Instance Variables and Methods in Java

Instance variables and methods belong to each object — every object gets its own copy.

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Instance Variables

Declared inside a class but outside any method. Each object has its own copy.

class Employee {
    String name;       // instance variable
    double salary;     // instance variable
    String department; // instance variable
}

Employee alice = new Employee();
alice.name = "Alice";
alice.salary = 95000;

Employee bob = new Employee();
bob.name = "Bob";
bob.salary = 78000;

// Changing alice's salary doesn't affect bob's
alice.salary = 100000;
System.out.println(bob.salary);  // still 78000

Default Values

Instance variables are auto-initialized:

Type	Default
int, long	0
double, float	0.0
boolean	false
char	'\u0000'
Objects (String, etc.)	null

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Instance Methods

Methods that operate on the object’s own data:

class Employee {
    private String name;
    private double salary;

    public Employee(String name, double salary) {
        this.name = name;
        this.salary = salary;
    }

    // Instance method — uses 'this' object's data
    public double calculateBonus() {
        return salary * 0.10;
    }

    public void giveRaise(double percentage) {
        this.salary += this.salary * (percentage / 100);
    }

    public String toString() {
        return name + " ($" + salary + ")";
    }
}

Employee alice = new Employee("Alice", 95000);
System.out.println(alice.calculateBonus());  // 9500.0

alice.giveRaise(5);
System.out.println(alice);  // Alice ($99750.0)

Each call to calculateBonus() uses that specific object’s salary.

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this — The Current Object

Inside an instance method, this refers to the object the method was called on:

class Employee {
    private String name;

    public void setName(String name) {
        this.name = name;  // this.name = field, name = parameter
    }

    public boolean isSameAs(Employee other) {
        return this.name.equals(other.name);  // compare this object with another
    }
}

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Instance vs Local Variables

	Instance Variable	Local Variable
Declared in	Class body	Method body
Scope	Entire object	Method only
Default value	Yes (0, null, false)	None — must initialize
Lifetime	Object lifetime	Method execution
Access via	this.varName	Directly

class Demo {
    int instanceVar = 10;  // instance — has default

    void method() {
        int localVar;  // local — NO default
        // System.out.println(localVar);  // ERROR: not initialized
        localVar = 20;
        System.out.println(instanceVar + localVar);  // 30
    }
}

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Passing Objects to Methods

When you pass an object to a method, you pass the reference:

public static void promote(Employee emp) {
    emp.giveRaise(15);  // modifies the ORIGINAL object
}

Employee alice = new Employee("Alice", 95000);
promote(alice);
System.out.println(alice);  // salary is now raised

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Quick Exercises

1. Create a Counter class with an instance variable count and methods increment(), decrement(), and getCount().
2. Create two Counter objects. Increment one 5 times and the other 3 times. Print both counts.
3. Write a method that takes two Employee objects and returns the one with the higher salary.

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Summary

• Instance variables: per-object, auto-initialized, live with the object
• Instance methods: operate on the calling object’s data via this
• Local variables: per-method, must be initialized, die when method ends
• Object references are passed by value — methods can modify object state

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What’s Next?

Some data belongs to the class itself, not individual objects. Let’s learn about Class Variables and Methods (static).