Reference 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 ofObject
.
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.