Change string in Java

Question

I want to send a string to a method and change the string there. The method should return void. Example:

String s = "Hello";
ChangeString(s);

String res = s;
//res = "HelloWorld"
-------------------------------------------

private void ChageString(String s){
s = s + "World";
}

How can I do it in Java? Can it be done without adding another class?

Thanks! PB

See http://stackoverflow.com/questions/11164675/mutable-strings-in-java — fdomig, Aug 12 '12 at 18:32
If you really need to do it in a method returning void there is a dirty hack to do so. -> http://blogs.atlassian.com/2008/07/magic_trick_in_java But that is usually not the way to go! — zip, Aug 12 '12 at 18:40
seems to be a duplicate of [enter link description here][1] [1]: http://stackoverflow.com/questions/9183703/passing-a-string-to-a-function — Cyber, Aug 12 '12 at 18:50

Mark Byers · Accepted Answer · 2012-08-12T21:13:54.273

15

Your method cannot work with the current interface because of two reasons:

Strings are immutable. Once you have created a string you cannot later change that string object.
Java uses pass-by-value, not pass-by-reference. When you assign a new value to s in your method it only modifies the local s, not the original s in the calling code.

To make your method work you need to change the interface. The simplest change is to return a new string:

private String changeString(String s){
    return s + "World";
}

Then call it like this:

String s = "Hello";
String res = changeString(s);

See it working online: ideone

edited Aug 12 '12 at 21:13

answered Aug 12 '12 at 18:31

Mark Byers

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1

private String changeString(...), return type shouldn't be void. – kosa Aug 12 '12 at 18:32
How can I return a value from a method that returns void? – bahar_p Aug 12 '12 at 18:32
They are, yes. But there is some nasty hack that works in some conditions, which makes them mutable :) – dantuch Aug 12 '12 at 18:33
please remove all the "bad idea" suggestions and just suggest using a wrapper class with a mutable member variable. i don't think any newbies out there need to see these "suggestions"! ;) – jtahlborn Aug 12 '12 at 19:43
4

Java is not pass by value. Please remove irrelavent resons. – Jack Jul 24 '17 at 15:41
agree with @Jack . The reason is just immutability of String. – AliReza19330 Feb 16 '18 at 02:11
Java is very much pass by value. See https://stackoverflow.com/questions/40480/is-java-pass-by-reference-or-pass-by-value . If Java was pass by reference, a new, immutable string would be created and stored in the mem reference that was passed into the method. This would result in the string's value changing for the calling method. – zerpsed Jun 20 '19 at 14:08

Alex · Answer 2 · 2014-03-20T18:07:18.820

Use StringBuffer instead of String it will solve your problem.

public static void main(String[] args) {
    StringBuffer s = new StringBuffer("Hello");
    changeString(s);
    String res = s.toString();
    //res = "HelloWorld"
}

private static void changeString(StringBuffer s){
    s.append("World");
}

Or if you really need to use only String so here is the solution using reflection:

public static void main(String[] args) {
    String s = "Hello";
    changeString(s);
    String res = s;
    //res = "HelloWorld"
}

private static void changeString(String s){
    char[] result = (s+"World").toCharArray();
    try {
        Field field = s.getClass().getDeclaredField("value");
        field.setAccessible(true);
        field.set(s, result);

    } catch (NoSuchFieldException | SecurityException | IllegalArgumentException |
            IllegalAccessException  e1) {
        e1.printStackTrace();
    }
}

score 1 · Answer 3 · answered Aug 12 '12 at 18:32

1

Nope, can't be done while returning void.

Java passes parameters by value. Strings are immutable, so you can't change its value. You're out of luck.

You can do this:

private String changeString(String original) {
    return original + " World";
}

answered Aug 12 '12 at 18:32

duffymo

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score 1 · Answer 4 · answered Aug 12 '12 at 18:44

You could wrap up your string in an object:

public class StringWrapper {
    String _str;
    public StringWrapper(String str) { set(str); }
    public void set(String str) { _str = str; }
    public String toString() { return _str; }
}

Allowing you to do:

StringWrapper s = new StringWrapper("Hello");
ChangeString(s);

String res = s + "";
//res = "HelloWorld"

private void ChageString(StringWrapper s) {
    s.set(s + "World");
}

score 0 · Answer 5 · answered Aug 12 '12 at 18:31

0

No, a Java String is immutable and cannot be changed. You can just return a new String and assign it to the same variable.

Even further: String is declared as final, so you can't extend String to add new behaviour

answered Aug 12 '12 at 18:31

klaustopher

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score 0 · Answer 6 · answered Aug 12 '12 at 18:34

Use the StringBuffer/StringBuilder to change the string. Strings in java are immutable, this means that once they are assigned a value, they cannot be changed, the code that seems to be changing the string actually makes a new object. Here's something on string buffer, hope it helps. :)

Change string in Java

6 Answers6