引用同一字符串对象的两个Java变量不同步

Question

From my novice perspective of Java object, if two variables referencing one object, updating one variable should do the same to the other, as the code shows below:

SomeObject s1 = new SomeObject("first");
SomeObject s2 = s1;
s2.setText("second");
System.out.println(s1.getText()); // print second
System.out.println(s2.getText()); // print second as well

I referenced the code from this thread .

However, this doesn't apply to String class. See code below:

String s_1 = new String("first");
String s_2 = s_1;
s_2 = "second";
System.out.println("s_1: " + s_1 + " s_2:  " + s_2); 
//s_1 appears to be "first" and s_2 "second"

Is this because the difference between String class and self-declared class? Thank you for your help!

Answer 1

No, the difference is in the kind of change you make.

This code:

s2.setText("second")

doesn't change the value of either s2 or s1 . Both s1 and s2 refer to the same object as they did before... but the contents of the object has changed.

Compare that with this code:

s_2 = "second";

Here you're changing the value of s2 to be a reference to a different object. Changing the value of s_2 doesn't change the value of s_1 at all - it still refers to the same object that it did before.

You probably want to read up on the differences between variables, objects and references - once you can get that clear in your head, everything else will make a lot more sense.

Answer 2

A String object can be initialized without the constructor being called. So when you initialize s_2 to s_1 , it is given the value of it and when you initialize s_2 = "second" it is overwritten. It is same as s2 = s1 and then s2 = new SomeObject ("second")