java_封装、继承、Object、多态

⒈封装

面向对象的封装把对象的属性和方法组合在一起，统一提供对外访问权限，封装可以将对象的使用者和设计者分开。

一般情况下，属性用private修饰，外部访问属性使用set/get方法（属性访问器）

只有get无set，则属性只读

只有set无get，则属性只写，示例代码见附录1

 

⒉继承

Java中继承使用extends关键字是实现，子类继承父类的所有属性和方法（构造方法除外）

不同修饰符下的属性和方法的访问权限

内容	public	protected	缺省	private
同类访问	√	√	√	√
同包其他类访问	√	√	√	×
同包子类访问	√	√	√	×
不同包子类访问	√	√	×	×
不同包非子类访问	√	×	×	×

子类继承父类时可以重载父类的方法

子类初始化时先调用父类不带参的构造方法，示例代码见附录2

子类使用super，指定使用父类的构造方法初始化父类，示例代码见附录2

 

⒊Object类

当一个类没有使用extends显式地继承其他类的时候，该类默认继承了Object类

能被外界访问的Object类的主要成员：

返回类型	方法名	方法说明
Object	clone()	创建并返回此对象的一个副本
boolean	equals(Object obj)	指示其他某个对象是否与此对象“相等”（地址） 若要比较大小，可重写该方法
void	finalize()	当垃圾回收器确定不存在对该对象的更多引用时，由对象的垃圾回收器调用此方法
int	hashCode()	返回该对象那个的哈希码值
String	toString	返回该对象的字符串表示

System.out.println()输出一个对象时，首先调用该对象的toString()方法，把调用的结果输出，Object类的toString方法默认输出的是该对象在内存中的地址。可以在子类中重写toString方法。示例代码见附录3。

 

⒋多态

把子类的对象赋值给父类，用父类的引用调用方法的时候，具体执行方法的是子类的对象

示例代码见附录3

父类对象赋值给子类时，首先要进行类型转换

 

 

 

附录1

private String favourite; //爱好 public String getFavourite() {         return favourite; } public void setFavourite(String favourite) {         this.favourite = favourite; }

 

附录2

Horse.java

public class Horse {         private String name;         public Horse(String name)         {                 this.name = name;                 System.out.println("Horse中带参数name的构造方法");         }         public Horse()         {                 System.out.println("Horse中不带参数的构造方法");         }         public String getName()         {                 return name;         }         public void setName(String name)         {                 this.name = name;         }         public void eat()         {                 System.out.println(name + "在吃饭......");         } }

WhiteHorse.java

public class WhiteHorse extends Horse {         public WhiteHorse()         {                 System.out.println("WhiteHorse中不带参数的构造方法......");         }         public WhiteHorse(String name)         {                 //this.name = name;会出错 父类的私有成员子类不可访问                 this.setName(name);                 System.out.println("WhiteHorse中带参数name的构造方法......");         }         public WhiteHorse(int number)         {                 super("白马"+number); //指定父类的构造方法初始化父类                 System.out.println("WhiteHorse中带参数的构造方法......");         }         public void eat(String food)  //重载父类中的eat()方法         {                  System.out.println(this.getName() + "在吃饭" + food);         }         public static void main(String[] args)         {                 WhiteHorse wHorse1 = new WhiteHorse(); //先调用父类Horse的无参构造方法，再调用子类WhiteHorse的无参构造方法                 wHorse1.setName("白马1"); //setName继承自父类                 wHorse1.eat();                 wHorse1.eat("草");                 WhiteHorse wHorse2 = new WhiteHorse("白马2"); //先调用父类Horse的无参构造方法，再调用子类WhiteHorse的带参构造方法                 wHorse2.eat();                 wHorse2.eat("草");                 WhiteHorse wHorse3 = new WhiteHorse(3); //先调用父类Horse的带参构造方法，再调用子类WhiteHorse的带参构造方法                 wHorse3.eat();                 wHorse3.eat("草");         } }

 

附录3

Shape.java public class Shape {         //计算形状的面积         public double area()         {                 return 0.0;         } }   Circle.java public class Circle extends Shape //圆 {         private double radius; // 圆的半径         public Circle()         {                 super();         }         public Circle(double r)         {                 super();                 this.radius = r;         }         public double area() //方法重写（override），与方法重载不同         {                 double pi = 3.14159;                 return pi*radius*radius;         }         public double getRadius()         {                 return radius;         }         public void setRadius(double radius)         {                 this.radius = radius;         } }   Square.java public class Square extends Shape  //矩形 {            private double width; //矩形的宽         private double height; //矩形的高         public Square() //构造方法         {                 super();         }         public Square(double width, double height) //构造方法         {                 super();                 this.width = width;                 this.height = height;         }         public double area() //重写方法area()         {                 return width*height;         }         public void setWidth(double width)         {                 this.width = width;         }         public double getWidth()         {                 return width;         }         public void setHeight(double height)         {                 this.height = height;         }         public double getHeight()         {                 return height;         } public String toString() //方法toString重写         {                 return "Square中的toString";         } }   Test.java public class Test {         public static double area(Shape shape)         {                 double result = shape.area();                 if(shape instanceof Circle)  //instanceof判断一个对象是否是某各类的对象                 {                         Circle c = (Circle)shape; //强制类型转换                         System.out.println("圆的半径是：" + c.getRadius() + ",面积是：" + result);                 }                 else if(shape instanceof Square)                 {                         Square s = (Square)shape;                         System.out.println("矩形的宽是：" + s.getWidth() + ",高是：" + s.getHeight() + ",面积是：" + result);                 }                 return result;         }         public static void main(String[] args)         {                 Shape shape1 = new Circle(3); //子类Circle对象赋值给父类对象shape1                 System.out.println("圆的面积是：" + shape1.area());  //此处父类引用shape1调用方法area()，具体执行方法的是子类的对象                 area(shape1);                 Shape shape2 = new Square(3,4); //子类Square对象赋值给父类对象shape2                 System.out.println("矩形的面积是：" + shape2.area());                 area(shape2);               System.out.println(shape1); //调用对象shape1的toString()方法，该方法默认输出的是shape1对象在内存中的位置                 System.out.println(shape2); //调用对象shape1的toString()方法，Square中给方法已经重写         } }