关于java:java中赋值问题

public class ReferenceTest {

static class Student{    public Bag b;    public int a;    public String s;    public String getS() {        return s;    }    public void setS(String s) {        this.s = s;    }    public int getA() {        return a;    }    public void setA(int a) {        this.a = a;    }    public Bag getB() {        return b;    }    public void setB(Bag b) {        this.b = b;    }}

static class Bag{

    public String name;    public String getName() {        return name;    }    public void setName(String name) {        this.name = name;    }}public static void main(String[] args) {    //援用

// Student student = new Student();
// Bag bag1= student.b;
// System.out.println(bag1);
// student.b = new Bag();
// System.out.println(bag1);
//8种根本变量
// Student student = new Student();
// int a= student.a;
// System.out.println(a);
// student.a = 1;
// System.out.println(a);
//字符串
// Student student = new Student();
// String s= student.s;
// System.out.println(s);
// student.s = "s";
// System.out.println(s);
//// int a=0;
// int b =a;
// System.out.println(b);
// a=1;
// System.out.println(b);
String s = "s";

    String s1 = s;    System.out.println(s1);    s="s2";    System.out.println(s1);}

}