Think Java: Java Programming Language Part 2 Chia James Chang [email protected] (Materials are...

Think Java:Java Programming Language Part 2

Chia James [email protected]

(Materials are based on: The Java Tutorials)

Overview

• The Java Tutorials: Learning the Java Language http://docs.oracle.com/javase/tutorial/index.ht

ml

• Quick Review (Language Basics, Classes, Objects)

• Interfaces• Numbers and Strings• Generics• Packages• What’s Next?

http://docs.oracle.com/javase/tutorial/index.html


References• The Java Tutorials


• Java SE 7 JDK (Windows, Unix/Linux, and Mac)

http://www.oracle.com/technetwork/java/javase/downloads/index.html

• Java Platform SE 7 Documentation

http://docs.oracle.com/javase/7/docs/

• Eclipse IDE for Java EE Developers

http://www.eclipse.org/downloads/

• Java Language Specification

http://docs.oracle.com/javase/specs/

• Java SE API

http://www.oracle.com/technetwork/java/javase/documentation/api-jsp-136079.html

Reading Materials

• All materials are based on The Java Tutorialshttp://docs.oracle.com/javase/tutorial/index.html

• Interfaceshttp://docs.oracle.com/javase/tutorial/java/IandI/createinterface.html

• Numbers and Stringshttp://docs.oracle.com/javase/tutorial/java/data/index.html

• Genericshttp://docs.oracle.com/javase/tutorial/java/generics/index.html

• Packageshttp://docs.oracle.com/javase/tutorial/java/package/index.html

Quick Review(Language Basics, Classes,

Objects)

Language Basics

• Java Reserved Words• Variables• Operators• Expressions• Statements• Blocks• Control Flow Statements

Java Reserved Words abstract assertboolean break bytecase catch char class const continuedefault do doubleelse enum extendsfalse final finally float forgotoif implements import in instanceof int interfacelongnative new nullpackage private protected publicreturnshort static strictfp super switch synchronizedthis throw throws transient try truevoid volatilewhile

Variables

• Instance Variables (Non-Static Fields) int speed = 0;

• Class Variables (Static Fields) static int numGears = 6;

• Local Variables (used in method) int count = 0;

• Parameters (used in method) public static void main(String[] args)

Naming

• Variable names are case-sensitive.• The name must not be a keyword or

reserved word.• One word

all lowercase letters (e.g. ratio)• Multiple words

capitalize the first letter of each subsequent word, aka, Camel Case (e.g. currentGear)

• Constant value capitalizing every letter and separating

subsequent words with the underscore character (e.g. static final int NUM_GEARS = 6)

Primitive Data Typesdata type size default

byte 8-bit signed 0

short 16-bit signed 0

int 32-bit signed 0

long 64-bit signed 0L

float 32-bit signed (single-precision) 0.0f

double 64-bit IEEE 754 (double-precision) 0.0d

boolean true and false false

char 16-bit Unicode char ‘\u0000’

OperatorsOperators Precedence

Postfix expr++ expr--

Unary ++expr –expr +expr –expr ~ !

Multiplicative * / %

Additive + -

Shift << >> >>>

Relational < > <= >= instanceof

Equality == !=

Bitwise AND &

Bitwise exclusive OR ^

Bitwise inclusive OR |

OperatorsOperators Precedence

Logical AND &&

Logical OR ||

Tenary ? :

Assignment = += -= *= /= %= &= ^= |= <<= >>= >>>=

Expressions

An expression is a construct made up of variables, operators, and method invocations.

int result = 1 + 2;

The data type of the value returned by an expression depends on the elements used in the expression.

String helloWorld = “hello” + “world”;

You can specify exactly how an expression will be evaluated using ( and ).

int result = (x + y) / 100;

Statements

• A statement forms a complete unit of execution terminated with a semicolon (;).

• Expression statements Assignment expression: speed = 0; Method invocations: System.out.println(“hello”); Object creation expressions

Point originOne = new Point(23, 94);

• Declaration statementsint speed = 0;

• Control flow statementsif (x > y) { ... }

Blocks

• A block is a group of zero or more statements between balanced braces and can be used anywhere a single statement is allowed.

if (x > 1) {

System.out.println(“true.”);

} else {

System.out.println(“false.”);

}

Control Flow Statements

• if• switch• while• do• for• break• continue• return• try/throw

Object-Oriented Programming (OOP)


• Classes• Objects• Inheritance• Interfaces• Abstract Classes• Packages


• A fundamental principle of object-oriented programming: Encapsulation

Inheritance

Polymorphism

Classes

• A class is a blueprint or prototype from which objects are created. Class header

Class body

class MyClass extends MySuperClass implements YourInterface{

// fields (variables)

// constructors

// methods

}

Objects

• An object is a software bundle of related behavior (methods) and state (fields).

• Referencing an object’s fields.objectReference.fieldName;

bike.speed;

• Calling an object’s methods.objectReference.methodName(argumentList);

bike.setSpeed(10);

Creating Objects

• A class provides the blueprint for objects; you create an object from a class.

Point originalOne = new Point(23, 40);

• The above statement has three parts: Declaration: variable declarations

Instantiation: the new keyword operator creates the object.

Initialization: the constructor initializes the new object.

Garbage Collector

• Garbage collector: The Java VM deletes objects when it determines that they are no longer being used.

• An object is eligible for garbage collection when there are no more references to that object. Or, you can explicitly drop an object reference by setting the variable to null.

Nested Classes

Classes

• Classes declared outside of any class are known as top-level classes.

• Classes declared as members of other classes are called as nested classes.

Nested Classes

• There are two kinds of nested classes: Static member class: declared static

Inner class: non-static member class, anonymous class and local class

class OuterClass {

static class StaticNestedClass {

…

}

class InnerClass {

…

}

}

Static Member Classes

• Static member class is a static member of an enclosing class.

• Although enclosed, it does not have an enclosing instance of that class, and cannot access the enclosing class’s instance fields and call its instance methods.

• It can access or call static members of the enclosing class, even those members that are declared private.

Non-Static Member Classes

• A non-static member class is a non-static member of an enclosing class.

• Each instance of the non-static member class’s instance methods can call instance method in the enclosing class and access the enclosing class instance’s non-static fields.

Anonymous Classes

• An anonymous class is a class without a name.

• It is not a member of it’s enclosing class.

• Instead, an anonymous class is simultaneously declared and instantiated any place where it is legal to specify an expression.

Local Class

• A local class is a class that is declared anywhere that a local variable is declared.

• It has the same scope as a local variable.

• A local class instance can access the surrounding scope’s local variables and parameters.

• However, the local variable and parameters that are accessed must be declared final.

Why Use Nested Classes?

• It’s a way of logically grouping classes that are only used in one place. Nesting such “helper classes” makes their package more streamlined.

• It increases encapsulation.class OuterClassA {

private int aVar;

class InnerClassB {

private int bMethod() {

return aVar;

}

}

}

Why Use Nested Classes?

• Nested classes can lead to more readable and maintainable code.

class OuterClassA {

private int aVar;

class InnerClassB {

private int bMethod() {

return aVar;

}

}

}

Question

• The following program doesn’t compile:public class Problem {

String s;

static class Inner {

void testMethod() {

s = "Set from Inner";

}

}

}

• What do you need to do to make it compile?

Answer

• Delete static in front of the declaration of the Inner class. An static inner class does not have access to the instance fields of the outer class.

Enum Types

• The constructor for an enum type must be package-private access. It automatically creates the constants that are defined at the beginning of the enum body. You cannot invoke an enum constructor yourself.

public enum Day {

SUNDAY, MONDAY, TUESDAY, WEDNESDAY,

THURSDAY, FRIDAY, SATURDAY

}

Enum Types

• An enum type is a type whose fields consist of a fixed set of constants.

• The enum declaration defines a class (called an enum type). The enum class body can include methods and other fields. The compiler automatically adds some special methods, e.g. values method, when it creates an enum.

public enum Day {

SUNDAY, MONDAY, TUESDAY, WEDNESDAY,

THURSDAY, FRIDAY, SATURDAY

}

Annotations

• Provide data about a program that is not part of the program itself.

• Apply to program’s declarations of classes, fields, methods, and other program elements.

• Appear first and may include elements with named or unnamed values.

@SuppressWarnings(value = “unchecked”)

@SuppressWarnings(“unchecked”)

@Override

@Deprecated

Defining Annotation Type

import java.lang.annotation.*;

@Documented

@interface ClassPreamble {

String author();

}

@ClassPreamble (

author = "John Doe"

}

Public class MyClass {

}

Interfaces

Interfaces

• An interface is a contract between a class and the outside world.

• When a class implements an interface, it promises to provide the behavior published by that interface.

• An interface is not a class.

• Writing an interface is similar to writing to a class, but they are two difference concepts: A class describes the attributes and behaviors of an

object.

An interface contains behaviors that a class implement.

Interfaces vs Classes

• An interface is not a class.

• Writing an interface is similar to writing to a class, but they are two difference concepts: A class describes the attributes and behaviors

of an object.

An interface contains behaviors that a class implement.

Interfaces and Classes: Similarities

An interface can contain any number of methods.

An interface is written in a file with a .java extension, with the name of the interface matching the name of the file.

The bytecode of an interface appears in a .class file.

Interfaces appear in packages, and their corresponding bytecode file must be in a directory structure that matches the package name.

Interfaces and Classes: Differences

• You cannot instantiate an interface.

• An interface does not contain any constructors.

• All of the methods in an interface are abstract.

• An interface cannot contain instance fields.

• The only fields that can appear in an interface must be declared both static and final – constant.

• An interface is not extended by a class; it is implemented by a class.

• An interface can extend multiple interfaces.

Interfaces

• Interface can contain only constants, method signatures, and nested types. There are no method bodies.

• All methods declared in an interface are implicitly public.

• All constants defined in an interface are implicitly public, static, and final.

• An interface declaration consists of modifiers, the keyword interface, the interface name, a comma-separated list of parent interfaces (if any), and the interface body.

Define Interfacepublic interface Bicycle {

// constant

int MAX_GEARS = 20;

// wheel revolutions per minute

void changeGear(int newValue);

void speedUp(int increment);

void applyBrakes(int decrement);

void changeCadence(int newValue);

}

Implements Interfacepublic interface Bicycle {

// wheel revolutions per minute

void changeGear(int newValue);

. . .

}

class MountainBike implements Bicycle {

void changeGear(int newValue)

gear = newValue;

}

. . .

} Bicycle

Mountain Bike

Road Bike Tandem Bike

Rewriting Interfaces

• Developed An InterfacePublic interface DoIt {

void doSomething(int i);

}

• Want to Add A MethodPublic interface DoIt {

void doSomething(int i);

int doSomethingElse(String s);

}

• Add A MethodPublic interface DoItPlus extends DoIt {

boolean doSomethingElse(String s);

}

Interfaces & Multiple Inheritance

• The Java programming language does not permit multiple inheritance, but interfaces provide an alternative.

• In Java, a class can inherit from only one class but it can implement more than one interface. Therefore, objects can have multiple types: the type of their own class and the types of all the interfaces that they implement. This means that if a variable is declared to be the type of an interface, its value can reference any object that is instantiated from any class that implements the interface.

Multiple Interface

public interface GroupedInterface extends Interface1, Interface2, Interface3 {

// constant declarations

// base of natural logarithms

double E = 2.718282;

// method signatures

void doSomething (int i, double x);

int doSomethingElse(String s);

}

Summary of Interfaces

• A protocol of communication between two objects

• Contains signatures and constant but not method implementation

• A class implements an interface must implement all methods

• An interface name can be used anywhere a type can be used

Question 1

• What’s wrong with the following interface?Public interface SomethingWrong {

void aMethod(int aValue) {

System.out.println(“Hi Mom”

}

}

Answer 1

• It has a method implementation in it. It should just have a declaration.

public interface SomethingWrong {

void aMethod(int aValue);

}

Question 2

• Is the following interface valid?Public interface Marker {

}

Answer 2

• Yes. Methods are not required.

Abstract Classes

Abstract Classes and Methods

• An abstract class is a class that is declared abstract—it may or may not include abstract methods. Abstract classes cannot be instantiated, but they can be subclassed.

• An abstract method is a method that is declared without an implementation (without braces, and followed by a semicolon).

abstract void moveTo(double x, double y);

Abstract Classes and Methods

• If a class includes abstract methods, the class itself must be declared abstract.public abstract class GraphicObject {

abstract void draw();

}

• An abstract class may have static fields and static methods. You can use these static members with a class reference as you would with any other class.AbstractClass.staticMethod();

Interfaces vs Abstract Classes

• An interface is a reference type, similar to a class, that can contain only constants, method signatures, and nested types. There are no method bodies. Interfaces cannot be instantiated—they can only be implemented by classes or extended by other interfaces.

• Unlike interfaces, abstract classes can contain fields that are not static and final, and they can contain implemented methods.


public interface GraphicObject {

int MIN_SIZE = 10;

void moveTo(int newX, int newY);

void draw();

}

class Circle implements GraphicObject {

void moveTo(int newX, int newY) {

. . .

}

void draw() {. . .}

}


abstract class GraphicObject {

int MIN_SIZE = 10;

int x, y;

void moveTo(int newX, int newY) {

. . .

}

abstract void draw();

}

class Circle extends GraphicObject {

void draw() {. . .}

}

Abstract Class Implements Interface

• A class that implements an interface must implement all of the interface’s methods.

• An abstract class does not need to implement all of the interface methods.abstract class X implements Y {

// implements all but one method of Y

}

class XX extends X {

// implements the remaining method in Y

}

Summary of Inheritance

• Except for the Object class, a class has exactly one direct superclass.

• A class inherits fields and methods from all its superclasses, whether direct or indirect.

• A subclass can override methods that it inherits, or it can hide fields or methods that it inherits.

• The Object class is the top of the class hierarchy. All classes are descendants from this class and inherit methods from it.

Summary of Abstract Classes

• You can prevent a class or a method from being subclassed by using the final keyword.

• An abstract class can only be subclassed; it cannot be instantiated.

• An abstract class can contain abstract methods-methods that are declared but not implemented. Subclasses then provide the implementations for the abstract methods.

Questionpublic class ClassA {

public void methodOne(int i) {}

public void methodTwo(int i) {}

public static void methodThree(int i) {}

public static void methodFour(int i) {}

}

public class ClassB extends ClassA {

public static void methodOne(int i) {}

public void methodTwo(int i) {}

public void methodThree(int i) {}

public static void methodFour(int i) {}

}

• What method overrides a method in the superclass?

Answer

Question 1a: Which method overrides a method in the superclass?

Answer 1a: methodTwo

Question 1b: Which method hides a method in the superclass?

Answer 1b: methodFour

Question 1c: What do the other methods do?

Answer 1c: They cause compile-time errors.

Numbers & Strings

Numbers Classes

• When working with numbers, most of the time you use the primitive types such as int, float, byte, etc. in your code.

• The Java platform provides wrapper classes for each of the primitive data types. These classes “wrap” the primitive in an object.

Boxing and Unboxing

• Often, the wrapping is done by the compiler: If you use a primitive where an object is expected,

the compiler boxes the primitive in its wrapper class for you.

If you use a number object when a primitive is expected, the compiler unboxes the object for you.

Integer x, y;

x = 12; // boxed

y = 15; // boxed

System.out.println(x+y); // unboxed

Number Wrapper Classes

Number

Byte Double Float

Integer Short Long

Why Use Number Object?

• As an argument of a method that expects an object (often used when manipulating collections of numbers).

• To use constants defined by the class, such as MIN_VALUE and MAX_VALUE, that provide the upper and lower bounds of the data type.

• To use class methods for converting values to and from primitive types, strings, and between number systems (decimal, octal, hexadecimal, binary).

Strings

• Strings, which are widely used in Java, are a sequence of characters and objects.

• Creating StringsString greeting = “Hello world!”;

String hello = new String(“Hello world!”);

• The String class is immutable, so that once it is created a String object cannot be changed.

Concatenating Strings

• String class includes a concat() method

String1.concat(string2);

• Use Concat() method with sting literals

“My name is “.concat(“John.”);

• More commonly concatenated with “+” operator

“Hello, “ + “world” + “!”;

Creating Format Strings

• Use printf() and format() to print output

System.out.printf(“%f, %d”, floatV, intV);

• String class includes a format() method

String fs;

fs = String.format(“%f, %d”, floatV, intV);

System.out.println(fs);

Converting Strings to Numbers

• Use valueOf() of Number subclassesoByte, Integer, Double, Float, Long, and

ShortString abc = “12.3”;

float a = (Float.valueOf(abc)).floatValue();

Converting Numbers to Strings

• Concatenate “i” with an empty stringint i = 5;

String s1 = “” + i;

• Use toString() of Number subclassesint i;

String s2 = Integer.toString();

More Methods in String

• More methods in String class• charAt(…)

• substring(…)

• split(…)

• trim()

• toLowerCase(), toUpperCase()

• indexOf(…)

• contains(…)

• …

StringBuilder Class

• StringBuilder objects are like String objects, except that they can be modified.

• length()

• capacity() // number of char spaces allocated

• append()

• insert()

• delete()

• replace()

• reverse()

• toString()

Questions

• What is the initial capacity of the following string builder? (Hints: string length = 26)

StringBuilder sb =

new StringBuilder("Able was I ere I saw Elba.");

Answers

• Initial string length + 16 = 26 + 16 = 42.

Generics

Why Use Generics?

• Generics enable types (classes and interfaces) to be parameters when defining classes, interfaces and methods.

• A generic type is a generic class or interface that is parameterized over types.

• Type parameters provide a way for you to re-use the same code with different inputs.

Why Use Generics? (continued)

• Strong type checks at compile time. A Java compiler applies strong type checking to generic code.

• Enabling programmers to implement generic algorithms (different types, type safe, and easier to read.)

Why Use Generics? (continued)

• Elimination of casts.// Without generics

List list = new ArrayList();

list.add(“hello”);

String s = (String) list.get(0);

// With generics

List<String> list = new ArrayList<String>();

list.add(“hello”);

String s = list.get(0);

Generics

• Non-Generic version of Box class:public class Box {

private Object obj;

public void add(Object obj) {

this.obj = obj;

}

public Object get() {

return obj;

}

}

Generics (continued)

• Non-Generic version of Box class:public class BoxDemo1 {

public static void main(String[] args) {

// ONLY place Integer objects into this box!

Box intBox = new Box();

intBox.add(new Integer(10));

Integer someInt = (Integer)intBox.get();

System.out.println(someInt);

}

}

Generics (continued)

• Without Generics – runtime ClassCastExceptionpublic class BoxDemo2 {

public static void main(String[] args) {

// ONLY place Integer objects into this box!

Box intBox = new Box();

intBox.add(“10”);

Integer someInt = (Integer)intBox.get();

System.out.println(someInt);

}

}

Generic Class

• A generic class is defined with the following format:class ClassName<T1, T2, …, Tn> { }

• class name

• type parameters (also called type variables) T1, …

• type parameter can be any non-primitive type:class type

interface type

array type

another type variable

Generic Types

• Generic version of Box class.public class Box<T> {

private T t; // T stands for “Type”

public void add(T t) {

this.t = t;

}

public T get() {

return t;

}

}

Generic Types

• Multiple type parameters (each parameter must be unique within its declaring class or interface)Box<T, U>

• Generic type innovation replaces T with some concrete typeBox<Integer> intBox = new Box<Integer>();

intBox.add(new Integer(10));

Integer someInt = intBox.get();

intBox.set(“10”); // compilation error

Type Parameter Naming Conventions

• By convention, type parameter names are single, uppercase letters.E – Element (used extensively by the Java

Collections Framework)

K – Key

N – Number

T – Type

V – Value

S, U, V etc. – 2nd, 3rd, 4th types

Type Inference

• Determines the type argument(s) that make the invocation applicable using the method invocation and corresponding declaration.

• Determines the types of the arguments and the return types.

• Finds the most specific type that works with all of the arguments.static <T> pick(T t1, T t2) { return t2; }

Serializable s = pick(“a”, new ArrayList<String>());

Type Inference

• Define a static generic methodpublic static void addBox(U u,List<Box> boxes) {

Box box = new Box();

box.add(u);

boxes.set(box);

}

• Use the method (with type inference)BoxDemo.<Integer>setBox(Integer.valueOf(10),

listOfIntBoxes);

// type inference: let compiler infer the type

BoxDemo.addBox(Integer.valueOf(20),listOfIntBoxes);

Bounded Type Parameters

• Bounded type parameter restricts the kind of types that are allowed to be passed to a type parameter

• To declare a bounded type parameter: list the type parameter’s name

followed by the extends keyword

followed by its upper boundpublic void inspect(U u) {

System.out.println(“T: “ + t.getClass().getName());

System.out.println(“U: “ + u.getClass().getName());

}

The Need for Wildcards

• Circle is a kind of (or subtype) Shape.

• String is a kind of Object.

• List<Object> is a kind of Collection<Object>.

• BUT, List<String> is NOT a kind of List<Object>.

Polymorphic behavior does not apply to multiple parameterized types the differ only in regard to one type parameter being a subtype of another type parameter.

Wildcards

• In generics, an unknown type is represented by the wildcard character “?”.

• Upper bounded wildcardClassName<? extends UpperBoundedClassName>

• Lower bounded wildcardClassName<? super LowerBoundedClassName>

• Unbounded wildcardClassName<?>

Upper Bounded Wildcards

• Upper bounded wildcard (“an unknown type that is a subtype of call”) e.g. Number: Integer, Double, Float, Number

ClassName<? extends UpperBoundedClassName>

List<? extends Number>

Unbounded Wildcards

• Unbounded wildcard: list of unknow type<?> OR <? extends Object>

// List<Object>,List<Integer>,List<String>,List<Double>

public static void printList(List<Object> list) {

for (Object e : list)

System.out.println(e + “ “);

System.out.println();

//

public static void printList(List<?> list) {

for (Object e : list)

System.out.println(e + “ “);

System.out.println();

Lower Bounded Wildcards

• Lower bounded wildcard (“an unknown type that is a super type of call”)

• e.g. Integer: Integer, Number, ObjectClassName<? super LowerBoundedClassName>

List<? super Integer>

public static void addNumbers(List<? super Integer> list) {

for (int i = 1; i <= 10; i++) {

list.add(i);

}

}

Type Erasure

• Type erasure: a process where the compiler removes all information related to type parameters and type arguments within a class or method.

• Type erasure enables Java applications that use generics to maintain binary compatibility with Java libraries and applications that were created before generics.

• For instance, Box<String> is translated to type Box, which is called the raw type.

Question 1

• What can you do with it?public class AnimalHouse<E> {

private E animal;

public void sendAnimal(E x) { animal = x; }

public E getAnimal() { return animal; }

}

public class Animal {}

public class Cat extends Animal {}

public class Dog extends Animal {}

AnimalHouse<Animal> house = new AnimalHouse<Cat>();

Answer 1

• Failed to compile. AnimalHouse<Cat> and AnimalHouse<Animal> are not compatible types, even though Cat is a subtype of Animal.

Question 2


private E animal;



}




AnimalHouse<Cat> house = new AnimalHouse<Animal>();

Answer 2

• Failed to compile. AnimalHouse<Cat> and AnimalHouse<Animal> are not compatible types, even though Cat is a subtype of Animal.

Question 3


private E animal;



}




AnimalHouse<?> house = new AnimalHouse<Cat>();

house.setAnimal(new Cat());

Answer 3

• Failed to compile.The first line is acceptable. It’s OK to define an

instance of unknown type.

However, the compiler doesn’t know the type of animal stored in house so the setAnimal method cannot be used.

Question 4


private E animal;



}




AnimalHouse house = new AnimalHouse ();

house.setAnimal(new Dog());

Answer 4

• Compiles with a warning.

• The compiler doesn’t know what type house contains. It will accept the code, but warn that there might be a problem when setting the animal to an instance of Dog.

Packages

Packages

• A package is a grouping of related types providing access protection and name space management.

• Placing your code into packages makes large software projects easier to manage.

Creating a Package

• To create a package,Choose a name for the package

Put a package statement at the top of every source file

The package statement must be the first line in the source file

There can be only one package statement in each source file, and it applies to all types in the file.

package graphics;

public abstract class Graphic {

}

Naming a Package

• Package names are written in all lower case

• Packages in the Java language itself begin with java or javax

• Companies use their reversed Internet domain name, e.g., com.acme.mypackage

• In some cases, the Internet domain name may not be a valid package name. The suggested convention is to add an underscore.

• Domain name: hyphenated-name.example.org

• Package name: org.example.hyphenated_name

Using Package Members

• To use a public package member from outside its package, you must do one of the following:Refer to the member by its fully qualified name

Import the package member

Import the member’s entire package

Referring By Its Qualified Name

• Use the member’s fully qualified name from a different packagepackage graphics;

public class Rectangle() {}

package othersomePackage;

graphics.Rectangle myRect =

new graphics.Rectangle();

Importing a Package Member

• To import a specific member into the current file, put an import statement at the beginning of a file before any type definitions but after the package statement.package otherPackage;

import graphics.Rectangle;

Rectangle myRect = new Rectangle();

Importing an Entire Package

• To import all the types contained in a particular package, use the import statement with the asterist (*) wildcard character.package otherPackage;

import graphics.*;

Rectangle myRect = new Rectangle();

Circle myCircle = new Circle();

Packages Imported Automatically

• For convenience, the Java compiler automatically imports three entire packages for each source file: the package with no name

the java.language package

the current package (the package for the current file)

Packages are not Hierarchical

• At first, packages appear to be hierarchical, but they are not.

• The prefix java.awt is used for a number of related packages to make the relationship evident, but not to show inclusion.

• You must import both packages with all their files:java.awt.*

java.awt.color.*;

Name Ambiguities

• If a member in one package shares its name with a member in another package and both packages are imported, you must refer to each member by its qualified name.For instance, Rectangle class in both graphics

and java.awt packages:

import java.awt.*

import graphics.*;

graphics.Rectangle myRect;

Static Import Statement

• The static import statement gives you a way to import the constants and static methods of a class without needing to prefix the name of their class.

• Use properly, static import makes code more readable by removing class name repetition.java.lang.Math:

public static final double PI = 3.14159;

double r = Math.cos(Math.PI * theta);

import static java.lang.Math.PI;

double r = cos(PI * theta);

Managing Source and Class Files

• Put the source code for a class, interface, enumeration, or annotation type in a text file whose name is the simple name of the type and whose extension is .java.//Rectangle.java

package com.acme.graphics;

public class Rectangle {}

• Put the source file in a directory whose name reflects the name of the package.c:\src\com\acme\graphics\Rectangle.java

Managing Source and Class Files

• The compiled .class file should be arranged in different directory.c:\classes\com\acme\graphics\Rectangle.class

• The full path to the classes directory is called the class path and is set with the CLASSPATH system variable.

• A class path may include several paths, separated by a semicolon (Windows) or colon (Unix).CLASSPATH=c:\classes;c:\Users\james\classes

Questions 1

• Assume that you have written some classes. Belatedly, you decide that they should be split into three packages, as listed in the table below.

• What line of code will you need to add to each source file to put each class in the right package?

Package Name Class Name

mygame.server Server

mygame.shared Utilities

mygame.client Client

Answer 1

• The first line of each source file must specify the package:

• In Client.java add:package mygame.client;

• In Server.java add:package mygame.server;

• In Utilities.java add:package mygame.shared;

Questions 2


• What subdirectories must you create? Which subdirectory does each source file go in?Package Name Class Name




Answer 2

• Within the mygame directory:

• In mygame/client/ place:Client.java

• In mygame/server/ place:Server.java

• In mygame/shared/ place:Utilities.java

Questions 3


• Do you think you'll need to make any other changes to the source files to make them compile correctly? If so, what?Package Name Class Name




Answer 3

• Yes, you need to add import statements.

• In Client.java and Server.java add:import mygame.shared.Utilities;

• In Server.java add:import mygame.client.Client;

Naming Conventionshttp://en.wikipedia.org/wiki/Naming_convention_(programming

)#Java

Naming Conventions - Classes

• Class names should be nouns in Upper CamelCase, with the first letter of every word capitalized. Use whole words — avoid acronyms and abbreviations (unless the abbreviation is much more widely used than the long form, such as URL or HTML).

class Bicycle { }

Naming Conventions - Methods

• Methods should be verbs in lower CamelCase; that is, with the first letter lowercase and the first letters of subsequent words in uppercase.

public in getGear() { }

Naming Conventions - Variables

• Local variables, instance variables, and class variables are also written in lower CamelCase.

• Variable names should not start with underscore (_) or dollar sign ($) characters, even though both are allowed. Certain coding conventions state that underscores should be used to prefix all instance variables, for improved reading and program understanding.

public int gear;

Naming Conventions - Variables

• Variable names should be short yet meaningful. The choice of a variable name should be mnemonic — that is, designed to indicate to the casual observer the intent of its use.

• One-character variable names should be avoided except for temporary "throwaway" variables. Common names for temporary variables are i, j, k, m, and n for integers; c, d, and e for characters.

public int speed;

Naming Conventions - Constants

• Constants should be written in uppercase characters separated by underscores. Constant names may also contain digits if appropriate, but not as the first character.

public final static int MAXIMUM_NUM_OF_SPEEDS = 10;

What’s Next?

What’s Next?• Classes from Technology Service Group (TSG)@ROLF

http://www.techsamaritan.org/ Future Java classes or non-Java classes such as C#, C++,

JavaScript, Objective-C, etc.• Java Collections Framework @ Java Tutorials

http://docs.oracle.com/javase/tutorial/collections/index.html

• Essential Java Classes @ Java Tutorials http://docs.oracle.com/javase/tutorial/essential/index.html

• Other Lessons from Java Tutorials http://docs.oracle.com/javase/tutorial/

• Android Development http://developer.android.com/

• Spring Framework the most popular app development framework for

enterprise Java http://www.springsource.org/

Thank you!謝謝 !

Think Java:Java Programming Language Part 2

Chia James [email protected]

Think Java: Java Programming Language Part 2 Chia James Chang [email protected] (Materials are...

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