Primitive Types vs. Reference Types

• Types are divided into primitive types and reference types.

• The primitive types are boolean, byte, char, short, int, long, float and double.

• All nonprimitive types are reference types.

• A primitive-type variable can store exactly one value of its declared type at a time.

• Primitive-type instance variables are initialized by default—variables of types byte, char, short, int, long, float and double are initialized to 0, and variables of type boolean are initialized to false.

• You can specify your own initial value for a primitive-type variable by assigning the variable a value in its declaration.

• Programs use variables of reference types (normally called references) to store the locations of objects in the computer’s memory. 

  • Such a variable is said to refer to an object in the program.

• Objects that are referenced may each contain many instance variables and methods.

• Reference-type instance variables are initialized by default to the value null. 

  • A reserved word that represents a "reference to nothing".

• When using an object of another class, a reference to the object is required to invoke (i.e., call) its methods. 

  • Also known as sending messages to an object.