Parameter Passing Mechanisms

Parameter Passing Mechanisms
An Introduction with Examples in Java

Computer Science Department

bernstdh@jmu.edu

Motivation

So Far:
- We've invoked methods and passed them parameters, but not considered what happens "under the hood"
Now:
- In order to understand what is and isn't possible in methods, we need to consider the details
Why Did We Wait Until Now?
- You need to understand references before you can understand parameter passing

Common Parameter Passing Mechanisms

Call by Value:
- A copy of the contents of the actual parameter is made and assigned to the formal parameter
Call by Reference:
- A reference to the actual parameter is assigned to the formal parameter (which makes the formal parameter an alias for the actual parameter)

Parameter Passing in Java

The Rule:
- All parameters are passed by value
The Confusion:
- For primitive types the parameter is a copy of the value
- For class types the parameter is a copy of the reference (and, hence, an alias)

Passing Value Types

Trying to Swap Two Values

javaexamples/basics/ParameterPassingExample1.java (Fragment: 0)

Parameter Reference Types (cont.)

Swapping the Attributes of Two Objects

javaexamples/basics/Pair.java (Fragment: 0)

javaexamples/basics/ParameterPassingExample1.java (Fragment: 1)

Parameter Reference Types (cont.)

Trying to Swap Two Objects

javaexamples/basics/Pair.java (Fragment: 0)

javaexamples/basics/ParameterPassingExample1.java (Fragment: 2)

Parameter Reference Types (cont.)

Swapping Two Elements of an Array

javaexamples/basics/ParameterPassingExample1.java (Fragment: 3)

Parameter Reference Types (cont.)

Trying to Swap Two Arrays

javaexamples/basics/ParameterPassingExample1.java (Fragment: 4)

There's Always More to Learn