Java面向对象笔记

多态

一种类型的变量可以引用多种实际类型的对象
如

package ooplearn;public class Test {public static void main(String[] args) {Animal[] animals = new Animal[2];animals[0] = new Dog();animals[1] = new Cat();for (Animal animal : animals){animal.eat();}}
}class Animal {public void eat() {System.out.println("Animal eat");}
}class Dog extends Animal{public void eat(){System.out.println("Dog eat");}
}class Cat extends Animal{public void eat(){System.out.println("Cat eat");}
}

Animal类型的变量animal可以引用Dog和Cat类型对象，称为多态。Animal就是animal变量的静态类型，Dog和Cat就是animal变量的动态类型。animal.eat()调用的是变量动态类型的方法，称为动态绑定。

静态绑定

package ooplearn;public class Test {public static void main(String[] args) {Dog d = new Dog();Animal a = d;System.out.println(d.title); // return DogSystem.out.println(a.title); // return Animal}
}
class Animal {public static String title = "Animal";
}class Dog extends Animal{public static String title = "Dog";
}

变量d的静态类型是Dog，访问Dog变量和方法，变量a的静态类型是Animal，访问Animal的变量和方法。访问绑定到变量的静态类型，称静态绑定。

实例方法调用顺序

package ooplearn;public class Test {public static void main(String[] args) {Animal[] animals = new Animal[2];animals[0] = new Dog();animals[1] = new Cat();for (Animal animal : animals){animal.eat();}}
}class Animal {public void eat() {System.out.println("Animal eat");}
}class Dog extends Animal{public void eat(){System.out.println("Dog eat");}
}
class Cat extends Animal{
}

上述代码返回

Dog eat
Animal eat

循环输出的过程animal变量的实际类型分别为Dog和Cat，先从实际类型找方法，找不到就去父类找。

内部类

定义在类里的类称为内部类，一般与外部类（包含内部类的类）关系密切，与其他类关系不大。一般对外隐藏，有更好的封装性。如Swing编程中为组件创建ActionListener:


import java.awt.BorderLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;public class TwoButtons {JFrame frame;JLabel label;public static void main(String[] args) {TwoButtons gui = new TwoButtons();gui.go();}public void go() {frame = new JFrame();frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);JButton labelButton = new JButton("Change Label");labelButton.addActionListener(new LabelListener());JButton colorButton = new JButton("Change Circle");colorButton.addActionListener(new ColorListener());label = new JLabel("I'm a label");MyDrawPanel drawPanel = new MyDrawPanel();}class LabelListener implements ActionListener {@Overridepublic void actionPerformed(ActionEvent e) {// TODO Auto-generated method stublabel.setText("Ouch!");}}class ColorListener implements ActionListener {@Overridepublic void actionPerformed(ActionEvent e) {// TODO Auto-generated method stubframe.repaint();}}
}