concat vs operator In Java for String concatenation

Question

Assuming String a and b:

a += b
a = a.concat(b)

Under the hood, are they the same thing?

Here is concat decompiled as reference. I'd like to be able to decompile the + operator as well to see what that does.

public String concat(String s) {

    int i = s.length();
    if (i == 0) {
        return this;
    }
    else {
        char ac[] = new char[count + i];
        getChars(0, count, ac, 0);
        s.getChars(0, i, ac, count);
        return new String(0, count + i, ac);
    }
}

Answer 1

Basically, there are two important differences between + and the concat method.

1. If you are using the concat method then you would only be able to concatenate strings while in case of the + operator, you can also concatenate the string with any data type.

   For Example:

   String s = 10 + "Hello";

   In this case, the output should be 10Hello.

   String s = "I";
   String s1 = s.concat("am").concat("good").concat("boy");
   System.out.println(s1);

   In the above case you have to provide two strings mandatory.

2. The second and main difference between + and concat is that:

   Case 1: Suppose I concat the same strings with concat operator in this way

   String s="I";
   String s1=s.concat("am").concat("good").concat("boy");
   System.out.println(s1);

   In this case total number of objects created in the pool are 7 like this:

   I
   am
   good
   boy
   Iam
   Iamgood
   Iamgoodboy

   Case 2:

   Now I am going to concatinate the same strings via + operator

   String s="I"+"am"+"good"+"boy";
   System.out.println(s);

   In the above case total number of objects created are only 5.

   Actually when we concatinate the strings via + operator then it maintains a StringBuffer class to perform the same task as follows:-

   StringBuffer sb = new StringBuffer("I");
   sb.append("am");
   sb.append("good");
   sb.append("boy");
   System.out.println(sb);

   In this way it will create only five objects.

So guys these are the basic differences between + and the concat method. Enjoy :)