[SOLVED] Java string value change in function

Issue

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”.

Solution

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.

Answered By – NullUserException

Answer Checked By – David Marino (BugsFixing Volunteer)