Java string value change in function

Question

I have this very awkward question...

void changeString(String str){
    str = "Hello world":
}

main(){
    String myStr = new String("");
    changeString(myStr);
}

When main returns, the value is still "" and not "Hello world". Why is that?

Also, how do I make it work? Let's say I want my function changeString to change the string it got to "Hello world".

Answer 1

Everyone explained why it doesn't work, but nobody explained how to make it work. Your easiest option is to use:

String changeString() {     return "Hello world"; }  main() {      String myStr = new String("");     myStr = changeString(); } 

Although the method name is a misnomer here. If you were to use your original idea, you'd need something like:

void changeString(ChangeableString str) {     str.changeTo("Hello world"); }  main() {      ChangeableString myStr = new ChangeableString("");     changeString(myStr); } 

Your ChangeableString class could be something like this:

class ChangeableString {     String str;          public ChangeableString(String str) {         this.str = str;     }      public void changeTo(String newStr) {         str = newStr;     }      public String toString() {         return str;     } } 

---

A quick lesson on references:

In Java method everything is passed by value. This includes references. This can be illustrated by these two different methods:

void doNothing(Thing obj) {     obj = new Something(); }  void doSomething(Thing obj) {     obj.changeMe(); } 

If you call doNothing(obj) from main() (or anywhere for that matter), obj won't be changed in the callee because doNothing creates a new Thing and assigns that new reference to obj in the scope of the method.

On the other hand, in doSomething you are calling obj.changeMe(), and that dereferences obj - which was passed by value - and changes it.

Answer 2

Java uses a call by value startegy for evaluating calls.

That is, the value is copied to str, so if you assign to str that doesn't change the original value.