

documentation in javadoc format.

The Java Class Libraries, 2nd Edition, Volumes 1 and 2, plus supplements for the Java 2 platform.

http://java.sun.com/products/jdk/1.2/docs/api

CIS 5930-04 – Spring 2001

Documents

Transcript of CIS 5930-04 – Spring 2001