System.out.println behavior when using String and int together

Question

Consider the below code snippet -

public class Student {

    public static void main(String args[]) {
        int a = 3;
        int b = 4;
        System.out.println(a + b +" ");
        System.out.println(a + b +" "+ a+b);
        System.out.println(""+ a+b);

    }
}

The output for the above snippet is coming as -

7
7 34
34

It is clear from the output that if we use String at first in the print statement then the integers are concatenated. But, if we use integer at first then the values are added and displayed.

Can someone please explain why is this behavior?

I even tried to look at the implementation of println() method in PrintStream class but could not figure out.

Answer 1

Actually it's not println() implementation who's causing that, this is Java way to treat the + operator when dealing with Strings.

In fact the operation is treated from left to right so:

• If the string comes before int (or any other type) in the operation String conversion is used and all the rest of the operands will be treated as strings, and it consists only of a String concatenation operation.
• If int comes first in the operation it will be treated as int, thus addition operation is used.

That's why a + b +" " gives 7 because Stringis in the end of the operation, and for other expressions a + b +" "+ a+b or ""+ a+b, the variables a and b will be treated as strings if the come after a String in the expression.

For further details you can check String Concatenation Operator + in Java Specs.