Types

Primitive Types

• byte
• short
• int
• long
• float
• double
• boolean
• char

Reference Types

Everything not belonging to [[#Primitive_Types.md|#Primitive_Types]] is a reference type, including arrays.

Class instantiation

When we instantiate(new) an Object (e.g. Dog, Walrus, Planet):

• Java first allocates a box of bits for each instance variable of the class and fills them with a default value (e.g. 0, null).
• The constructor then usually fills every such box with some other value.

Can think of new as returning the address of the newly created object

• Addresses in Java are 64 bits.
• Example (rough picture): If object is created in memory location

So the new- allocating memory and returning the address, similar to the malloc in C.

Reference Type Variable Declarations

When we declare a variable of any reference type (Walrus, Dog, Planet):

• Java allocates exactly a box of size 64 bits, no matter what type of object.
• These bits can be either set to:
  • Null (all zeros).
  • The 64 bit “address” of a specific instance of that class (returned by new).

The Golden Rule of Equals(GRoE) (and Parameter Passing)

y = x copies all the bits (pass by value) from x into y.

So does reference types, in terms of our visual metaphor, we “copy” the arrow by making the arrow in the b box point at the same instance as a.

Summary

(Variables that store arrays are reference variables just like any other.)

Casting