My favourite Java puzzler 2 + 1 = 4

Daniel ShayaJanuary 13th, 2015Last Updated: January 12th, 2015

3 65 1 minute read

Here’s my current favourite Java puzzler. How can you get your code to do this?

Integer b = 2;
Integer c = 1;
 
System.out.println("b+c : " + (b+c) ); // output: 'b+c : 4' !!

There are no tricks with Sytem.out.println() i.e. you would be able to see the same value in a debugger. Clue: You need to add a few lines of code somewhere before this in your program. Scroll down for the solution.
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public static void main(String... args)throws Exception{
        Integer a = 2;
 
        Field valField = a.getClass().getDeclaredField("value");
        valField.setAccessible(true);
        valField.setInt(a, 3);
 
        Integer b = 2;
        Integer c = 1;
 
        System.out.println("b+c : " + (b+c) ); // b+c : 4
}

As you can see (and I would encourage you to go to the source code for Integer) there is a static cache (look for the Inner class IntegerCache) where the value of the Integer is mapped to its corresponding int value. The cache will store all numbers from -128 to 127 although you can tune this using the property java.lang.Integer.IntegerCache.high.

Reference:

My favourite Java puzzler 2 + 1 = 4 from our JCG partner Daniel Shaya at the Rational Java blog.