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Java继承和多态


 

  父类和子类

  如果类C1扩展自另一个类C2,那么C1称为子类或派生类,C2称为父类或基类。派生类可以从它的基类中继承可访问的数据域和方法,还可添加新数据域和新方法

  例如:实现一个几何图形基类;

  class GeometricObject1 {

  private String color = "white";

  private boolean filled;

  private java.util.Date dateCreated;

  public GeometricObject1() {

  dateCreated = new java.util.Date();

  }

  public GeometricObject1(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 boolean isFilled() {

  return filled;

  }

  public void setFilled(boolean filled) {

  this.filled = filled;

  }

  public java.util.Date getDateCreated() {

  return dateCreated;

  }

  public String toString() {

  return "created on " + dateCreated + " color: " + color + " and filled: " +

  filled;

  }

  }

  一个派生类Circle:

  class Circle extends GeometricObject1 {

  private double radius;

  public Circle(double radius) {

  this.radius = radius;

  }

  public Circle(double radius, String color, boolean filled) {

  this.radius = radius;

  setColor(color);

  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 getDiameter() {

  return 2 * radius;

  }

  public double getPerimeter() {

  return 2 * Math.PI * radius;

  }

  public void printCircle() {

  System.out.println("The circle is creatd " + getDateCreated() +

  " and the radius is " + radius);

  }

  }

  一个派生类Rectangle :

  class Rectangle extends GeometricObject1 {

  private double width;

  private double height;

  public Rectangle() {

  }

  public Rectangle(double width, double height) {

  this.height = height;

  this.width = width;

  }

  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);

  }

  }

  创建Circle与Rectangle对象:

  public class Main

  {

  public static void main(String args[])

  {

  Circle circle = new Circle(1);

  System.out.println("A circle " + circle.toString());

  System.out.println("The radius is " + circle.getRadius());

  System.out.println("The area is " + circle.getArea());

  System.out.println("The diameter is " + circle.getDiameter());

  Rectangle rectangle = new Rectangle(2, 4);

  System.out.println(" A rectangle " + rectangle.toString());

  System.out.println("The area is " + rectangle.getArea());

  System.out.println("The perimeter is " + rectangle.getPerimeter());

  }

  }

  注意:

  1、派生类并不是基类的一个子集,事实上比父类包含更多的信息和方法

  2、父类中的私有数据域在该类之外是不可访问的,如果父类中定义了公共的访问器/修改器,那么可以通过这些公共的访问器/修改器来访问和修改它们

  3、不是所有的“是”关系(is-a)都该用继承来建模。例如:一个正方形和矩形。如果要用类B去扩展类A,那么A应该要比B包含更多的信息

  4、java中不允许多重继承

  使用super关键字

  关键字super的用途:

  1、调用父类的构造方法

  2、调用父类的方法

  调用父类的构造方法的语法:super(), or super(parameters)

  语句super(), or super(parameters)必须出现在子类构造方法的第一行,这是显式调用父类构造方法的唯一方式

  上面代码Circle类中的构造方法可以使用下面的代码替换:

  public Circle(double radius, String color, boolean filled) {

  super(color, filled);

  this.radius = radius;

  }

  super不仅可以引用父类的构造方法,也可以引用父类的方法:

  super.方法名(参数)

  改写Circle类中的printCircle()方法:

  public void printCircle() {

  System.out.println("The circle is creatd " + super.getDateCreated() +

  " and the radius is " + radius);

  }

  覆盖方法

  子类从父类继承方法,有时候需要修改父类中定义的方法的实现,称为方法覆盖GeometricObject类中的toString方法返回表示几何对象的字符串。这个方法可以被覆盖,返回表示圆的字符串,下面是新的方法:

  public String toString() {

  return super.toString() + " radius is " + radius;

  }

  覆盖与重载

  重载方法意味着可以定义多个同名的方法,但是这些方法具有不同的签名;覆盖方法以为着为子类中的方法提供一个全新的实现,该方法已经在父类中定义。

  覆盖的例子:

  public class Main

  {

  public static void main(String args[])

  {

  A a = new A();

  a.p(10);

  a.p(10.0);

  }

  }

  class B {

  public void p(double i) {

  System.out.println(i * 2);

  }

  }

  class A extends B {

  public void p(double i) { //覆盖

  System.out.println(i);

  }

  }

  运行结果:

  10.0

  10.0

  重载的例子:

  public class Main

  {

  public static void main(String args[])

  {

  A a = new A();

  a.p(10);

  a.p(10.0);

  }

  }

  class B {

  public void p(double i) {

  System.out.println(i * 2);

  }

  }

  class A extends B {

  public void p(int i) { //重载

  System.out.println(i);

  }

  }

  运行结果:

  10

  20.0

  多态

  可以将子类的实例传给需要父类类型的参数

  public class Main

  {

  public static void main(String args[])

  {

  displayObject(new Circle(1, "red", false));

  displayObject(new Rectangle(1, 1, "black", true));

  }

  }

  public static void displayObject(GeometricObject object) {

  System.out.println("Created on " + object.getDateCreated() +

  ".Color is " + object.getColor());

  }