Java Implementation: Part 1 S1 2015
CompSci 230 Software Construction Agenda COMPSCI 230: Impl1 2 Topics: Interfaces in Java Reference data types Abstract classes in Java Java syntax: five important keywords Reading In The Java Tutorials: What is an Interface?, in the Object-Oriented Programming Concepts Lesson The Interfaces and Inheritance Lesson Learning objectives: Java Implementation COMPSCI 230: Impl1 3 Students will be competent at implementing OO designs in Java Interfaces, reference data types, abstract classes, intro to generics Visibility, packages, static & dynamic typing, conversion & casting The lectures will give you the basic “theory”, but they won’t give you a “working understanding” – you have to do the hard-yards of putting these ideas into practice. You won’t even understand the theory, if you listen passively to lectures. I’ll try to help you “learn how to learn” from the Java tutorials. You’ll get many chances to develop your understanding in your lab assignments for this course. Interfaces, in UML COMPSCI 230: Impl1 4 Interfaces specify behaviour (a public contract), without data or implementation. Interfaces are drawn like classes, but without attributes, and with the keyword <> A dotted open-triangle arrow, from a class to an interface, means that “the class implements this interface”. We also say that “the class fulfils the contract specified by this interface”, or that it “realizes the interface.” Note that interfaces define methods but not attributes. A password allows a secureLogin(). Interfaces in Java 7 An Interface is like a Class, with no bodies in the methods. It may define constants (public static final) but no runtime variables. Usually, an Interface is public. An interface provides a standard way to access a class which could be implemented in many different ways. The Java Tutorials: “There are a number of situations in software engineering when it is important for disparate groups of programmers to agree to a ‘contract’ that spells out how their software interacts.” “Each group should be able to write their code without any knowledge of how the other group's code is written.” “Generally speaking, interfaces are such contracts.” COMPSCI 230: Impl1 5 Interfaces in Java 8 COMPSCI 230: Impl1 6 In Java 8, an interface may contain default implementations of instance methods, and implementations of static methods. In any OO language, an interface cannot be instantiated, and defines a “contract” which any realization of the interface must fulfil. Java is a strongly-typed language. Java compilers can enforce contracts, by refusing to compile classes whose implementations might “partially realize” an interface. Java is a tightly-specified language. If a compiler allows instantiations of incompletely-implemented interfaces, then it is not a Java compiler. Implementations as contracts A class which realizes an interface must provide an implementation of every method defined within the interface A class may implement some additional methods (but these extra methods aren’t accessible through this interface) Beware: adding another method to an existing Interface will “break” every current implementation of this Interface! A class can implement many interfaces. An Interface can extend other Interfaces. Extension is the preferred way to add new methods to an Interface. (Do you understand why?) In Java, classes are less extendible than interfaces, because a Class can extend at most one other Class (“single inheritance”). class MountainBike extends Bicycle { … } COMPSCI 230: Impl1 7 Interfaces in Java 8 COMPSCI 230: Impl1 8 In Java 8, an interface may contain default implementations of instance methods, and implementations of static methods. In any OO language, an interface cannot be instantiated, and defines a “contract” which any realization of the interface must fulfil. In Java, a realization is denoted by the keyword implements. Example 1 COMPSCI 230: Impl1 9 public interface Bicycle { void changeCadence(int newValue); void changeGear(int newValue); void speedUp(int increment); void applyBrakes(int decrement); } class ACMEBicycle implements Bicycle { int cadence = 0; void changeCadence(int newValue) { cadence = newValue; } \\ note: an implementation may be incorrect! void changeGear(int newValue) {} void speedUp(int increment) {} void applyBrakes(int decrement) {} } \\ an implementation may have variables Example 2 COMPSCI 230: Impl1 10 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 ); } Example 3 COMPSCI 230: Impl1 11 public interface EventListener { // No constants // No method signatures! } “A tagging interface that all event listener interfaces must extend.” [http://docs.oracle.com/javase/6/docs/api/java/util/EventListener.html] Why? At first glance, this is worse than useless! One more name for the Java programmer to remember… This interface allows programmers, and the Java compiler, to distinguish event-listeners from all other types of classes and interfaces. Event-listeners are important, and they behave quite differently to a regular class. (Later, you’ll learn about inversion of control…) MouseListener in java.awt.event public interface MouseListener extends EventListener The listener interface for receiving “interesting” mouse events (press, release, click, enter, and exit) on a component. (To track mouse moves and mouse drags, use the MouseMotionListener.) All Known Subinterfaces: MouseInputListener All Known Implementing Classes: AWTEventMulticaster, BasicButtonListener, BasicComboPopup.InvocationMouseHandler, BasicComboPopup.ListMouseHandler, BasicDesktopIconUI.MouseInputHandler, … COMPSCI 230: Impl1 12 public interface MouseListener extends EventListener { mouseClicked( MouseEvent e ); mouseEntered( MouseEvent e ); mouseExited( MouseEvent e ); mousePressed( MouseEvent e ); mouseReleased( MouseEvent e ); } public interface MouseMotionListener extends EventListener { mouseDragged( MouseEvent e ); mouseMoved( MouseEvent e ); } public interface MouseInputListener extends MouseListener, MouseMotionListener { // this interface has 7 method signatures, can you list them? } Using an Interface as a Type COMPSCI 230: Impl1 13 “When you define a new interface, you are defining a new reference data type. “You can use interface names anywhere you can use any other data type name. “If you define a reference variable whose type is an interface, any object you assign to it must be an instance of a class that implements the interface.” [http://docs.oracle.com/javase/tutorial/java/IandI/interfaceAsType.html] Example on the next slide: A method for finding the largest object in a pair of objects, for any objects that are instantiated from a class that implements Relatable. public interface Relatable { public int isLargerThan( Relatable other ); } Using an Interface as a Type COMPSCI 230: Impl1 14 public Object findMax(Object object1, Object object2) { Relatable obj1 = (Relatable)object1; Relatable obj2 = (Relatable)object2; if( (obj1).isLargerThan(obj2) > 0 ) return object1; else return object2; } If comparisons are important in your application, then you’ll be able to write very elegant code! You can write z.findMax(x, y), if x and y are instances of any class which extends Relatable. Using an Interface as a Type: Mismatches COMPSCI 230: Impl1 15 public Object findMax( Object object1, Object object2 ) { Relatable obj1 = (Relatable)object1; Relatable obj2 = (Relatable)object2; if( (obj1).isLargerThan(obj2) > 0 ) return object1; else return object2; } We’d get errors at compile-time (or at runtime) if (object1).isLargerThan(object2) were in the body of this method, if we invoked it as z.findMax(x,y), for any instance x of a class that doesn’t extend Relatable, or if we invoked it as x.findLargest(y,z), if y.isLargerThan() does not accept z as a parameter. Typing is complex… we’ll keep looking at it, in different ways… Typing Rules The typing rules for interfaces are similar to those for classes. A reference variable of interface type T can refer to an instance of any class that implements interface T or a sub-interface of T. Through a reference variable of interface type T, methods defined by T and its super interfaces can be called. COMPSCI 230: Impl1 16 interface I1 { public void m1(); public void m2(); } class c1 implements I1 { public void m1() {} public void m2() {} } interface I2 extends I1 { public void m3(); } class c2 implements I2 { public void m1() {} public void m2() {} public void m3() {} } C1 a = new C1(); \\ a is a reference variable of type C1 C1 b = new C1(); I1 p = a; \\ p is a reference variable of type I1 p.m1(); C2 c = new C2(); I2 q = c; \\ q is a reference variable of type I2 q.m1(); q.m2(); «realize» «realize» instanceof COMPSCI 230: Impl1 17 You can use the instanceof operator to test an object to see if it implements an interface, before you invoke a method in this interface. This might improve readability and correctness. This might be a hack. Where possible, you should extend classes and interfaces to obtain polymorphic behaviour, rather than making a runtime check. if( b instanceof Bounceable ) { b.hitWall( "Wall A" ); } else { \\ abort, with an error message to the console throw new AssertionError( b ); } Date somedate = new Date(); \\ throw an exception if somedate is not Relatable. assert( Date instanceof Relatable ); \\ See http://docs.oracle.com/javase/1.4.2/docs/guide/lang/assert.html Abstract Classes COMPSCI 230: Impl1 18 Sometimes, it’s appropriate to partly-implement a class or interface. Abstract classes allow code to be reused in similar implementations. Abstract classes may include some abstract methods. If there are no abstract methods, then the class is usually (but not always) implemented fully enough to be used by an application. Sometimes it’s helpful to have multiple implementations that differ only in their type, but this is quite an advanced concept in design. public abstract class MyGraphicObject { // declare fields – these may be non-static private int x, y; // declare non-abstract methods // (none) // declare methods which must be implemented later abstract void draw(); } Example COMPSCI 230: Impl1 19 public class Rectangle extends Shape { private int width, height; public int area() { return (width * height); } ... abstract class Shape { Point p; Shape(){ this(0, 0); } Shape(x, y){ p = new Point(x, y); } public abstract void draw(Graphics g); public abstract int area(); } public class Triangle extends Shape { private int base, height; public int area() { return (base * height) / 2; } ... An abstract method is defined with a signature but no implementation. Abstract methods Concrete subclasses must implement all abstract methods. public class Circle extends Shape { private int radius; public int area() { return (int) (Math.PI * radius * radius); } ... Super! COMPSCI 230: Impl1 20 If your method overrides one of its superclass's methods, you can invoke the overridden method through the use of the keyword super. You can also use super to refer to a hidden field (although hiding fields is discouraged). Example below. Can you determine what will be printed to System.out when main() is executed? public class Superclass { public void printMethod() { System.out.println("Printed in Superclass."); } } public class Subclass extends Superclass { public void printMethod() { // overrides super.printMethod super.printMethod(); System.out.println("Printed in Subclass"); } } public static void main(String[] args) { Subclass s = new Subclass(); s.printMethod(); } } Printed in Superclass. Printed in Subclass Hiding vs overriding COMPSCI 230: Impl1 21 If a subclass defines a static method with the same signature as a static method in the superclass, then the method in the subclass hides the one in the superclass. The distinction between hiding a static method and overriding an instance method has important implications: The version of the overridden instance method that gets invoked is the one in the subclass. The version of the hidden static method that gets invoked depends on whether it is invoked from the superclass or the subclass. Hmmm… this could be confusing! So … I don’t encourage you to hide methods. Overriding methods is an important part of OO design. this COMPSCI 230: Impl1 22 Within an instance method or a constructor, this is a reference to the current object — the object whose method or constructor is being called. You can refer to any member of the current object from within an instance method or a constructor by using this. The most common reason for using the this keyword is because a field is shadowed by a method or constructor parameter. Is shadowing a good idea? COMPSCI 230: Impl1 23 A parameter can have the same name as one of the class's fields. If this is the case, the parameter is said to shadow the field. Shadowing fields can make your code difficult to read and is conventionally used only within constructors and methods that set a particular field. For example, consider the following Circle class … Source: http://docs.oracle.com/javase/tutorial/java/javaOO/arguments.html Example: using this.x COMPSCI 230: Impl1 24 public class Point { public int x = 0; public int y = 0; public Point(int a, int b) { x = a; y = b; } } Equivalently: public class Point { public int x = 0; public int y = 0; public Point(int x, int y) { this.x = x; // this.x refers to the shadowed instance variable this.y = y; } } Using this() COMPSCI 230: Impl1 25 From within a constructor, you can also use the this keyword to call another constructor in the same class. Doing so is called an explicit constructor invocation. [https://docs.oracle.com/javase/tutorial/java/javaOO/thiskey.html] (Let’s look at an example of this in Eclipse.) (I also want to show you how to import a JARfile.) Final COMPSCI 230: Impl1 26 The final keyword can be applied to prevent the extension (over-riding) of a field, argument, method, or class. Final field: constant Final argument: cannot change the data within the called method Final method: cannot override method in subclasses Final class: cannot be subclassed (all of its methods are implicitly final as well) class ChessAlgorithm { . . . final void nextMove( ChessPiece pieceMoved, BoardLocation newLocation ) { \\ body of nextMove – can’t be overriden } } Review COMPSCI 230: Impl1 27 Interfaces in Java Types in Java Abstract classes in Java Six important keywords: interface implements abstract super this final