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Tip of the Day
Language: Java
Expertise: Beginner
Aug 29, 2000

Be Careful Before Returning Mutable Objects Through Accessor Methods

Accessor methods are those methods which provide access to the values stored in instance field. Mutable objects are those objects which providen certains methods which can change the instance data stored in them.

You should always be careful before returning references to mutable objects from the accessor methods of your class.

The reason why you should be careful before returning mutable objects through accessor methods is that the caller can call a mutable method on the object reference that is returned by accessor method.

This breaks encapsulation as the caller can change the value of instance fields your object without even using any of the mutator methods of your class.

Here is an example that makes use of object of mutable class StringBuffer. The caller of this class can call "getStringBuffer()" method and then it can call "append()" method of "StringBuffer" class to append some data to it. There are two versions of "getStringBuffer()" method; the first one is not correct, whereas the second one is correct because it returns a reference to a different StringBuffer (like clone).

 
class MyClass
{
This is wrong because we are returning reference to mutable object of class StringBuffer.
 
public StringBuffer getStringBuffer() 
{
return myStringBuffer;
}
This is correct, because we are returning a copy of mutable object of class StringBuffer.
 
public StringBuffer getStringBuffer() 
{
StringBuffer tempBuffer;
		
tempBuffer = new
StringBuffer(myStringBuffer.toString());
return tempBuffer;
}

private StringBuffer myStringBuffer;
}
Deepak Pant
 
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