I want to replace a entire String with new value using String replaceAll method in Java

Question

String x = "Welcome to Java World";
System.out.println(x.replaceAll(".*","JAVA"));
Actual Output = "JAVAJAVA" .
Excepted Output = "JAVA".

Can anybody help why it replace like this . ".*" all characters in a original string and replace this with "JAVA" . Why this returns "JAVAJAVA" .

Answer 1

Your regular expression can match 0 to all characters. First, it matches the entire string "Welcome to Java World", then it matches the end of the string "", replacing both with "JAVA".

To make this work how you expect it, you have a couple options.

String x = "Welcome to Java World";
System.out.println(x.replaceAll(".+","JAVA"));

Notice the + instead of the *, this means 1 or many, so the end won't be matched.

or

String x = "Welcome to Java World";
System.out.println(x.replaceFirst(".*","JAVA"));

This will only replace the entire string with "JAVA", the empty end of the string won't be replaced.

Answer 2

You don't need replaceAll for your mission. The exact same semantics are achieved by simply stating

System.out.println("JAVA");

Since String is immutable in Java, you cannot avoid getting a new object.