Primitive vs. Reference Data Types in Java

• primitive types are the basic types of data
  • primitive variables store primitive values
• reference types are any instantiable class as well as arrays
  • String, Scanner, Random, Ratio, int[], String[], etc.
  • reference variables store addresses

• copies the contents of RHS variable into LHS variable
  • primitives: the primitive value is copied
  • references: the address is copied
• implications: for references the object is not copied, it is shared (reference variables are aliases)

• compares the contents of the variables
  • primitives: the primitive values are compared
  • references: the addresses are compared
• implications: for references the contents of the objects are not compared

• terminology (not universal, but what I like to use and find helpful):
  • parameter: the variable that is listed (along with its type) in the method declaration
  • argument: the value that is given when the method is called
• copies the contents of actual parameter into the formal parameter (i.e., pass-by-value)
  • primitives: the primitive value is copied
  • references: the address is copied
• implications: for references the object is not copied, it is shared (i.e., actual parameter and formal parameter are aliases)
• primitives: changing the formal parameter's value doesn't affect the actual parameter's value
• references: changing the formal parameter's address doesn't affect the actual parameter's address but changing the formal parameter's object does change the actual parameter's object since they refer to the same object

• returns a result to where the method was called
  • primitives: the primitive value is returned
  • references: the address is returned
• recall: local variables and parameters are destroyed when the method finishes execution
• implications: a locally created object can survive if it is returned or if it is stored in a data member

Courtesy of Timothy Bahls