Objects: Instances of Classes: Practice Questions (Computer Science A 1.12)

These are original Marlbridge practice questions, not past exam questions. All code is invented for these questions and was compiled and run before publication. The rubrics are a suggested Marlbridge rubric to help you check your work; they are not an official scoring guideline. No calculator is allowed on the exam, so work everything by hand. Code fragments are assumed to be inside a main method. The questions use the class Counter below; an object of this class stores a count that starts at 0, increment() adds 1 to it and getCount() returns it.

class Counter {
    private int count;

    public Counter() {
        count = 0;
    }

    public void increment() {
        count++;
    }

    public int getCount() {
        return count;
    }
}

Question 1 (multiple choice · foundation)

Which statement correctly describes the relationship between a class and an object?

(A) An object is the blueprint, and a class is one thing built from it
(B) A class is the blueprint, and an object is a specific instance of it
(C) A class and an object are two names for the same thing
(D) A class is stored in a variable and an object is stored in a method

Answer and explanation

Answer: (B). A class is the formal implementation, or blueprint, of attributes and behaviors. An object is a specific instance of the class with defined attributes.

(A) reverses the two ideas.
(C) ignores that many objects can come from one class.
(D) mixes up variables and methods with classes and objects.

Question 2 (multiple choice · foundation)

What does the variable s hold after the following statement?

String s = "hello";

(A) A reference to a String object
(B) The five characters h, e, l, l, o stored directly in the variable
(C) The int 5
(D) A copy of the String class

Answer and explanation

Answer: (A). String is a class, so s is a reference variable. It holds an object reference, which can be thought of as the memory address of the object.

(B) describes a primitive variable, which holds its value directly.
(C) confuses the object with its length.
(D) confuses the class with an object of the class.

Question 3 (multiple choice · core)

Which statement about Counter c; is true?

(A) It creates a Counter object whose count is 0
(B) It declares a variable that holds the int 0
(C) It declares a variable that can hold a reference to a Counter, but creates no object
(D) It does not compile, because an object cannot be declared without new

Answer and explanation

Answer: (C). Declaring a reference variable creates the variable only. An object is created when you use new with a constructor (Topic 1.13).

(A) assumes that declaring creates an object.
(B) confuses a reference variable with a primitive one.
(D) is wrong: the declaration compiles (the variable simply cannot be used until it has been assigned a value).

Question 4 (multiple choice · core)

What is printed by the following code segment?

Counter p = new Counter();
Counter q = p;
q.increment();
q.increment();
System.out.println(p.getCount());

(A) 0
(B) 1
(C) The code does not compile
(D) 2

Answer and explanation

Answer: (D). q = p; copies the reference, so p and q refer to the same object. Both increments act on that single object, whose count becomes 2, and p.getCount() reports 2.

(A) assumes that q is a separate object, so p is unaffected.
(B) counts only one of the two increments.
(C) is wrong: the code compiles.

Question 5 (multiple choice · core)

Which statement about the Object class is true?

(A) Only classes whose names begin with Object extend it
(B) All classes in Java are subclasses of the Object class
(C) A programmer must write extends Object in every class
(D) The Object class is a subclass of String

Answer and explanation

Answer: (B). Every class is a subclass of Object, directly or through other classes. As a result, every object has Object’s methods, such as equals and toString.

(A) is wrong: the name of the class has nothing to do with it.
(C) is wrong: the relationship exists automatically.
(D) reverses the hierarchy: String is a subclass of Object.

Question 6 (multiple choice · core)

What is printed by the following code segment?

int x = 3;
int y = x;
x = x + 10;
Counter a = new Counter();
Counter b = a;
b.increment();
System.out.println(x + " " + y + " " + a.getCount());

(A) 13 13 1
(B) 13 3 0
(C) 3 3 1
(D) 13 3 1

Answer and explanation

Answer: (D). y received a copy of 3, so changing x to 13 leaves y at 3. a and b refer to the same Counter, so b.increment() raises the count seen through a to 1.

(A) treats the primitive copy as if y followed x.
(B) treats the reference copy as if a were a separate object from b.
(C) ignores the update to x.

Question 7 (classifying · core)

A school program has a class Student. Classify each of the following as a class, an object, an attribute or a behavior, and give one-line reasons.

(a) Student (b) the student called Amira, as stored by the program (c) a student’s date of birth (d) calculating a student’s age (e) the student called Tariq, as stored by the program

Worked solution

(a) Class: the blueprint that describes every student.
(b) Object: one specific instance of Student, with its own data.
(c) Attribute: data related to the class, stored in a variable.
(d) Behavior: something an instance can do, defined by a method.
(e) Object: a different instance of the same class, with different attribute values but the same behaviors.

Suggested Marlbridge rubric (5 marks): one mark each for (a) to (e): the correct classification and a reason in terms of blueprint, instance, data or method.

Question 8 (tracing references · core)

Trace the following code segment.

Counter x = new Counter();
Counter y = new Counter();
Counter z = y;
x.increment();
z.increment();
z.increment();
System.out.println(x.getCount() + " " + y.getCount() + " " + z.getCount());

(a) How many Counter objects are created?

(b) State the output and explain why y and z give the same value.

Worked solution

(a) Two. new Counter() is executed twice. The statement Counter z = y; copies a reference and creates no object.

(b) The output is 1 2 2. x refers to the first object, which was incremented once. y and z refer to the same second object, which was incremented twice (both times through z). Whichever name is used to ask, the answer is that object’s count.

Suggested Marlbridge rubric (4 marks):

Mark	Awarded for
1	Two objects created
1	`Counter z = y;` identified as copying a reference
1	Output `1 2 2`
1	Explanation that `y` and `z` are two names for one object

Question 9 (primitive and reference copying · stretch)

Two code segments are shown.

int m = 7;
int n = m;
m++;
System.out.println(m + " " + n);

Counter r = new Counter();
Counter s = r;
r.increment();
System.out.println(r.getCount() + " " + s.getCount());

(a) State the output of each.

(b) Explain, in terms of what each variable holds, why the first segment leaves the two values different while the second leaves them the same.

Worked solution

(a) The first prints 8 7. The second prints 1 1.

(b) A primitive variable holds its value. int n = m; copies the value 7 into n, so n has its own 7, and m++ changes only m. A reference variable holds an object reference. Counter s = r; copies the reference, so r and s refer to the same object; incrementing through r changes that object, and s sees the change.

Suggested Marlbridge rubric (4 marks):

Mark	Awarded for
1	First output `8 7`
1	Second output `1 1`
1	Primitive explanation: the value is copied, so the variables are independent
1	Reference explanation: the reference is copied, so both refer to one object

How did you do?

Questions 1 to 3 and 7: re-read “Blueprints and the things built from them” in the study guide.
Questions 4, 6, 8 and 9: draw the diagram: variables on the left, objects on the right, an arrow for each reference.
Question 5: re-read “Class hierarchies and the Object class”.

Then try the checklist.