Worked Example: Parameter Passing

Section 9.8 Worked Example: Parameter Passing

Subsection 9.8.1

Problem: Evaluate the following code - what is the output?

public class Parms {
    public static void primMethod (int beta, Person mm, Person mm2) {
        System.out.println("Inside method primMethod:");
        System.out.println("beta is " + beta + "  mm is " + mm + "  mm2 is " + mm2);

        // change values
        beta = -22;
        mm.setName("Daisy Duck");
        mm.setId(932);
        mm2 = new Person("Pluto", 87);

        System.out.println("At end of method primMethod:");
        System.out.println("beta is " + beta + "  mm is " + mm + "  mm2 is " + mm2);
    }

    public static void main(String[] args) {
        int alpha = 99;
        Person mickey = new Person("Mickey Mouse", 111);
        Person dupe = mickey;
        System.out.println("Before method call:");
        System.out.println("alpha is " + alpha + "  mickey is " + mickey +     "  dupe is " + dupe);
        primMethod(alpha, mickey, dupe);
        System.out.println("After method call:");
        System.out.println("alpha is " + alpha + "  mickey is " + mickey +     "  dupe is " + dupe);
    }
}

This code also makes use of a Person class, which is defined here:

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public class Person {
    private String name;    // name of person
    private int id;         // person's id

    // overloaded constructor
    public Person(String name, int id) {
        setName(name);
        setId(id);
    }

    // default constructor
    public Person() {
    }

    // Accessors and Mutators
    public String getName() {
        return name;
    }

    public void setName(String name) {
        if (name.length() != 0)     // name must not be null
            this.name = name;
    }

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    // toString to allow conversion to String
    public String toString() {
        return "Person{" + "name='" + name + '\'' + ", id=" + id + '}';
    }

    // determines if two Person instances are the same
    // true if both the name (regardless of case) and id are the same
    public boolean equals(Person person) {
        if (getId() != person.getId()) return false;
        if (getName().equalsIgnoreCase(person.getName()))
            return true;
        return false;
    }
}

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Subsection 9.8.2 Evaluate code

Let’s look at exactly what this code is doing.

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int alpha = 99;
Person mickey = new Person("Mickey Mouse", 111);
Person dupe = mickey;

The first three lines are just declaring and initializing some variables. Once these statements have executed, here is a memory representation:

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Notice that the object references mickey and dupe both refer to the same Person instance.

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The next lines are prints:

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System.out.println("Before method call:");
System.out.println("alpha is " + alpha + "  mickey is " + mickey + "  dupe is " + dupe);

Which will produce the output:

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Before method call:
alpha is 99  mickey is Person{name='Mickey Mouse', id=111}  dupe is Person{name='Mickey Mouse', id=111}

Because mickey and dupe are referring to the same Person instance, they print the same values.

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The next line

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primMethod(alpha, mickey, dupe);

is calling the method primMethod and passing the arguments of alpha, mickey, and dupe. Here is a memory representation once primMethod has been called:

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In Java, all arguments are pass-by-value, meaning that a copy of the value is made. So the variable beta has a copy of alpha’s value, mm has a copy of mickey’s value, and mm2 has a copy of dupe’s value. However, because mm and mm2 are object references, they are still referencing the same Person instance that was created

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We are now inside the primMethod, where the first two lines are printing values of the parameters:

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System.out.println("Inside method primMethod:");
System.out.println("beta is " + beta + "  mm is " + mm + "  mm2 is " + mm2);

They will generate this output:

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Inside method primMethod:
beta is 99  mm is Person{name='Mickey Mouse', id=111}  mm2 is Person{name='Mickey Mouse', id=111}

Next is code that changes the values:

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beta = -22;
mm.setName("Daisy Duck");
mm.setId(932);
mm2 = new Person("Pluto", 87);

Here is a memory representation:

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The value of beta has been changed due to the assignment statement.
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The contents of the Person instance referenced by mm (and mickey and dupe) have been updated.
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The mm2 object reference now refers to a completely new Person instance (at a different memory location).
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So the final statements inside primMethod

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System.out.println("At end of method primMethod:");
System.out.println("beta is " + beta + "  mm is " + mm + "  mm2 is " + mm2);

will generate this output:

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At end of method primMethod:
beta is -22  mm is Person{name='Daisy Duck', id=932}  mm2 is Person{name='Pluto', id=87}

We are now back in the main() method immediately after the call to primMethod. Look at the previous memory representation. Has alpha’s value changed? What about mickey’s value or dupe’s value?

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Answer.

No, none of the values of those variables have changed, because Java simply makes a copy of the variable values.

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However, the object that mickey and dupe refer to has had its content updated by primMethod.

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So the final print statements in main()

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System.out.println("After method call:");
System.out.println("alpha is " + alpha + "  mickey is " + mickey + "  dupe is " + dupe);

Will generate this output:

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After method call:
alpha is 99  mickey is Person{name='Daisy Duck', id=932}  dupe is Person{name='Daisy Duck', id=932}

Where alpha has the same value as before the method call and mickey and dupe still refer to the same Person instance, however the contents of that instance have been updated.

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Subsection 9.8.3

Answer.

Before method call:
alpha is 99  mickey is Person{name='Mickey Mouse', id=111}  dupe is Person{name='Mickey Mouse', id=111}
Inside method primMethod:
beta is 99  mm is Person{name='Mickey Mouse', id=111}  mm2 is Person{name='Mickey Mouse', id=111}
At end of method primMethod:
beta is -22  mm is Person{name='Daisy Duck', id=932}  mm2 is Person{name='Pluto', id=87}
After method call:
alpha is 99  mickey is Person{name='Daisy Duck', id=932}  dupe is Person{name='Daisy Duck', id=932}

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