High/Low Guessing Game
Naming

Handler is too generic and doesn't explain what this class is about. GameHandler would be better. Or maybe GameManager even better.

Methods should be named camelCase, so this violates the convention: PrintGuesses

The gameOver and gameWin methods' main job is to print messages.
So it would be better to name them as such, for example printGameOverMessage, printGameWinMessage.

Visibility

A good general rule of thumb:
you shouldn't manipulate the fields of objects directly like this: gameHandler.gameOver = true

All the fields in Handler should be private:


They should not be writable by other classes
Other classes should not need to see these implementation details 


This is related to good encapsulation and information hiding:
other classes should not need to know how the game handler works,
they should be able to interact with it using public interface methods.

General coding style


The formatting is a bit inconsistent. Use your IDE's function to reformat the code nicely and consistently. 
The empty public constructor of Handler is unnecessary, you can delete it, the compiler will add it automatically