Instance Variables

An instance variable (also called a field) is a variable declared in the body of a class but outside any method. Every object built from the class gets its own independent copy of every instance variable.

Instance variables hold state: data that persists for the lifetime of the object. They are different from local variables (declared inside methods), which disappear when the method returns.

On the AP exam, instance variables are tested in tracing problems, in FRQ class design, and in MCQ questions about scope and default values.

Declaring instance variables

public class Dog {
    private String name;
    private int age;
}

Instance variables are declared inside the class body but outside any method. They should be private to support encapsulation.

Each object has its own copy

Dog a = new Dog();   // a has its own name and age
Dog b = new Dog();   // b has its own name and age
// changing a.name does not affect b.name

Default values

If you don't initialize an instance variable, Java assigns a default based on its type:

• int, double → 0 or 0.0
• boolean → false
• Object types (String, int[], etc.) → null

This is different from local variables, which have no default and must be assigned before use.

Lifetime and scope

Instance variables exist as long as their object exists. They are accessible from any method of the class. Local variables exist only inside the block where they are declared.

Accessing fields inside the class

public class Dog {
    private String name;

    public void setName(String n) {
        name = n;       // accesses the field directly
        // or: this.name = n;
    }
}

The keyword this refers to the current object. Use this.name when you need to distinguish the field from a parameter of the same name.