Why does Java's concat() method not do anything?

Question

This code:

String s = "TEST";
String s2 = s.trim();

s.concat("ING");
System.out.println("S = "+s);
System.out.println("S2 = "+s2);

results in this output:

S = TEST
S2 = TEST
BUILD SUCCESSFUL (total time: 0 seconds)

Why are "TEST" and "ING" not concatenated together?

Answer 1

a String is immutable, meaning you cannot change a String in Java. concat() returns a new, concatenated, string.

String s = "TEST";
String s2 = s.trim();
String s3 = s.concat("ING");

System.out.println("S = "+s);
System.out.println("S2 = "+s2);
System.out.println("S3 = "+s3);

Answer 2

Because String is immutable - class String does not contain methods that change the content of the String object itself. The concat() method returns a new String that contains the result of the operation. Instead of this:

s.concat("ING");

Try this:

s = s.concat("ING");