Activity 4.6

Object References and Default Values

Topics

• Default initialization of instance variables
• Primitive vs object defaults
• What null means
• Object references
• Why NullPointerException occurs

Default Initialization

Instance variables are automatically assigned default values before the constructor runs.

1
class Example
2
{
3
    private int count;
4
    private double price;
5
    private boolean active;
6
    private String name;
7
    private Product featuredProduct;
8
}

Default Values

• int → 0
• double → 0.0
• boolean → false
• char → '\u0000'
• Any object → null

Constructors Run After Defaults

Default values are assigned before the constructor executes.

1
class Example
2
{
3
    private int count;
4
    private String name;
5

6
    public Example()
7
    {
8
        // Constructor runs AFTER default initialization
9
        name = "Default";
10
    }
11
}

What Does null Mean?

null means a variable does not reference any object.

• null is not an object
• null is only used with object variables
• null is different from an empty String

null vs Empty String

1
String a = null;
2
String b = "";
3

4
System.out.println(a);
5
System.out.println(b.length());

NullPointerException

If a variable is null, there is no object to use.

1
Product p = null;
2
p.applyDiscount(10); // NullPointerException

Preventing NullPointerException

1
if (p != null) {
2
    p.applyDiscount(10);
3
}

Object References

Object variables store references to objects, not the objects themselves.

1
Dog dog1 = new Dog("Fido", "Greyhound", 3, "gray");
2
Dog dog2 = dog1;

Why This Matters

• Many fields start as null by default
• Constructors often prevent null problems
• Composition (next lesson) relies heavily on object references
• Data structures (next module) are also reference types