AP Computer Science a Quiz: Methods Passing And Returning References

What this quiz covers

This quiz focuses on Methods Passing And Returning References, giving you a quick way to practice the rules, question types, and explanations that matter most for AP Computer Science a.

How to use this quiz

Try each quiz question before looking at the correct answer. Use the explanations to review missed ideas, then come back to similar questions until the pattern feels familiar.

Question 1

public class BankAccount {
    private String accountNumber;
    private double balance;

    public BankAccount(String accountNumber, double balance) {
        this.accountNumber = accountNumber;
        this.balance = balance;
    }

    // Adds funds to this account
    public void deposit(double amount) {
        balance += amount;
    }

    // Deposits then returns this same account reference
    public BankAccount depositAndReturn(double amount) {
        deposit(amount);
        return this;
    }

    public double getBalance() {
        return balance;
    }

    public static void main(String[] args) {
        BankAccount acct = new BankAccount("A100", 50.0);
        BankAccount ref = acct.depositAndReturn(25.0);
        System.out.println(acct.getBalance());
        System.out.println(ref.getBalance());
    }
}

1. A new BankAccount object with the updated balance.
2. null, because depositAndReturn has no explicit return value.
3. A reference to the same modified BankAccount object (this). (correct answer)
4. A reference to a temporary local parameter that becomes invalid.

Explanation: This question tests AP Computer Science A skills: understanding methods passing and returning object references. In Java, the 'this' keyword refers to the current object instance, and returning 'this' from a method returns a reference to that same object, enabling method chaining. In this scenario, the depositAndReturn method first calls deposit to add funds, then returns 'this' - a reference to the current BankAccount object that was just modified. Choice C is correct because it identifies that the method returns a reference to the same modified BankAccount object, allowing the caller to continue working with the updated object through the returned reference. Choice A is incorrect because it suggests a new object is created and returned, which doesn't happen - the same object reference is returned after modification. To help students: Demonstrate method chaining with visual representations of how 'this' points to the current object. Show examples where returning 'this' enables fluent interfaces like builder patterns.

Question 2

public class BankAccount {
    private String accountNumber;
    private double balance;

    public BankAccount(String accountNumber, double balance) {
        this.accountNumber = accountNumber;
        this.balance = balance;
    }

    // Adds funds to this account
    public void deposit(double amount) {
        balance += amount;
    }

    // Deposits then returns this same account reference
    public BankAccount depositAndReturn(double amount) {
        deposit(amount);
        return this;
    }

    public double getBalance() {
        return balance;
    }

    public static void main(String[] args) {
        BankAccount acct = new BankAccount("A200", 100.0);
        BankAccount ref = acct.depositAndReturn(40.0);
        ref.deposit(10.0);
        System.out.println(acct.getBalance());
    }
}

1. acct balance is 100.0 because ref modifies only its own copy.
2. acct balance is 140.0 because only depositAndReturn changes it.
3. acct balance is 150.0 because acct and ref reference the same object. (correct answer)
4. acct becomes null after returning this from depositAndReturn.

Explanation: This question tests AP Computer Science A skills: understanding methods passing and returning object references. In Java, when a method returns 'this', it returns a reference to the same object, meaning multiple variables can reference the same object in memory. In this scenario, acct starts with balance 100.0, depositAndReturn adds 40.0 (making it 140.0) and returns 'this' to ref, then ref.deposit(10.0) adds another 10.0 to the same object, resulting in a final balance of 150.0. Choice C is correct because both acct and ref reference the same BankAccount object in memory - any modifications through either reference affect the same object, so the balance becomes 150.0 after both deposits. Choice B is incorrect because it fails to account for the second deposit through ref, missing that both variables point to the same object. To help students: Use box-and-arrow diagrams to show how multiple references can point to the same object. Trace through the code step-by-step, updating the object's state to show cumulative effects.

Question 3

public class BankAccount {
    private String accountNumber;
    private double balance;

    public BankAccount(String accountNumber, double balance) {
        this.accountNumber = accountNumber;
        this.balance = balance;
    }

    // Adds funds to this account
    public void deposit(double amount) {
        balance += amount;
    }

    // Attempts to reassign the parameter to a new object
    public static void tryReassign(BankAccount target) {
        target = new BankAccount("NEW", 999.0);
        target.deposit(1.0);
    }

    public double getBalance() {
        return balance;
    }

    public static void main(String[] args) {
        BankAccount acct = new BankAccount("A500", 10.0);
        tryReassign(acct);
        System.out.println(acct.getBalance());
    }
}

1. acct balance becomes 1000.0 because target reassigns the original reference.
2. acct balance remains 10.0 because only the parameter target was reassigned. (correct answer)
3. acct becomes null because target is set to a new object.
4. acct balance becomes 11.0 because deposit always affects the caller object.

Explanation: This question tests AP Computer Science A skills: understanding methods passing and returning object references. In Java, reassigning a parameter reference inside a method only affects the local parameter variable, not the original reference passed from the caller. In this scenario, tryReassign receives a copy of the reference to acct, but when it reassigns target to a new BankAccount object, only the local parameter target is changed - the original acct reference remains unchanged and still points to the original object with balance 10.0. Choice B is correct because reassigning the parameter target doesn't affect the caller's reference acct - the reassignment is local to the method, so acct still references the original object with balance 10.0. Choice A is incorrect because it assumes reassigning a parameter can change the caller's reference, which is impossible in Java since references are passed by value. To help students: Emphasize that Java is always pass-by-value, including for references - the value being passed is the reference itself. Use diagrams to show how reassigning a parameter creates a new arrow from the parameter to a different object, leaving the original reference unchanged.

Question 4

public class BankAccount {
    private String accountNumber;
    private double balance;

    public BankAccount(String accountNumber, double balance) {
        this.accountNumber = accountNumber;
        this.balance = balance;
    }

    // Adds funds to this account
    public void deposit(double amount) {
        balance += amount;
    }

    // Deposits then returns this same account reference
    public BankAccount depositAndReturn(double amount) {
        deposit(amount);
        return this;
    }

    public double getBalance() {
        return balance;
    }

    public static void main(String[] args) {
        BankAccount acct = new BankAccount("A600", 5.0);
        BankAccount ref = acct.depositAndReturn(2.5);
        ref.deposit(1.5);
        System.out.println(acct.getBalance());
    }
}

1. 5.0
2. 7.5
3. 9.0 (correct answer)
4. A compilation error occurs due to returning this.

Explanation: This question tests AP Computer Science A skills: understanding methods passing and returning object references. In Java, when a method returns 'this', it enables multiple operations on the same object, and all references to that object see the cumulative changes. In this scenario, acct starts with balance 5.0, depositAndReturn adds 2.5 (making it 7.5) and assigns the returned reference to ref, then ref.deposit(1.5) adds another 1.5 to the same object, resulting in a final balance of 9.0. Choice C is correct because both acct and ref reference the same BankAccount object - the first deposit increases the balance to 7.5, and the second deposit through ref increases it further to 9.0, which is displayed when accessing the balance through acct. Choice B is incorrect because it only accounts for the first deposit, missing that the second deposit also affects the same object since ref and acct point to the same instance. To help students: Trace the execution step-by-step, showing the balance after each operation. Emphasize that returning 'this' doesn't create a new object but provides another way to reference the same object.

Question 5

public class BankAccount {
    private String accountNumber;
    private double balance;

    public BankAccount(String accountNumber, double balance) {
        this.accountNumber = accountNumber;
        this.balance = balance;
    }

    // Moves money from this account into another account
    public BankAccount transferTo(BankAccount other, double amount) {
        this.balance -= amount;
        other.balance += amount;
        return other;
    }

    public double getBalance() {
        return balance;
    }

    public static void main(String[] args) {
        BankAccount a = new BankAccount("A", 50.0);
        BankAccount b = new BankAccount("B", 10.0);
        a.transferTo(b, 15.0);
        System.out.println(a.getBalance() + "," + b.getBalance());
    }
}

1. a is 50.0 and b is 10.0 because objects are passed by value.
2. a is 35.0 and b is 25.0 because both referenced objects were mutated. (correct answer)
3. a is 65.0 and b is -5.0 because amount is added to the sender.
4. a is 35.0 and b is 10.0 because only this.balance can change.

Explanation: This question tests AP Computer Science A skills: understanding methods passing and returning object references. In Java, methods can modify multiple objects when given references to them, and changes to object state through any valid reference are permanent. In this scenario, transferTo is called on object a, which decreases a's balance by 15.0 (from 50.0 to 35.0) and increases b's balance by 15.0 (from 10.0 to 25.0), effectively transferring funds between the two accounts. Choice B is correct because both referenced objects were mutated - the method modifies this.balance (object a) by subtracting the amount and other.balance (object b) by adding the amount, resulting in balances of 35.0 and 25.0 respectively. Choice A is incorrect because it claims objects are passed by value in a way that prevents modification, but Java passes references by value, allowing methods to modify the objects those references point to. To help students: Step through the code showing how 'this' refers to object a and 'other' refers to object b. Use visual representations of bank accounts with arrows showing money movement to make the transfer concept concrete.

Question 6

public class BankAccount {
    private String accountNumber;
    private double balance;

    public BankAccount(String accountNumber, double balance) {
        this.accountNumber = accountNumber;
        this.balance = balance;
    }

    // Moves money from this account into another account
    public BankAccount transferTo(BankAccount other, double amount) {
        this.balance -= amount;
        other.balance += amount;
        return other;
    }

    public double getBalance() {
        return balance;
    }

    public static void main(String[] args) {
        BankAccount a = new BankAccount("A", 80.0);
        BankAccount b = new BankAccount("B", 20.0);
        BankAccount result = a.transferTo(b, 30.0);
        System.out.println(result.getBalance());
    }
}

1. A reference to other, enabling chained operations on the recipient account. (correct answer)
2. A reference to this, because methods always return the caller object.
3. A new BankAccount containing the combined balances.
4. null, since returning an object after field updates is not allowed.

Explanation: This question tests AP Computer Science A skills: understanding methods passing and returning object references. In Java, methods can modify multiple objects through their references and return any reference, enabling flexible object interactions and method chaining. In this scenario, transferTo modifies both the calling object (this) by decreasing its balance and the parameter object (other) by increasing its balance, then returns the reference to other, which now has balance 50.0 (20.0 + 30.0). Choice A is correct because the method returns a reference to the other account after transferring funds to it, enabling continued operations on the recipient account - this design allows for chaining operations on the account that received the transfer. Choice B is incorrect because methods don't automatically return the caller object - the explicit 'return other' statement determines what reference is returned. To help students: Trace through the method showing how both objects are modified and how the return value is determined. Demonstrate practical uses of returning specific object references for method chaining and fluent interfaces.

Question 7

public class BankAccount {
    private String accountNumber;
    private double balance;

    public BankAccount(String accountNumber, double balance) {
        this.accountNumber = accountNumber;
        this.balance = balance;
    }

    // Adds funds to this account
    public void deposit(double amount) {
        balance += amount;
    }

    // Helper method: passes an object reference into a method
    public static void applyDeposit(BankAccount target, double amount) {
        target.deposit(amount);
    }

    public double getBalance() {
        return balance;
    }

    public static void main(String[] args) {
        BankAccount acct = new BankAccount("A400", 60.0);
        applyDeposit(acct, 15.0);
        System.out.println(acct.getBalance());
    }
}

1. It changes target's balance, which updates acct since they reference the same object. (correct answer)
2. It changes only a copied balance value; acct remains 60.0.
3. It reassigns target, making acct reference a different BankAccount.
4. It throws a syntax error because methods cannot accept object references.

Explanation: This question tests AP Computer Science A skills: understanding methods passing and returning object references. In Java, when an object reference is passed as a parameter, the method receives a copy of the reference value, but this copy still points to the same object in memory. In this scenario, applyDeposit receives a reference to the same BankAccount object that acct references, and calling target.deposit(amount) modifies the balance of that shared object from 60.0 to 75.0. Choice A is correct because the method changes the balance field of the object that both target and acct reference - since they point to the same object in memory, the change is visible through the acct reference. Choice B is incorrect because it suggests only a copy is modified, misunderstanding that while the reference is copied, both references point to the same mutable object. To help students: Draw diagrams showing how parameter passing creates a new reference variable that points to the same object. Demonstrate that modifications through any reference to an object are visible through all references to that object.

Question 8

public Rectangle(int w, int h) {
    width = w;
    height = h;
}

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

public Rectangle clone() {
    return new Rectangle(width, height);
}

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

1. rect2 and rect3 have been modified; rect1 retains original dimensions
2. rect1 and rect2 have been modified; rect3 retains original dimensions
3. Only rect2 has been modified; rect1 and rect3 retain original dimensions (correct answer)
4. All rectangles have been modified from their original dimensions

Explanation: When you encounter questions about object modification and method calls, focus on whether methods modify the original object or create new copies, and trace through each code path carefully. Let's analyze what happens to each rectangle through the resize method: For rect1 with factor 1.0: The condition factor == 1.0 is true, so the method simply returns r (the original rectangle) without any modifications. The original rect1 keeps its 10×8 dimensions. For rect2 with factor 2.0: Since 2.0 > 1.0, the method calls r.scale(factor) directly on the original rectangle, then returns it. The scale method modifies rect2's width and height in place, changing them to 20×16. The original rect2 is permanently modified. For rect3 with factor 0.5: Since 0.5 < 1.0, the method creates a copy using r.clone(), then calls scale on that copy. The original rect3 remains unchanged at 10×8, while only the copy gets modified to 5×4. Answer choice A incorrectly claims rect3 was modified, but the scaling happened to its clone. Answer choice B wrongly states rect1 was modified, when it was returned unchanged. Answer choice D incorrectly suggests all rectangles were modified. Only answer choice C correctly identifies that solely rect2 was modified from its original dimensions. Study tip: When tracing object modifications, always distinguish between methods that modify the original object versus those that work on copies. Pay special attention to conditional logic that determines which path executes.

Question 9

public Student(String n, int g) {
    name = n;
    grade = g;
}

public void setGrade(int g) {
    grade = g;
}

public int getGrade() {
    return grade;
}

public String getName() {
    return name;
}

public static void main(String[] args) {
    Student alice = new Student("Alice", 85);
    Student result = updateStudent(alice);
    System.out.println(alice.getName() + ": " + alice.getGrade());
    System.out.println(result.getName() + ": " + result.getGrade());
}

1. Alice: 95 Updated: 100 (correct answer)
2. Alice: 85 Updated: 100
3. Updated: 100 Updated: 100
4. Alice: 95 Alice: 95

Explanation: When alice is passed to updateStudent(), the reference is copied. First, s.setGrade(95) modifies the original alice object through the reference. Then s is reassigned to point to a new Student object, but this doesn't affect the original alice reference. The method returns the new Student object. Output: Alice: 95, Updated: 100.

Question 10

public Box(int v) {
    value = v;
}

public void setValue(int v) {
    value = v;
}

public int getValue() {
    return value;
}

1. result1 and box1 reference the same object, while result2 and box2 reference different objects
2. result1 and box1 reference different objects, while result2 and box2 reference the same object (correct answer)
3. Both result1 and box1, and result2 and box2 reference the same objects respectively
4. Both result1 and box1, and result2 and box2 reference different objects respectively

Explanation: For box1 (value 30): Since 30 ≤ 50, processBox creates and returns a new Box object with value 55. For box2 (value 60): Since 60 > 50, processBox modifies the original object (setting value to 120) and returns the same reference. Therefore, result1 and box1 are different objects, while result2 and box2 are the same object.

Question 11

public Counter(int c) {
    count = c;
}

public void increment() {
    count++;
}

public int getCount() {
    return count;
}

1. original: 6, result: 12 (correct answer)
2. original: 5, result: 10
3. original: 12, result: 12
4. original: 6, result: 6

Explanation: In doubleCounter(), the parameter c initially refers to the same object as original. c.increment() modifies the original object (count becomes 6). A new Counter is created with value 12 (6 * 2). The local variable c is reassigned to this new object and returned. The original object retains its modified value of 6, while result references the new object with value 12.

Question 12

public Account(String owner, double balance) {
    this.owner = owner;
    this.balance = balance;
}

public void deposit(double amount) {
    balance += amount;
}

public double getBalance() {
    return balance;
}

public String getOwner() {
    return owner;
}

1. acc1 balance becomes -50.0, result references the same object as acc1 with balance -50.0
2. acc1 balance becomes 0.0, result references the same object as acc1 with balance 0.0
3. acc1 balance remains 100.0, result references a new Account object with balance 0.0
4. acc1 balance becomes -50.0, result references a new Account object with balance 0.0 (correct answer)

Explanation: When you encounter questions about object references and method calls that modify objects, you need to carefully trace how the original objects change and what new objects might be created. Let's trace through the transfer method step by step. Starting with acc1 (Alice, $100) and acc2 (Bob, $50), we're transferring $150 from acc1 to acc2. First, from.deposit(-amount) calls acc1.deposit(-150), which subtracts 150 from acc1's balance: 100 + (-150) = -50. So acc1's balance becomes -50.0. Next, to.deposit(amount) adds 150 to acc2's balance: 50 + 150 = 200. The crucial part is the conditional check: since from.getBalance() returns -50.0, which is less than 0, the method executes return new Account(from.getOwner(), 0). This creates a completely new Account object with Alice's name and a 0 balance. Answer choice A incorrectly assumes the method returns the original acc1 object when the balance goes negative. Answer choice B wrongly suggests that acc1's balance itself gets reset to 0, but the original object remains at -50.0. Answer choice C mistakenly claims acc1's balance doesn't change, but the deposit method definitely modifies the original object's balance. The correct answer is D: acc1's balance becomes -50.0 (the original object is modified), while result references a new Account object with balance 0.0. Key strategy: Always distinguish between modifying an existing object's state versus creating and returning a new object. The original objects can still change even when methods return new objects.

Question 13

public Node(int data) {
    this.data = data;
    this.next = null;
}

public void setNext(Node next) {
    this.next = next;
}

public Node getNext() {
    return next;
}

public int getData() {
    return data;
}

public void setData(int data) {
    this.data = data;
}

1. node1: 8, node2: 3, node3: 7, node4: 2
2. node1: 8, node2: 10, node3: 9, node4: 2
3. node1: 5, node2: 8, node3: 7, node4: 9
4. node1: 8, node2: 3, node3: 9, node4: 2 (correct answer)

Explanation: When you encounter linked list traversal problems, focus on carefully tracking how the loop variable advances and which nodes get modified. This question tests your ability to trace through a while loop that processes every other node. Let's trace through processNodes step by step. The method starts with current pointing to node1(5). The while loop continues as long as current isn't null AND current.getNext() isn't null. First iteration: current points to node1(5). Since node1 exists and has a next node (node2), we enter the loop. We update node1's data: 5 + 3 = 8. Then current advances by TWO positions to node3(7). Second iteration: current points to node3(7). Since node3 exists and has a next node (node4), we continue. We update node3's data: 7 + 2 = 9. Then current advances by TWO positions to null (beyond node4). Loop ends: current is null, so the condition fails. The key insight is that current = current.getNext().getNext() skips every other node, so only nodes at odd positions (1st, 3rd, etc.) get their data modified. Choice A incorrectly assumes only the first node changes. Choice B mistakenly thinks node2 also gets modified and that changes cascade through the list. Choice C wrongly believes node2 and node4 get updated instead of node1 and node3. Study tip: When tracing linked list algorithms, draw out the nodes and carefully track which pointer moves where after each iteration. Pay special attention to how many positions the loop variable advances—it's often more than one!

Question 14

public Point(int x, int y) {
    this.x = x;
    this.y = y;
}

public void move(int dx, int dy) {
    x += dx;
    y += dy;
}

public Point copy() {
    return new Point(x, y);
}

public int getX() { return x; }
public int getY() { return y; }

1. p1 coordinates: (15, 25), p2 coordinates: (15, 25), result1 and p1 are the same object
2. p1 coordinates: (10, 20), p2 coordinates: (15, 25), result1 and p1 are different objects (correct answer)
3. p1 coordinates: (15, 25), p2 coordinates: (10, 20), result2 and p2 are different objects
4. p1 coordinates: (10, 20), p2 coordinates: (10, 20), both results reference new objects

Explanation: For p1 with shouldCopy=true: A copy is made and moved to (15, 25), original p1 remains (10, 20), result1 references the copy. For p2 with shouldCopy=false: The original p2 is moved to (15, 25) and returned, so result2 and p2 reference the same object. Final state: p1(10, 20), p2(15, 25), result1 and p1 are different objects.

Question 15

public Card(String suit, int value) {
    this.suit = suit;
    this.value = value;
}

public void setValue(int v) {
    value = v;
}

public int getValue() {
    return value;
}

public String getSuit() {
    return suit;
}

1. card1 value: 1, card2 value: 1, result1 and card1 are the same object
2. card1 value: 11, card2 value: 11, result1 and card1 are different objects
3. card1 value: 11, card2 value: 1, result1 and card1 are different objects (correct answer)
4. card1 value: 11, card2 value: 1, result1 and card1 are the same object

Explanation: When you encounter questions about object references and method calls, you need to carefully trace through the code execution while tracking both the values of object fields and whether methods return the same object or create new ones. Let's trace through each method call step by step. For CardGame.playCard(card1, true) where card1 starts with value 10: First, the method increments card1's value to 11. Since isSpecialRound is true and the value (11) is greater than 10, the method creates and returns a new Card object with suit "Hearts" and value 1. The original card1 still exists with its modified value of 11. For CardGame.playCard(card2, false) where card2 starts with value 10: The method increments card2's value to 11. Since isSpecialRound is false, we skip the first if-statement. The second condition checks if the value equals 11, which it does, so the method calls setValue(1) on card2 itself, changing its value to 1. The method then returns card2 (the same object). Therefore, card1 has value 11, card2 has value 1, and result1 is a different object from card1. Choice A incorrectly states card1's value is 1 and that result1 and card1 are the same object. Choice B incorrectly claims card2's value is 11 and that result1 and card1 are different objects (the relationship is wrong). Choice D incorrectly states that result1 and card1 are the same object. Remember: when a method returns new ClassName(), it creates a different object; when it returns the parameter directly, it's the same object reference.