Reference data types in Java
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.
Instantiating a reference type
Object obj = new Object(); // Note the 'new' keyword
- 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.
- 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.