Java基本功练习十九（抽象类和接口【抽象类、接口（监听器、克隆、可比较）、包装类、有理数类实现】）

在展示范例之前，先介绍一些相关的概念和注意点，这是抽象类和接口的基础预备知识。

在继承的层次结构中，随着每个新子类的出现，类会变的越来越明确和具体。如果从一个子类追溯到父类，类就会变得更通用、更加不明确。类的设计应该确保父类包含子类的共同特征。

有时候，一个父类设计的非常抽象，以至于它没有任何具体的实例。这样的类称为抽象类。

抽象类中不实现，只声明的方法称为抽象方法。在方法头中用abstract修饰符表示。在类头使用abstract修饰符表示该类为抽象类。在UML类图中，抽象类和方法的名字用斜体表示。

抽象类和常规类很像，但是不能使用new操作符创建它的实例。抽象方法只有定义而没有实现。它的实现由子类提供。一个包含抽象方法的类必须声明为抽象类。

抽象类的构造方法定义为protected，因为它只被子类使用。创建一个具体子类的实例时，它的父类构造方法被调用以初始化父类中定义的数据域。

关于抽象类的几个注意点：

1）抽象方法不能包含在非抽象类中。如果抽象父类的子类不能实现所有的抽象方法，那么子类必须定义为抽象的。换句话说，在抽象类扩展的非抽象类中，必须实现所有的抽象方法。并且，抽象方法是非静态的。

2）包含抽象对象的类必须是抽象的。但是，可以定义一个不包含抽象方法的抽象类。在这种情况下，不能使用new操作符创建该类的实例。这种类是用来定义新子类的基类的。

3）即使子类的父类是具体的，这个子类也可以是抽象的。

4）子类可以覆盖父类的方法并将它定义为abstract。这是很少见的，但是它在当父类的方法实现在子类中变得不合法时是很有用的。在这种情况下，子类必须定义为abstract。

5）不能使用new操作符从一个抽象类创建一个实例，但是抽象类可以用作一种数据类型。

接口是一种与类相似的结构，只包含常量和抽象方法。它在很多方面都与抽象类很相似，但是它的目的是指明多个对象的共同行为。如，使用正确的接口，可以指明这些对象是可以比较的、可食用的或可克隆的。

Java中，接口被看作是一种特殊的类。就像常规类一样，每个接口都被编译为独立的字节码文件。与抽象类相似，不能使用new操作符创建接口的实例，但是大多数情况下，使用接口或多或少有点像使用抽象类。如可以使用接口作为引用变量的数据类型或类型转换的结果等等。

接口和抽象类的区别

或多或少可以使用和抽象类一样的方式使用接口，但是定义一个接口和定义一个抽象类有所不同，如下表所示：

  Java只允许为类的扩展做单一继承，但是允许使用接口做多重扩展，如下所示的定义是合法的。

接口可以扩展其他接口而不是类。一个类可以扩展它的父类同时实现多个接口 。

所有的类共享同一个根类Object，但是接口没有共同的根。与类类似，接口也可以定义一种类型。一个接口类型的变量可以引用任何实现该接口的类的实例。如果一个类实现了一个接口，那么这个接口就类似于该类的一个父类。可以将接口当作一种数据类型使用，将接口类型的变量转换成他的子类，反过来也可以。

范例一：抽象类的使用范例。

运行效果如图：

实现的源代码如下所示：

package Blog;public class blogTryProject {public static void main(String[] args) {TestChouXiang tcx = new TestChouXiang();tcx.main(args);}}//抽象类测试程序class TestChouXiang{public static void main(String[]args){Rectangle rt = new Rectangle(4,5);System.out.println(rt.toString());System.out.println("Rectangle 的面积为："+rt.getArea());System.out.println("Rectangle 的周长为："+rt.getPerimeter());Circle c = new Circle(3);System.out.println(c.toString());System.out.println("Circle 的面积为："+c.getArea());System.out.println("Circle 的周长为："+c.getPerimeter());}}//抽象类GeometricObjectabstract class GeometricObject{private String color = "White";private boolean filled;private java.util.Date dateCreated;protected GeometricObject(){dateCreated = new java.util.Date();}protected GeometricObject(String color,boolean filled){dateCreated = new java.util.Date();this.color = color;this.filled = filled;}public String getColor(){return color;}public void setColor(String color){this.color = color;}public void setFilled(boolean filled){this.filled = filled;}public java.util.Date getDateCreated(){return dateCreated;}public String toString(){return "created on "+dateCreated+"\ncolor: "+color+"\t\tfilled: "+filled;}public abstract double getArea();public abstract double getPerimeter();}//矩形类class Rectangle extends GeometricObject{private double width;private double height;public Rectangle(){}public Rectangle(double width,double height){this.width = width;this.height = height;}public Rectangle(double width,double height,String color,boolean filled){this.width = width;this.height = height;setColor(color);setFilled(filled);}public double getWidth(){return width;}public void setWidth(double width){this.width = width;}public double getHeight(){return height;}public void setHeight(double height){this.height = height;}public double getArea(){return width * height;}public double getPerimeter(){return 2 * (width + height);}public String toString(){return super.toString()+"\n宽为："+getWidth()+"\t\t长为："+getHeight();}}//圆类class Circle extends GeometricObject{private double radius;public Circle(){}public Circle(double radius){this.radius = radius;}public Circle(double radius,String color,boolean filled){this.radius = radius;setColor(color);//这里不能用this.color = color;因为它是父类的私有数据域，在GeometricObject//类外只能通过访问器和修改器对它进行操作setFilled(filled);}public double getRadius(){return radius;}public void setRadius(double radius){this.radius = radius;}public double getArea(){return radius * radius * Math.PI;}public double getPerimeter() {return 2 * radius * Math.PI;}public double getDiameter(){return 2 * radius;}public String toString(){return super.toString()+"\n半径为： "+radius;}}

范例二：接口的简单范例。

运行效果如图所示：

实现的源代码如下所示：

package Blog;public class blogTryProject {public static void main(String[] args) {TestEdible te = new TestEdible();te.main(args);}}//接口的简单范例interface Edible{public abstract String howToEat();}class TestEdible{public static void main(String[]args){Object[] objects = {new Tiger(),new Chicken(),new Apple(),new Orange()};for(int i = 0;i < objects.length;i++)if(objects[i] instanceof Edible)System.out.println(((Edible)objects[i]).howToEat());}}class Animal{}class Chicken extends Animal implements Edible{public String howToEat(){return "Chicken: Fry it";}}class Tiger extends Animal{}abstract class Fruit implements Edible{}class Apple extends Fruit{public String howToEat(){return "Apple: Make apple cider";}}class Orange extends Fruit{public String howToEat(){return "Orange: Make orange juice";}}

范例三：监听器接口（ActionListener），克隆接口（Cloneable），可比较接口（Comparable）。

ActionListener

运行效果如图所示：

实现的源代码如下所示：

package Blog;import javax.swing.*;public class blogTryProject {public static void main(String[] args) {HandleEvent he = new HandleEvent();he.main(args);}}//ActionListener监听器接口class HandleEvent extends JFrame{public HandleEvent(){JButton jbtOk = new JButton("Ok");JButton jbtCancel = new JButton("Cancel");JPanel p = new JPanel();p.add(jbtOk);p.add(jbtCancel);add(p);OkListenerClass listener1 = new OkListenerClass();CancelListenerClass listener2 = new CancelListenerClass();//条件二：使用source.addActionListener(listener)注册源对象jbtOk.addActionListener(listener1);jbtCancel.addActionListener(listener2);}public static void main(String[]args){JFrame frame = new HandleEvent();frame.setTitle("Handle Event");frame.setSize(200, 150);frame.setLocation(200, 100);frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);frame.setVisible(true);}}//条件一：使得OkListenerClass成为监听器类的实例，并实现其方法class OkListenerClass implements java.awt.event.ActionListener{//事件actionPerformed方法的实现public void actionPerformed(java.awt.event.ActionEvent e){System.out.println("Ok button clicked");}}class CancelListenerClass implements java.awt.event.ActionListener{public void actionPerformed(java.awt.event.ActionEvent e){System.out.println("Cancel button clicked");}}

Cloneable（注意克隆不成功会有异常抛出，可参考《Java基本功练习十八》）

运行效果如图所示：

实现的源代码如下所示：

package Blog;public class blogTryProject {public static void main(String[] args) throws CloneNotSupportedException {House house = new House(12,89);house.main(args);}}//Cloneable接口范例class House implements Cloneable,Comparable{private int id;private double area;private java.util.Date whenBuilt;public House(int id,double area){this.id = id;this.area = area;whenBuilt = new java.util.Date();}public int getId(){return id;}public double getArea(){return area;}public java.util.Date getWhenBuilt(){return whenBuilt;}//覆盖Object中保护类型的clone()方法，并增强其可见性为publicpublic Object clone() throws CloneNotSupportedException{return super.clone();}public int compareTo(Object o){if(area > ((House)o).area)return 1;else if(area < ((House)o).area)return -1;elsereturn 0;}public String toString(){return "House的信息，ID为："+id+"\t面积为："+area;}public boolean equals(Object o){if(id == ((House)o).id && area == ((House)o).area)return true;elsereturn false;}public static void main(String[]args) throws CloneNotSupportedException{House house1 = new House(502, 123);House house2 = new House(503, 223);House house3 = (House)house1.clone();System.out.println("house1的信息为："+house1.toString());System.out.println("house2的信息为："+house2.toString());System.out.println("house3的信息为："+house3.toString());System.out.println("house1 与 house2 的面积大小比较："+house1.compareTo(house2));System.out.println("house1 与 house3 是否内容相同? "+house1.equals(house3));System.out.println("house1 与 house3 是否引用变量相同? "+(house1 == house3));}}

Comparable

运行效果如图所示：

实现的源代码如下所示：

package Blog;public class blogTryProject {public static void main(String[] args){TestComparableRectangle tcr = new TestComparableRectangle();tcr.main(args);}}//Comparable接口实现范例/*Comparable的接口定义如下package java.lang;public interface Comparable{  public int compareTo(Object o);}*/class ComparableRectangle extends Rectangle implements Comparable{//构造函数public ComparableRectangle(double width,double height){super(width,height);}//Comparable接口的实现public int compareTo(Object o){if(getArea() > ((ComparableRectangle)o).getArea())return 1;else if(getArea() < ((ComparableRectangle)o).getArea())return -1;elsereturn 0;}//利用接口新定义一个比较两个矩形面积的静态方法public static Comparable max(Comparable o1,Comparable o2){if(o1.compareTo(o2) > 0)return o1;elsereturn o2;}}//测试Comparable接口程序class TestComparableRectangle{public static void main(String[]args){ComparableRectangle tcr1 = new ComparableRectangle(2, 5);ComparableRectangle tcr2 = new ComparableRectangle(3, 6);System.out.println("\nComparable接口演示范例");System.out.println("两个矩形的信息如下");System.out.println(tcr1.toString());System.out.println(tcr2.toString());System.out.println("tcr1 和 tcr2 比较结果为："+tcr1.compareTo(tcr2));System.out.println("面积大者的信息如下\n"+tcr1.max(tcr1, tcr2));}}//矩形类class Rectangle extends GeometricObject{private double width;private double height;public Rectangle(){}public Rectangle(double width,double height){this.width = width;this.height = height;}public Rectangle(double width,double height,String color,boolean filled){this.width = width;this.height = height;setColor(color);setFilled(filled);}public double getWidth(){return width;}public void setWidth(double width){this.width = width;}public double getHeight(){return height;}public void setHeight(double height){this.height = height;}public double getArea(){return width * height;}public double getPerimeter(){return 2 * (width + height);}public String toString(){return super.toString()+"\n宽为："+getWidth()+"\t\t长为："+getHeight();}}//抽象类GeometricObjectabstract class GeometricObject{private String color = "White";private boolean filled;private java.util.Date dateCreated;protected GeometricObject(){dateCreated = new java.util.Date();}protected GeometricObject(String color,boolean filled){dateCreated = new java.util.Date();this.color = color;this.filled = filled;}public String getColor(){return color;}public void setColor(String color){this.color = color;}public void setFilled(boolean filled){this.filled = filled;}public java.util.Date getDateCreated(){return dateCreated;}public String toString(){return "created on "+dateCreated+"\ncolor: "+color+"\t\tfilled: "+filled;}public abstract double getArea();public abstract double getPerimeter();}

范例四：包装类范例。Java提供一个方便的办法，将基本数据类型并入对象或包装成对象，称为包装类。

运行效果如图所示：

实现的源代码如下所示：

package Blog;public class blogTryProject {public static void main(String[] args){GenericSort gc = new GenericSort();gc.main(args);}}//对象数组排序范例class GenericSort{public static void main(String[]args){System.out.println("对象数组排序范例");Integer[] intArray = {new Integer(2),new Integer(4),new Integer(3)};Double[] doubleArray = {new Double(3.4),new Double(1.3),new Double(-22.1)};Character[] charArray = {new Character('a'),new Character('J'),new Character('r')};String[] stringArray = {"Tom","John","Fred"};sort(intArray);sort(doubleArray);sort(charArray);sort(stringArray);System.out.println("Sorted Integer objects:");printList(intArray);System.out.println("Sorted Double objects:");printList(doubleArray);System.out.println("Sorted Character objects:");printList(charArray);System.out.println("Sorted String objects:");printList(stringArray);}public static void sort(Comparable[] list){Comparable currentMin;int currentMinIndex;for(int i = 0;i < list.length - 1;i++){currentMin = list[i];currentMinIndex = i;for(int j = i + 1;j < list.length;j++){if(currentMin.compareTo(list[j]) > 0){currentMin = list[j];currentMinIndex = j;}}if(currentMinIndex != i){list[currentMinIndex] = list[i];list[i] = currentMin;}}}public static void printList(Object[] list){for(int i = 0;i < list.length;i++)System.out.print(list[i]+" ");System.out.println();}}

范例五：有理数类的加减乘除实现。

运行效果如图所示：

实现的源代码如下所示：

package Blog;public class blogTryProject {public static void main(String[] args){TestRational tr = new TestRational();tr.main(args);}}//有理数类测试class TestRational{public static void main(String[]args){Rational r1 = new Rational(4,2);Rational r2 = new Rational(2,3);System.out.println(r1+" + "+r2+" = "+r1.add(r2));System.out.println(r1+" - "+r2+" = "+r1.subtract(r2));System.out.println(r1+" * "+r2+" = "+r1.multiply(r2));System.out.println(r1+" / "+r2+" = "+r1.divide(r2));System.out.println(r2+" is "+r2.doubleValue());}}//有理数类实现class Rational extends Number implements Comparable{private long numerator = 0;private long denominator = 1;public Rational(){this(0,1);}public Rational(long numerator,long denominator){long gcd = gcd(numerator,denominator);this.numerator = ((denominator > 0) ? 1 : -1) * numerator / gcd;this.denominator = Math.abs(denominator) / gcd;}private static long gcd(long n,long d){long n1 = Math.abs(n);long n2 = Math.abs(d);int gcd = 1;for(int k = 1;k <= n1 && k <= n2;k++){if(n1 % k == 0 && n2 % k == 0)gcd = k;}return gcd;}public long getNumerator(){return numerator;}public long getDenominator(){return denominator;}public Rational add(Rational secondRational){long n = numerator * secondRational.getDenominator() +denominator * secondRational.getNumerator();long d = denominator * secondRational.getDenominator();return new Rational(n,d);}public Rational subtract(Rational secondRational){long n = numerator * secondRational.getDenominator() -denominator * secondRational.getNumerator();long d = denominator * secondRational.getDenominator();return new Rational(n,d);}public Rational multiply(Rational secondRational){long n = numerator * secondRational.getNumerator();long d = denominator * secondRational.getDenominator();return new Rational(n,d);}public Rational divide(Rational secondRational){long n = numerator * secondRational.getDenominator();long d = denominator * secondRational.numerator;return new Rational(n,d);}public String toString(){if(denominator == 1)return numerator +"";elsereturn numerator+"/"+denominator;}public boolean equals(Object parm1){if((this.subtract((Rational)(parm1))).getNumerator() == 0)return true;elsereturn false;}public int intValue(){return (int)doubleValue();}public float floatValue(){return (float)doubleValue();}public double doubleValue(){return numerator * 1.0 / denominator;}public long longValue(){return (long)doubleValue();}public int compareTo(Object o){if((this.subtract((Rational)o)).getNumerator() > 0)return 1;else if((this.subtract((Rational)o)).getNumerator() < 0)return -1;elsereturn 0;}}

范例六：接口和抽象类简单实例的实现对比。

运行效果如图所示：

实现的源代码如下所示：

package Blog;public class blogTryProject {public static void main(String[] args){ChouXiangAnimal cxa = new ChouXiangAnimal();cxa.main(args);JieKouAnimal jka = new JieKouAnimal();jka.main(args);}}//接口和抽象类的优劣比较//抽象类实现的方式class ChouXiangAnimal{public static void main(String[]args){//抽象类实现AnimalSystem.out.println("抽象类实现Animal");NewAnimal animal = new NewChicken();eat(animal);animal = new Duck();eat(animal);}public static void eat(NewAnimal animal){System.out.println(animal.howToEat());}}abstract class NewAnimal{public abstract String howToEat();}class NewChicken extends NewAnimal{public String howToEat(){return "NewChicken: Fry it";}}class Duck extends NewAnimal{public String howToEat(){return "Duck: Roast it";}}//接口实现方式class JieKouAnimal{public static void main(String[]args){//接口实现的AnimalSystem.out.println("接口实现的Animal");NewEdible stuff = new NewChicken1();eat(stuff);stuff = new Duck1();eat(stuff);stuff = new Broccoli();eat(stuff);}public static void eat(NewEdible stuff){System.out.println(stuff.howToEat());}}interface NewEdible{public String howToEat();}class NewChicken1 implements NewEdible{public String howToEat(){return "NewChicken1: Fry it";}}class Duck1 implements NewEdible{public String howToEat(){return "Duck1: Fry it";}}class Broccoli implements NewEdible{public String howToEat(){return "Broccoli: Stir-fry it";}}