Parameter Passing Mechanisms

Parameter Passing Mechanisms
An Introduction with Examples in Java

Prof. David Bernstein
James Madison University

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)

    /**
     * This function will only swap the local copies
     * of the parameters a and b
     */
    public static void swapNot(int a, int b)
    {
        int temp;


        temp = a;
        a    = b;
        b    = temp;
    }

Parameter Reference Types (cont.)

Swapping the Attributes of Two Objects

javaexamples/basics/Pair.java

(Fragment: 0)

public class Pair
{
    public int  a, b;
}

javaexamples/basics/ParameterPassingExample1.java

(Fragment: 1)

    /** This function will swap the values of
     *  the instance variables a and b in the object p
     */
    public static void swapAttributes(Pair p)
    {
        int temp;


        temp = p.a;
        p.a  = p.b;
        p.b  = temp;
    }

Parameter Reference Types (cont.)

Trying to Swap Two Objects

javaexamples/basics/Pair.java

(Fragment: 0)

public class Pair
{
    public int  a, b;
}

javaexamples/basics/ParameterPassingExample1.java

(Fragment: 2)

    /** This method will only swap the local copies
     *  of the references to m and n
     */
    public static void swapNotObject(Pair m, Pair n)
    {
        Pair temp;


        temp = m;
        m    = n;
        n    = temp;
    }

Parameter Reference Types (cont.)

Swapping Two Elements of an Array

javaexamples/basics/ParameterPassingExample1.java

(Fragment: 3)

    /** This function will swap the values of
     *  elements 0 and 1 of the array data
     */
    public static void swapElements(int[] data)
    {
        int temp;


        temp    = data[0];
        data[0] = data[1];
        data[1] = temp;
    }

Parameter Reference Types (cont.)

Trying to Swap Two Arrays

javaexamples/basics/ParameterPassingExample1.java

(Fragment: 4)

    /** This method will only swap the local copies
     *  of the references to the arrays m and n
     */
    public static void swapNotArrays(int[] m, int[] n)
    {
        int[] temp;


        temp = m;
        m    = n;
        n    = temp;
    }