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November 7, 2007
Interfaces
ComS 207: Programming I (in Java)Iowa State University, FALL 2007Instructor: Alexander Stoytchev
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Quick Review of Last Lecture
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Objects – instances of a class with a static variable ‘size’
int weight= 170; int weight= 130;
obj1 obj2
static int size;
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Objects – instances of classes
• Note that the variables can have different values in the two objects
int size =5;int weight= 170;
int size =10;int weight= 130;
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Static Class Members
• The order of the modifiers can be interchanged, but by convention visibility modifiers come first
• Recall that the main method is static – it is invoked by the Java interpreter without creating an object
• Static methods cannot reference instance variables because instance variables don't exist until an object exists
• However, a static method can reference static variables or local variables
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Static Class Members• Recall that a static method is one that can be
invoked through its class name
• For example, the methods of the Math class are static:
result = Math.sqrt(25);
• Variables can be static as well
• Determining if a method or variable should be static is an important design decision
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Static Methods
class Helper{ public static int cube (int num) { return num * num * num; }}
Because it is declared as static, the methodcan be invoked as
value = Helper.cube(5);
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myMethod();
myMethodcompute
Method Control Flow
• If the called method is in the same class, only the method name is needed
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myMethod();
myMethodcompute
Accessing Variables
• If the called method is in the same class, only the method name is needed
int myVariable;
myVariable=5; OK
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myMethod();
myMethodcompute
Accessing Variables
• Static methods cannot use non static class variables.
int myVariable;
myVariable=5; Error
static
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myMethod();
myMethodcompute
Accessing Variables
• Static methods can use static class varables
static int myVariable;
myVariable=5; OK
static
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Class Helper
static cube
static helpMe
helpMe();
Helper.cube();
main
Method Control Flow
• Static methods can only call other static methods within the same classs
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Chapter 6
Section 6.4
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Class Relationships
• Classes in a software system can have various types of relationships to each other
• Three of the most common relationships:
Dependency: A uses B
Aggregation: A has-a B
Inheritance: A is-a B
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Dependency
• A dependency exists when one class relies on another in some way, usually by invoking the methods of the other
• We've seen dependencies in many previous examples
• We don't want numerous or complex dependencies among classes
• Nor do we want complex classes that don't depend on others
• A good design strikes the right balance
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Dependency Example: Client-Server
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Dependency
• Some dependencies occur between objects of the same class
• A method of the class may accept an object of the same class as a parameter
• For example, the concat method of the String class takes as a parameter another String object
str3 = str1.concat(str2);
• This drives home the idea that the service is being requested from a particular object
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concat(String s)
Concatenation Example
str1 str2
concat(String s)
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Dependency
• The following example defines a class called Rational to represent a rational number
• A rational number is a value that can be represented as the ratio of two integers
• Some methods of the Rational class accept another Rational object as a parameter
• See RationalTester.java (page 297)• See Rational.java (page 299)
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Representing Rational Numbers
• public class RationalNumber{ private int numerator, denominator;
// …
• }
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Adding Two rational numbers
public RationalNumber add (RationalNumber op2){
int commonDenominator = denominator *op2.getDenominator();
int numerator1 = numerator * op2.getDenominator();
int numerator2 = op2.getNumerator() * denominator;
int sum = numerator1 + numerator2;
return new RationalNumber (sum, commonDenominator);
}
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Aggregation
• An aggregate is an object that is made up of other objects
• Therefore aggregation is a has-a relationship
A car has a chassis
A student has an address
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Aggregation
• In software, an aggregate object contains references to other objects as instance data
• The aggregate object is defined in part by the objects that make it up
• This is a special kind of dependency – the aggregate usually relies on the objects that compose it
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Aggregation Example: Copmonents of a Student
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Student
Home Address School Address
First Name Last Name
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john
21 Jump Street 800 Lancaster Ave.
John Smith
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marsha
123 Main Street 800 Lancaster Ave.
Marsha Jones
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Aggregation
• In the following example, a Student object is composed, in part, of Address objects
• A student has an address (in fact each student has two addresses)
• See StudentBody.java (page 304)• See Student.java (page 306)• See Address.java (page 307)
• An aggregation association is shown in a UML class diagram using an open diamond at the aggregate end
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Other Stuff from Section 6.4
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A More Complicated Student Example
Home Address School Address
Degree ProgramUniversityName
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How would you write the code for the more complicated student example?
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Inheritance is discussed in Chapter 8
Man Woman
AbstractPerson
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Condo
Mansion
5 bedroom house
AbstractHome
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The this Reference
• The this reference allows an object to refer to itself
• That is, the this reference, used inside a method, refers to the object through which the method is being executed
• Suppose the this reference is used in a method called tryMe, which is invoked as follows:
obj1.tryMe();
obj2.tryMe();
• In the first invocation, the this reference refers to obj1; in the second it refers to obj2
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The this reference
• The this reference can be used to distinguish the instance variables of a class from corresponding method parameters with the same names
• The constructor of the Account class (from Chapter 4) could have been written as follows:
public Account (Sring name, long acctNumber, double balance){ this.name = name; this.acctNumber = acctNumber; this.balance = balance;}
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The this reference
public Account (Sring name, long acctNumber, double balance){ this.name = name; this.acctNumber = acctNumber; this.balance = balance;}
public Account (Sring owner, long account, double initial){ name = owner; acctNumber = account; balance = initial;}
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Chapter 6
Section 6.5 – 6.6
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Interfaces
• A Java interface is a collection of abstract methods and constants
• An abstract method is a method header without a method body
• An abstract method can be declared using the modifier abstract, but because all methods in an interface are abstract, usually it is left off
• An interface is used to establish a set of methods that a class will implement
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Interfaces
public interface Doable{ public void doThis(); public void doThat(); public void doThis2 (float value, char ch); public boolean doTheOther (int num);}
interface is a reserved wordNone of the methods inan interface are given
a definition (body)
A semicolon immediatelyfollows each method header
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Interfaces
• An interface cannot be instantiated
• Methods in an interface have public visibility by default
• A class formally implements an interface by:
stating so in the class header
providing implementations for each abstract method in the interface
• If a class asserts that it implements an interface, it must define all methods in the interface
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Interfaces
public class CanDo implements Doable{ public void doThis () { // whatever }
public void doThat () { // whatever }
// etc.}
implements is areserved word
Each method listedin Doable is
given a definition
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Interfaces
• A class that implements an interface can implement other methods as well
• See Complexity.java (page 310)
• See Question.java (page 311)
• See MiniQuiz.java (page 313)
• In addition to (or instead of) abstract methods, an interface can contain constants
• When a class implements an interface, it gains access to all its constants
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Interfaces
• A class can implement multiple interfaces
• The interfaces are listed in the implements clause
• The class must implement all methods in all interfaces listed in the header
class ManyThings implements interface1, interface2
{
// all methods of both interfaces
}
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Interfaces
• The Java standard class library contains many helpful interfaces
• The Comparable interface contains one abstract method called compareTo, which is used to compare two objects
• We discussed the compareTo method of the String class in Chapter 5
• The String class implements Comparable, giving us the ability to put strings in lexicographic order
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Where ca you find the standard Java interfaces• C:\Program Files\Java\jdk1.5.0\src.zip
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The Comparable Interface
• Any class can implement Comparable to provide a mechanism for comparing objects of that type
if (obj1.compareTo(obj2) < 0)
System.out.println ("obj1 is less than obj2");
• The value returned from compareTo should be negative is obj1 is less that obj2, 0 if they are equal, and positive if obj1 is greater than obj2
• When a programmer designs a class that implements the Comparable interface, it should follow this intent
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The Comparable Interface
• It's up to the programmer to determine what makes one object less than another
• For example, you may define the compareTo method of an Employee class to order employees by name (alphabetically) or by employee number
• The implementation of the method can be as straightforward or as complex as needed for the situation
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The Iterator Interface
• As we discussed in Chapter 5, an iterator is an object that provides a means of processing a collection of objects one at a time
• An iterator is created formally by implementing the Iterator interface, which contains three methods
• The hasNext method returns a boolean result – true if there are items left to process
• The next method returns the next object in the iteration
• The remove method removes the object most recently returned by the next method
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The Iterator Interface
• By implementing the Iterator interface, a class formally establishes that objects of that type are iterators
• The programmer must decide how best to implement the iterator functions
• Once established, the for-each version of the for loop can be used to process the items in the iterator
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Interfaces
• You could write a class that implements certain methods (such as compareTo) without formally implementing the interface (Comparable)
• However, formally establishing the relationship between a class and an interface allows Java to deal with an object in certain ways
• Interfaces are a key aspect of object-oriented design in Java
• We discuss this idea further in Chapter 9
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Interface Example:
Sortable.javaSortableIntArray.java
SortableStrigArray.javaSortingTest.java
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Enumerated Types (read on your own)
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Enumerated Types
• In Chapter 3 we introduced enumerated types, which define a new data type and list all possible values of that type
enum Season {winter, spring, summer, fall}
• Once established, the new type can be used to declare variables
Season time;
• The only values this variable can be assigned are the ones established in the enum definition
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Enumerated Types
• An enumerated type definition is a special kind of class
• The values of the enumerated type are objects of that type
• For example, fall is an object of type Season
• That's why the following assignment is valid
time = Season.fall;
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Enumerated Types
• An enumerated type definition can be more interesting than a simple list of values
• Because they are like classes, we can add additional instance data and methods
• We can define an enum constructor as well
• Each value listed for the enumerated type calls the constructor
• See Season.java (page 318)• See SeasonTester.java (page 319)
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Enumerated Types
• Every enumerated type contains a static method called values that returns a list of all possible values for that type
• The list returned from values is an iterator, so a for loop can be used to process them easily
• An enumerated type cannot be instantiated outside of its own definition
• A carefully designed enumerated type provides a versatile and type-safe mechanism for managing data
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THE END