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.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.

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.