Ever wondered how the JVM debugger 'Evaluate' feature works in IDEs? Long ago I thought that the Java debug interface (JDI) actually allows you to specify the expression and get back its value.

However, this is not quite possible: there's no Java language compiler inside the JVM. When the process is paused JDI allows you to read or write any variables and fields or even execute an existing method with given arguments. But nothing like addition or multiplication.

On the other hand, #IntelliJIDEA debugger allows you to evaluate even statements! You can write there `for`, `if`, `while`, `switch` - whatever. You can even declare local variables. So how this works?

The answer is simple: IDEA debugger has a built-in interpreter for Java! If you access an existing variable or a field, call a method or instantiate an object, it queries JDI. Otherwise, IDE itself does all the calculations.

The amusing thing is that debugger authors didn't bother to make it as strict as Java itself, so the language inside the interpreter is a much more permissive version of Java. For example, you don't need to initialize variables there! Default values like 0 are used automatically.