Java 基础知识

1. 接口只能用public 来修饰

2 http：超文本传输协议    端口 80 

　　smtp：简单邮件传输协议 端口25

　　ftp 21 ssh 22 

3 public int aMethod(){
static int i=0;
i++;
}

编译出错，不能在类中声明静态变量

4 class Super {
public Integer getLenght() { return new Integer(4); }
}

public class Sub extends Super {
public Long getLenght() { return new Long(5); }
public static void main(String[] args) {
Super sooper = new Super();
Sub sub = new Sub();
System.out.println(sooper.getLenght().toString() + "," +
 sub.getLenght().toString() );
}
}

编译失败.---------重定时不能改变返回类型

5

class Parent{
    String name="parent_name";
    static {
        System.out.println("parent_static");
    }
    public Parent() {
        // TODO Auto-generated constructor stub
        System.out.println("parent_constructor");
    }
    public void method(){
        System.out.println("parent");
    }
    public static void smethod(){
        System.out.println("parent_1");
    }
}
class Child extends Parent{
    String name="child_name";
    static {
        System.out.println("child_static");
    }
    public Child() {
        // TODO Auto-generated constructor stub
        System.out.println("child_constructor");
    }
    public void method(){
        System.out.println("child");
    }
    public static void smethod(){
        System.out.println("child_1");
    }
}
public class Test5 {

    
    public static void main(String[] args) {
        Child c=new Child();
        System.out.println("---------------");
        Parent p=new Parent();
        System.out.println("------------------");
        p.method();
        System.out.println("--------------");
        c.method();
        System.out.println("--------------");
        Parent ddd=new Child();
        System.out.println("--------------");
        System.out.println(ddd.name);
        ddd.method();
    }
}