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Certification Java Programming SCJP

String concatenation operator

The string concatenation operator is a bit like a hedgehog, it looks cute and sweet, but try to grab hold of it quickly and you’ll soon know about it…

It’s actually quite simple, however there are some traps that are very easily overlooked (just got caught out with it on a mock test, so I’m here to rant).

The rule of thumb is, that if either one of the operands being “+” is a String, then concatenation will occur, but if they are both numbers, arithmetic addition will occur. Sounds simple right? What do you think the following equate to?

package operators;
public class MockTest
{
    public static void main(String[] args)
    {
        System.out.println(3 + 2);
        System.out.println(3 + "s");
        System.out.println("s" + 2);
        System.out.println("hello " + "world");
    }
}

Think you know it? Well its going to be : 5, 3s, s2. The first one is an arithmetic addition, since both sides of the operator are numeric. The second and third are String concatenation, because at least one of them is a String. The fourth one are both Strings, so concatenation once again.

those are the simple ones, however it can get a lot more tricky, have a look at these common ones that those exam creators try to catch you with!

package operators;

public class MockTest
{
    public static void main(String[] args)
    {
        // Inside brackets are evaluated first, so it'll be 7String
        System.out.println( (3 + 4) + "String");

        /* This is the little bugger that caught me, I vouched for 34String, but its actually 7String.
         Concatenation happens from left to right, one at a time, so it'll break it down to:
            3 + 4 then...
            <thing on left> + "String"

         Careful on these ones!
        */
        System.out.println(3 + 4 + "String");

    }
}


So remember, it’ll evaluate from left to right, one “+” operator at a time. Brackets are always evaluated first however.

Take my examples and have a play!

Happy coding (or raging at the screen if the mock tests catch you out…)

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