AP CSA – 1.14 Calling Instance Methods

1. What Is an Instance Method?

An instance method is a method that belongs to an object created from a class. To use an instance method, you must have an object reference.

• Instance methods operate on the data stored inside an object.
• You must use the dot operator . to call the method.

Syntax:

objectName.methodName(arguments);

2. Instance Methods vs. Static Methods

Static method example:

Math.sqrt(16);

Instance method example:

str.length();

3. Calling Methods on a String Object

String str = "Hello";
int len = str.length();

Other common String instance methods:

str.toUpperCase();     // "HELLO"
str.substring(1, 4);   // "ell"
str.indexOf("l");      // 2

4. Calling Instance Methods on Custom Objects

Example class:

public class Dog {
    private int age;

    public void setAge(int a) {
        age = a;
    }

    public int getAge() {
        return age;
    }
}

Using the class:

Dog d = new Dog();
d.setAge(5);
int a = d.getAge();

5. Dot Notation

Dot notation is used for all instance method calls:

objectReference.methodName(parameters);

Examples:

student.getName();
car.startEngine();
scanner.nextInt();

6. Methods That Return Values vs. Methods That Don’t

Methods that return values:

int len = str.length();
String upper = str.toUpperCase();

Void methods (do not return values):

Dog d = new Dog();
d.setAge(3);

7. Chaining Instance Method Calls

String result = str.toUpperCase().substring(1, 3);

This calls multiple instance methods in a row.

8. Common Mistakes

• Calling a method on a null reference:

String s = null;
s.length();   // ERROR: NullPointerException

• Forgetting to instantiate an object:

Scanner sc;
int x = sc.nextInt();  // ERROR: sc not initialized

• Confusing static and instance methods:

Math.random();  // correct
myRandom.random();  // incorrect unless myRandom is an object with that method

9. Why This Matters in AP CSA

• Instance method calls appear frequently on the AP Exam.
• Understanding how objects and methods work together is essential.
• String, Scanner, ArrayList, and custom classes all rely on instance methods.

10. Quick Summary

• Instance methods always require an object.
• Use objectName.methodName() syntax.
• Instance methods work with the object’s data.
• You must instantiate the object before calling its methods.