How to replace a substring of a string [duplicate]

You need to use return value of replaceAll() method. replaceAll() does not replace the characters in the current string, it returns a new string with replacement.

• String objects are immutable, their values cannot be changed after they are created.
• You may use replace() instead of replaceAll() if you don't need regex.

    String str = "abcd=0; efgh=1";
    String replacedStr = str.replaceAll("abcd", "dddd");

    System.out.println(str);
    System.out.println(replacedStr);

outputs

abcd=0; efgh=1
dddd=0; efgh=1

2 things you should note:

1. Strings in Java are immutable to so you need to store return value of thereplace method call in another String.
2. You don't really need a regex here, just a simple call to String#replace(String) will do the job.

So just use this code:

String replaced = string.replace("abcd", "dddd");

You need to create the variable to assign the new value to, like this:

String str = string.replaceAll("abcd","dddd");