Reference Types in java

Java is an object-oriented language. An object is a collection of variables and associated methods that is described by a class. The concepts in this section that relate to objects are discussed in detail in Object-Orientation Java Style.

The name of a class can be used as a type, so you can declare an object-type variable or specify that a method returns an object. If you declare a variable using the name of a class for its type, that variable can contain a reference to an object of that class. Such a variable does not contain an actual object, but rather a reference to the class instance, or object, the variable refers to. Because using a class name as a type declares a reference to an object, such types are called reference types. Java also allows the use of an interface name to specify a reference type. In addition, array types in Java are reference types because Java treats arrays as objects.

The two main characteristics of objects in Java are that:

• Objects are always dynamically allocated. The lifetime of the storage occupied by an object is determined by the program's logic, not by the lifetime of a procedure call or the boundaries of a block. The lifetime of the storage occupied by an object refers to the span of time that begins when the object is created and ends at the earliest time it can be freed by the garbage collector.
• Objects are not contained by variables. Instead, a variable contains a reference to an object. A reference is similar to what is called a pointer in other languages. If there are two variables of the same reference type and one variable is assigned to the other, both variables refer to the same object. If the information in that object is changed, the change is visible through both variables.

Java references are very similar to pointers in C/C++, but they are not at all related to the C++ notion of a reference. The main difference between Java references and C++ pointers is that Java does not allow any arithmetic to be done with references. This, coupled with Java's lack of any way to explicitly deallocate the storage used by reference type values, guarantees that a reference can never point to an illegal address.