Java LanguageReference Data Types

Instantiating a reference type

Object obj = new Object(); // Note the 'new' keyword

Where:

  • Object is a reference type.
  • obj is the variable in which to store the new reference.
  • Object() is the call to a constructor of Object.

What happens:

  • Space in memory is allocated for the object.
  • The constructor Object() is called to initialize that memory space.
  • The memory address is stored in obj, so that it references the newly created object.

This is different from primitives:

int i = 10;

Where the actual value 10 is stored in i.

Dereferencing

Dereferencing happens with the . operator:

Object obj = new Object();
String text = obj.toString(); // 'obj' is dereferenced.

Dereferencing follows the memory address stored in a reference, to the place in memory where the actual object resides. When an object has been found, the requested method is called (toString in this case).


When a reference has the value null, dereferencing results in a NullPointerException:

Object obj = null;
obj.toString(); // Throws a NullpointerException when this statement is executed.

null indicates the absence of a value, i.e. following the memory address leads nowhere. So there is no object on which the requested method can be called.