赋值操作后，对象的引用发生变化

// 此用例来源于Thinking in java 

class Tank{

int level;

String msg;

Tmp tmp;

@Override

public String toString() {

return "level = "+level+" msg="+msg+" tmp="+tmp;

}

}

class Tmp{

private String name;

public Tmp(String name){

this.name=name;

}

@Override

public String toString() {

return this.name;

}

}

-----------------

Tank t1 = new Tank();

t1.level =1;

t1.msg="message 1";

t1.tmp = new Tmp("tmp 1");

System.out.println(t1);

Tank t2 = new Tank();

t2.level =2;

t2.msg="message 2";

t2.tmp = new Tmp("tmp 2");

System.out.println(t2);

t1 = t2 ;

t2赋值给t1后，t2中原对象的引用也指向了t1对象的引用，原先t2对象的引用被覆盖，信息丢失。
此时t2与t1指向同一个引用。