Question

Why is the answer "a is something else!" and NOT "a is 0!" when running the program?

String a = "";
a += "0";
if (a == "0") {
    System.out.println("a is 0!");
} else if (a == "1") {
    System.out.println("a is 1!");
} else if (a == "a") {
    System.out.println("a is a!");
} else {
    System.out.println("a is something else!");
}

Answer 1

The output is "a is something else!", because every condition is getting false, i. e. else block is getting executed.

The only case where the else block will execute is a is not 0, not 1 or not a.

Thus it is clear that a is something else, but what it is?

The answer is a is " ", a null string. Its because String in Java is immutable, they can not be changed once they are declared.

At very first line we have declared String a = " ", thus it will remain a null string throughout the program.

To changes it, we need to use more advanced features like Java String builder.