how string concat works internally in java code example

Example: string concat in java

public class ConcatenationExample {
   public static void main(String args[]) {
       //One way of doing concatenation
       String str1 = "Welcome";
       str1 = str1.concat(" to ");
       str1 = str1.concat(" String handling ");
       System.out.println(str1);

       //Other way of doing concatenation in one line
       String str2 = "This";
       str2 = str2.concat(" is").concat(" just a").concat(" String");
       System.out.println(str2);
   }
}