Strings in android/java, replace() does nothing

In android/java, I'm trying to replace the space in some strings with a +, but开发者_JAVA百科 it doesn't seem to work. Am I doing it wrong?

String string="Hello world";
string.replace(" ", "+");

String objects are immutable, so the replace method doesn't change the string but creates a new one that you have to re-save:

String string="Hello world";
string = string.replace(" ", "+");

In Java, the StringBuffer class provides a mutable string. The replace method will return the same object.