How Java JDK create new Strings in String Pool

Question

I have learned that when you have assigned the value with strings, it use a string pools. So that, they are using same address

String S1 = “Hello World”;
String S2 = “Hello World”;

String S3 = “Hello”;
String S4 = “ World”;

System.out.println(S1 == S2); // Returns true

But when I tried to do as follows

S3 = S3 + S4;

And check address with S1...

System.out.println(S1 == S3); // Returns false

Why does it returns false? Why do the JDK doesn’t use the same address with S1 or S2?

Thanks!