Java this/static/super/abstract

1:this 

 

 

 

public class Leaf {
    int i=0;
    Leaf(int i) { this.i = i ;}
    Leaf increament() {
        i++;
        return this;
    void print() { System.out.println("i = " + i); }
    
    public static void main(String[] agrs) {
        Leaf leaf = new Leaf(100);
        leaf.increament().increament().print();
     }
    
}

// print result :102;

 

2: static

public class Cat {
   private static  int sId = 0;
   private String name;
   int id;
   Cat(String name) {
       this.name = name;
       id = sId++;   
    }
    void info() {
       System.out.println(name+"   "  + id);
    }
   public  static void main(String[] args) {
       Cat.sId = 100;//use SId without "new"...
       Cat mimi = new Cat("mimi");
       Cat keke = new Cat("keke");
       mimi.info();
       keke.info();
    }
}

//print :      mimi   100
//             keke   101

3:super

 

class Father {
    public int value;
    public void f() {
        value = 100;
        System.out.println("father.vaue is "+value);
    }
}
class Child extends Father {
    public int value;
    public void f() {
        super.f();
        value = 200;
        System.out.println("child.value is "+value);
      System.out.println("no super.value is"+value);
        System.out.println("super.value is"+super.value);
    }
}
public class TestSuper {
  public static void main(String[] args) {
      Child cc=new Child();
      cc.f();
  } 
}
//  father.value is 100
//  child.value is 200
//  no super.value is 200
//  super.value is 100

在 继承构造函数中，先使用Super(**,**)调用父类的构造函数;再使用this(**,**)调用自身

没写super的话默认为无参构造函数

 4:abstarct

 抽象方法必须被继承重写，抽象类不能被实例化

 5：final

 

public class TestFinal {
    public static void main(String[] args) {
        T t = new T();
        //t.i = 2;    //final变量无法赋值；
        System.out.println(t.i);
    }
}

/*final*/  class T {        //如果类是final，无法做基类
    final int i = 2;    
    public void m(final Dog j) {  //final 无法改变一个对象
        //
    }
    public  final void n() {} //final无法被重载
}
class Dog {
}
class TT extends T {
   //    public void n(){}
}