You have not copied the object. You have two references to the same object, set up with Tester t2 = t1; For copying, you should override the clone() method and implement the Cloneable interface, or maybe better, give your Tester class a copy constructor.
This happens because variables in Java do not represent the objects themselves directly (like in C++); in Java, variables are references to objects. Line 4 in your code makes variable t2 refer to the same object that variable t1 refers to. It doesn't copy the object itself.