1.14 Calling Instance Methods

1. Reference Guide

Term	Definition / Example	College Board Requirement
Instance method	A method that works on one object’s own data, called on that object: `myDog.bark();`	Unit 1, Topic 1.14, p. 48
Dot operator	Connects an object to the method you call on it: `object.method()`	Unit 1, Topic 1.14, p. 48
Static (class) method	A method that belongs to the class, called on the class name: `Math.sqrt(9)`	Unit 1, Topic 1.10
`void` method	Does something but returns nothing, so its call is a statement: `c.add(5);`	Unit 1, Topic 1.9
Non-`void` method	Returns a value you can store or use: `int n = c.getCount();`	Unit 1, Topic 1.9
`NullPointerException`	Run-time error from calling a method on a `null` reference: `Dog d = null; d.bark();`	Unit 1, Topic 1.14, p. 48

An instance method reads and changes the fields of the object it was called on
Each object has its own fields, so the same call on two objects can give two different answers

Instance Methods vs. Static Methods

Feature	Instance method	Static (class) method
How you call it	`myCookie.addChips(2)`	`Cookie.ovenTempF()`
Needs an object?	Yes	No
Can use the object’s fields?	Yes	No
Common uses	Behavior tied to one object’s state	Utilities and class-wide information

Rules for Calling Instance Methods (AP Exam Focus)

You need an object reference: Dog.bark() does not compile unless bark is static.
The reference cannot be null: calling a method on null throws a NullPointerException at run time.
Arguments must match the parameters in number, order, and type (or widen to the type).
A void method returns nothing: int x = d.bark(); does not compile if bark is void.
A non-void method’s result can be stored or used in an expression: r1.getArea() + r2.getArea().
Inside the class, an instance method can call another instance method of the same object without a dot: forward(50);.
Parameter names in the method header do not matter to the caller. Only the arguments’ values, order, and types do.

Predicting a Call

The call is on a class name → it compiles only if the method is static
The variable was never given an object → it does not compile if it is a local variable that was never assigned; it throws a NullPointerException if it holds null
The result is stored in a variable → the method must not be void, and its return type must fit the variable
Two objects call the same method → each one uses its own fields

College Board Requirements

AP CSA Unit 1, Topic 1.14 Calling Instance Methods. Quoted from the course and exam description (College Board, 2025, p. 48):

“Instance methods are called on objects of the class. The dot operator is used along with the object name to call instance methods.”
“A method call on a null reference will result in a NullPointerException.”

The Java language specification defines the dot-operator call as a method invocation expression (Gosling et al., 2023, section 15.12).

Reference: College Board, AP Computer Science A Course and Exam Description (2025), Unit 1, Topic 1.14, p. 48.

2. Real-Life Application

One Order, Many Phones

When you tap “Order” in a food delivery app, the app does not call a method on “orders” in general. It calls methods on your one order object: myOrder.addItem(...), myOrder.getTotal(). Thousands of people order at the same time, and every order keeps its own items and total. Everyone guess: what would go wrong if getTotal() belonged to the class instead of to each order?

Real Case	What Happened	Java Idea in This Lesson
Food delivery orders	Every customer’s order keeps its own items and total	Each object has its own fields, so `a.getTotal()` and `b.getTotal()` differ
Game characters	`player.jump()` moves one character, not every character	An instance method acts on the object it is called on
A missing account	The app reads a profile that was never loaded and crashes	Calling a method on `null` throws a `NullPointerException`
A calculator app	`Math.sqrt(9)` needs no object	Static methods are called on the class name

The takeaway: an instance method call says which object to use. Make sure that object exists, then use what the method gives back.

3. Tech Talk

Learning Objective: Call instance methods on objects with the dot operator, use the values non-void methods return, and tell instance methods from static methods.

Success Criteria: You can say whether a call compiles, whether it needs an object first, what it returns, and whether a null reference would make it throw a NullPointerException.

Class = the recipe. Object = one cookie made from it with new.
A static method belongs to the recipe: Cookie.ovenTempF() is the same for every cookie.
An instance method needs one cookie: myCookie.addChips(3); changes that cookie only.


Counter c = new Counter();   // create the object first
c.add(5);                    // void: a statement on its own
int value = c.getCount();    // non-void: store what it returns

sequenceDiagram
    participant Main
    participant c as c (a Counter)

    Main->>c: new Counter()
    activate c
    Main->>c: add(5)
    c-->>Main: (void)
    Main->>c: getCount()
    c-->>Main: 5
    deactivate c

Code Examples

A. Calling methods on two objects. The Dog class shows that each object uses its own fields

bark uses name, and each Dog has its own name:


fido.bark();   // Fido says: Woof!
rex.bark();    // Rex says: Woof!

Terminology:

Dot operator: the object before the dot decides whose fields the method uses.

Code Runner Challenge

Predict both lines, run it, then make a third Dog and call bark on it

View IPYNB Source

// CODE_RUNNER: Predict both lines, run it, then make a third Dog and call bark on it
public class Dog {
    private String name;

    public Dog(String name) {
        this.name = name;
    }

    public void bark() {
        System.out.println(name + " says: Woof!");
    }

    public static void main(String[] args) {
        Dog fido = new Dog("Fido");
        Dog rex = new Dog("Rex");
        fido.bark();
        rex.bark();
    }
}

Dog.main(null);

Lines: 1 Characters: 0

Output

Click "Run" in code control panel to see output ...

B. Arguments and return values. The Counter class mixes void and non-void calls

add changes the object and returns nothing. getCount returns a value, so its call can be stored or printed:


c.add(5);                    // void
int val = c.getCount();      // non-void: returns an int
int x = c.add(5);            // does not compile: add is void

Terminology:

Return value: what a non-void method sends back to the line that called it.

Code Runner Challenge

Predict the three counts, run it, then call add on c2 as well

View IPYNB Source

// CODE_RUNNER: Predict the three counts, run it, then call add on c2 as well
public class Counter {
    private int count;

    public void add(int x) {
        count += x;
    }

    public int getCount() {
        return count;
    }

    public static void main(String[] args) {
        Counter c1 = new Counter();
        Counter c2 = new Counter();
        c1.add(5);
        c1.add(3);

        int val = c1.getCount();
        System.out.println("c1: " + val);
        System.out.println("c2: " + c2.getCount());
        System.out.println("Total: " + (c1.getCount() + c2.getCount()));
    }
}

Counter.main(null);

Lines: 1 Characters: 0

Output

Click "Run" in code control panel to see output ...

Hacks & Practice Tasks


Lesson: CSA 1.14 Calling Instance Methods
MCQ 1.14: <paste the copied result line, such as 3/4 | answers: B,C,A,D>
Popcorn: breed field, describe method, two dogs described with method calls, runs and prints (yes/no); does BreedDog.describe() compile = <your answer>
Homework Hack 1: subtract, multiply, and divide added, both checks print true (yes/no)
Homework Hack 2: compiler error read = <the error line>, all three rectangle checks print true (yes/no)
Homework Hack 3: Student tracker, all four checks print true (yes/no)

Submission Safety Rules (Read First)

One class per cell, ending with ClassName.main(null);.
Label the homework hacks 1 through 3.
Call every instance method on an object, never on the class name.
Run each Java cell and leave the output showing.
Include your MCQ score.
Use ## headings or smaller.

Popcorn Hack (In-Class)

3-minute challenge: give Dog a breed and a method that uses it.

Add a breed field and set it in the constructor
Write a non-void method describe() that returns "My dog <name> is a <breed>!"
In main, make two dogs and print describe() for each one
Answer the question in the Popcorn checker under the runner before you run. When every check passes, copy its line into your submission notes

Code Runner Challenge

Add breed and describe(), describe two dogs, then run it

View IPYNB Source

// CODE_RUNNER: Add breed and describe(), describe two dogs, then run it
// Practice #1 - Try it yourself first!
public class BreedDog {
    private String name;
    // TODO: add a breed field

    public BreedDog(String name) {  // TODO: also take and store the breed
        this.name = name;
    }

    // TODO: add describe(), which returns "My dog <name> is a <breed>!"

    public static void main(String[] args) {
        BreedDog first = new BreedDog("Fido");
        // TODO: make a second dog, then print describe() for both dogs
    }
}

BreedDog.main(null);

Lines: 1 Characters: 0

Output

Click "Run" in code control panel to see output ...

Popcorn checker press Run to check

Before you press Run, answer this.

Would BreedDog.describe(); compile? (yes/no)

Pass every check to finish the Popcorn Hack.

4. MCQ Check

4 questions, one at a time. Answer, check, then go to the next one. At the end, copy the score line into your submission notes.

5. Homework Hack

Task: Do all three hacks. Hack 2 starts with a compiler error on purpose: run it, read the error, then fix it. The other starters compile, so you can run the initial version and compare its incorrect output with the expected output. Keep going until every check prints true.

Optional: The past 1.14 homework has five more practice questions and the full Student Grade Tracker assignment.

Solution Skeleton:

Hack 1: Grow the Counter (5 minutes)

Writing and calling instance methods. Fill in subtract, multiply, and divide. Each one follows the same pattern as add: combine count with the parameter and store the result back in count. The calls in main are already written.

Code Runner Challenge

Fill in subtract, multiply, and divide so both checks print true.

View IPYNB Source

// CODE_RUNNER: Fill in subtract, multiply, and divide so both checks print true.
public class CounterHack {
    static class Counter {
        private int count;

        public void add(int x) {
            count += x;
        }

        // TODO: subtract x from count
        public void subtract(int x) {
        }

        // TODO: multiply count by x
        public void multiply(int x) {
        }

        // TODO: divide count by x
        public void divide(int x) {
        }

        public int getCount() {
            return count;
        }
    }

    public static void main(String[] args) {
        Counter c = new Counter();
        c.add(10);
        c.subtract(4);
        c.multiply(3);
        c.divide(2);
        System.out.println("Count: " + c.getCount());

        Counter other = new Counter();
        other.add(7);
        System.out.println(c.getCount() == 9);
        System.out.println(other.getCount() == 7);
    }
}
CounterHack.main(null);

Lines: 1 Characters: 0

Output

Click "Run" in code control panel to see output ...

Hack 2: Void or Not? (5 minutes)

Return values. Run it first: one line tries to store the result of a void method, so it does not compile. Read the error, then fix that line so it calls scale as a statement. Then write getPerimeter, which returns 2 * (length + width), and use the return values of both rectangles in total.

Code Runner Challenge

Run it to see the compiler error, fix the scale line, write getPerimeter, then run until all three checks print true.

View IPYNB Source

// CODE_RUNNER: Run it to see the compiler error, fix the scale line, write getPerimeter, then run until all three checks print true.
public class RectangleHack {
    static class Rectangle {
        private int length;
        private int width;

        public Rectangle(int length, int width) {
            this.length = length;
            this.width = width;
        }

        public int getArea() {
            return length * width;
        }

        public void scale(int factor) {
            length *= factor;
            width *= factor;
        }

        // TODO: return 2 * (length + width)
        public int getPerimeter() {
            return 0;
        }
    }

    public static void main(String[] args) {
        Rectangle r1 = new Rectangle(3, 4);
        Rectangle r2 = new Rectangle(1, 2);
        int result = r1.scale(2);  // TODO: scale is void, so call it as a statement

        int total = 0;  // TODO: the area of r1 plus the area of r2, using getArea()

        System.out.println("r1 area: " + r1.getArea() + ", perimeter: " + r1.getPerimeter());
        System.out.println(r1.getArea() == 48);
        System.out.println(r1.getPerimeter() == 28);
        System.out.println(total == 50);
    }
}
RectangleHack.main(null);

Lines: 1 Characters: 0

Output

Click "Run" in code control panel to see output ...

Hack 3: Student Grade Tracker (10 minutes)

Combined. Each Student tracks its own grades. Fill in the four methods:

addGrade(int points) (void): adds points to totalPoints and counts one more assignment
getAverage() (non-void): returns the average as a double, or 0.0 with no assignments. Cast before dividing (Topic 1.5)
getLetterGrade() (non-void): "A" for 90 and up, "B" for 80 and up, "C" for 70 and up, "D" for 60 and up, otherwise "F"
printReport() (void): prints the name, average, and letter grade

The two students must stay independent: adding a grade to one never changes the other.

Code Runner Challenge

Fill in the four Student methods so all four checks print true.

View IPYNB Source

// CODE_RUNNER: Fill in the four Student methods so all four checks print true.
public class GradeTracker {
    static class Student {
        private String name;
        private int totalPoints;
        private int numAssignments;

        public Student(String name) {
            this.name = name;
        }

        // TODO: add points to totalPoints and count one more assignment
        public void addGrade(int points) {
        }

        // TODO: return the average as a double, or 0.0 with no assignments
        public double getAverage() {
            return 0.0;
        }

        // TODO: return A, B, C, D, or F from the average
        public String getLetterGrade() {
            return "F";
        }

        // TODO: print the name, average, and letter grade on one line
        public void printReport() {
        }
    }

    public static void main(String[] args) {
        Student emma = new Student("Emma");
        Student james = new Student("James");
        emma.addGrade(95);
        emma.addGrade(88);
        emma.addGrade(92);
        emma.addGrade(85);
        james.addGrade(78);
        james.addGrade(82);
        james.addGrade(75);
        emma.printReport();
        james.printReport();

        System.out.println(emma.getAverage() == 90.0);
        System.out.println(emma.getLetterGrade().equals("A"));
        System.out.println(Math.abs(james.getAverage() - 78.33) < 0.01);
        System.out.println(james.getLetterGrade().equals("C"));
    }
}
GradeTracker.main(null);

Lines: 1 Characters: 0

Output

Click "Run" in code control panel to see output ...

Grading Plan (1 Point Total)

Part	Points	What earns the points
Popcorn	0.2	A `breed` field set by the constructor, a `describe()` method that returns a String, and two dogs described with calls on objects. The Popcorn checker line is in the notes.
MCQ	0.2	4 correct. 0.15 for 2 or 3, 0.1 if every question was answered.
Homework Hack 1	0.15	`subtract`, `multiply`, and `divide` change `count`, and both checks print `true`.
Homework Hack 2	0.2	The compiler error is recorded, `scale` is called as a statement, `getPerimeter` and `total` use return values, and all three checks print `true`.
Homework Hack 3	0.25	All four `Student` methods work on each object’s own fields, the average is cast before dividing, and all four checks print `true`.
Total	1.0

Quick Validation Checklist

Each cell ends with ClassName.main(null); and shows output.
Popcorn checker line in the notes.
MCQ score in the notes.
No instance method called on a class name.
No void method’s result stored in a variable.
Every homework check prints true.

6. LxD Cycle Process

Empathize: When students see a class for the first time, they often call its methods on the class name (Dog.bark()), the way they call Math.sqrt(). They have not yet separated what belongs to the class (static) from what belongs to one object (instance), and they try to store the result of void methods.

Define:

POV: CSA students need one reliable test for when a call needs an object and what it returns, because the AP exam repeatedly shows static and instance calls that look almost the same.
Learning Goal: Students will call instance methods with the dot operator, use the values non-void methods return, tell instance methods from static methods, and predict when a call fails to compile or throws a NullPointerException.

Ideate:

HMW Question: How might we give students one question to ask about any call: “which object is this for?”
HMW Question: How might we make the difference between a void and a non-void call something students check every time?
Activity: A cookie-recipe analogy, then add methods to Dog and Counter, fix a call that stores a void result, and build a grade tracker with two independent students.

Prototype:

A reference guide with seven calling rules, runnable two-object and return-value examples, a method-writing Popcorn Hack with a checker, an MCQ knowledge check, three scaffolded homework hacks with self-checking output, and a grading rubric.
A trial run showed that students who missed the first MC question all called deposit on the class name, so the cookie analogy and the instance-versus-static table come before any questions, and the Popcorn checker asks whether the class-name call compiles.
Excellence means every call is on the right object and every return value is either used or deliberately ignored.

Test:

Ask peers in peer review to complete the practice without additional explanation.
Observe whether peers call methods on objects rather than on the class name, and whether they store results only from non-void methods.
Compare your lesson with another that is posted.
Use the findings to revise any instruction or rubric criterion that did not guide students clearly.
On submission, collect evidence from runner output, MCQ results, AI grading and student explanations.
After teaching, grading and analysis, come back and revise lesson to complete teaching cycle for continuous improvement.

7. References/Citations

College Board. (2025). AP Computer Science A course and exam description [Effective fall 2025]. https://apcentral.collegeboard.org/media/pdf/ap-computer-science-a-course-and-exam-description-effective-fall-2025.pdf

Gosling, J., Joy, B., Steele, G., Bracha, G., Buckley, A., Smith, D., & Bierman, G. (2023). The Java® language specification (Java SE 21 ed., section 15.12, Method invocation expressions). Oracle. https://docs.oracle.com/javase/specs/jls/se21/html/jls-15.html

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1. Reference Guide

Instance Methods vs. Static Methods

Rules for Calling Instance Methods (AP Exam Focus)

Predicting a Call

College Board Requirements

2. Real-Life Application

One Order, Many Phones

3. Tech Talk

Code Examples

A. Calling methods on two objects. The Dog class shows that each object uses its own fields

Code Runner Challenge

B. Arguments and return values. The Counter class mixes void and non-void calls

Code Runner Challenge

Hacks & Practice Tasks

Submission Safety Rules (Read First)

Popcorn Hack (In-Class)

Code Runner Challenge

4. MCQ Check

5. Homework Hack

Hack 1: Grow the Counter (5 minutes)

Code Runner Challenge

Hack 2: Void or Not? (5 minutes)

Code Runner Challenge

Hack 3: Student Grade Tracker (10 minutes)

Code Runner Challenge

Grading Plan (1 Point Total)

Quick Validation Checklist

6. LxD Cycle Process

7. References/Citations

Submit Assignment