Java Reference Data Types
Instantiating a reference type[edit | edit source]
Object obj = new Object(); // Note the 'new' keyword
Objectis a reference type.
objis the variable in which to store the new reference.
Object()is the call to a constructor of
- 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
Dereferencing[edit | edit source]
Dereferencing happens with the
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.