Question

When I pass an immutable type object(String, Integer,.. ) as final to a method I can achieve the characters of a C++ constant pointer. But how can I enforce such behavior in objects which are mutable?

public void someMethod(someType someObject){  /*   * code that modifies the someObject's state   *    */ } 

All I want is to prevent someMethod from modifying the state of someObject without making any change in someType. Is this possible?

Answer 1

No, I don’t think this is possible. The normal approach is to create an adapter for SomeType where all the methods changing state throws UnsupportedOperationException. This is used by for instance java.util.Collections.unmodifiable*-functions.

There are several approaches to this:

• you can let SomeType be an interface, and when you need it to be read only, just create a wrapper delegating all the read-methods to the original object and implementing all the write-methods to throw an exception.
• or you can create a subclass of SomeType overriding all the write methods

This will of course only give you run-time checking, not compiletime. If you want compile-time, you can let SomeType be an interface (or a superclass) with no write-methods, only read.