3.6 Methods: Passing and Returning References of an Object
1. Exam Points
Pass the an object as a parameter into a method.Return an object as a returned value from a method.- For a reference type (mutable) parameter, changes to the parameter will reflect on the object passed to the parameter. But String is an exception, since String is an immutable class.
Methods from external classes can not access private instance variables or methods of a reference type parameter.Methods from internal classes can access private instance variables or methods of a reference type parameter
2. Knowledge Points
(1) Methods: Passing and Returning References of an Object
When an argument is an object reference, the parameter is initialized with a copy of that reference; it does not create a new independent copy of the object.
- In this example, x and y refer to the same object, only one object is created using the new keyword.
- If the parameter refers to a mutable object (String is immutable), the method or constructor can use this reference to alter the state of the object.
- In this example, the age of the dog is changed since we pass dog1(an object) to d.
- When the
return expression evaluates to an object reference, the reference is returned, not a reference to a new copy of the object. 
(2) Access private instance variables or methods
- Methods
cannot access the private data and methods of a parameter that holds a reference to an object unless the parameter is the same type as the method’s enclosing class. 
- In this example, since age is
private, so it can not be accessed using the parameter x.
3. Exercises
