1 package study;
 2
 3 class Person{
 4     String name;
 5     public Person(String name) {
 6         this.name  = name;
 7     }
 8     public Person() {}
 9 }
10
11 class Student extends Person{
12
13 }
14
15 public class star {
16     public static void main(String[] args) {
17         Person star = new Person("星星");
18         boolean result1 = star instanceof Person;
19         System.out.println(result1);//ture
20
21         Student s = new Student();//默认父类的无参构造
22
23         boolean result2 = s instanceof Person;
24         System.out.println(result2);//ture
25     }
26
27 }

 1 package study;
 2
 3 class Yuan{
 4     int r;
 5     final double pai = 3.14;//应该是一个不变的量，不然下面参数 给了 不是3.14 就会被修改。类似常量
 6
 7     public Yuan(int r/*,double pai*/) {
 8         this.r = r;
 9     /*    this.pai = pai;*/
10     }
11
12     public void area(){
13         System.out.println("圆的面积是"+r*r*pai);
14     }
15 }
16
17 public class star {
18     public static void main(String[] args) {
19         Yuan yuan = new Yuan(10/*,3.14*/);//不给修饰会被修改
20         yuan.area();
21     }
22 }

 1 package study;
 2
 3 public class star {
 4     public static void main(String[] args) {
 5
 6         //需求：定义两个整形的数，用一个方法来交换两个数
 7         int a = 10;
 8         int b = 20;
 9         change(a, b);
10         System.out.println("a = "+a+",b = "+b);//10.20
11
12         //需求：定义一个数组，交换两个数的位置
13         int[] arr = {10,20};
14         changeArr(arr);//20,10
15         System.out.println("arr[0] = "+arr[0]+"arr[1] = "+arr[1]);
16     }
17
18     public static void change(int a,int b){
19         int temp = a;
20         a = b;
21         b = temp;
22     }
23     public  static void changeArr(int[] arr){
24         int temp = arr[0];
25         arr[1] = arr[0];
26         arr[0] = temp;
27     }
28 }