Scope and Access: Practice Questions (Computer Science A 3.8)

These are original Marlbridge practice questions, not past exam questions. All scenarios are fictional. Mark points are a suggested Marlbridge rubric, not an official scoring guideline. No calculator is allowed. The classes Pet, Scorer and Calc are the ones printed in the study guide. All code was compiled and run.

Question 1 (multiple choice · foundation)

A variable is declared inside the braces of an if statement. Where can it be used?

(A) Anywhere in the class
(B) Anywhere in the method that contains the if
(C) Only inside the block of that if statement, after its declaration
(D) Only in other classes

Answer and explanation

Answer: (C). A local variable can be accessed only in the block in which it is declared.

(A) describes instance variables.
(B) would apply to a variable declared at the top of the method, not inside the block.
(D) is not how scope works.

Question 2 (multiple choice · foundation)

Which statement about the parameters of a method is true?

(A) They are local variables that can be used only within the method.
(B) They are instance variables shared by all methods.
(C) They may be declared private to hide them from other classes.
(D) They keep their values after the method returns.

Answer and explanation

Answer: (A). Parameters are local variables of the method or constructor.

(B) is wrong: they are not shared.
(C) is wrong: local variables cannot be public or private.
(D) is wrong: they disappear when the method ends.

Question 3 (multiple choice · core)

public class Pet
{
    private String name;
    private int age;

    public Pet(String name, int age)
    {
        name = name;
        age = age;
    }

    public String describe()
    {
        return name + " " + age;
    }
}

What is printed by System.out.println(new Pet("Rex", 4).describe());?

(A) Rex 4
(B) null 0
(C) Rex 0
(D) The code does not compile.

Answer and explanation

Answer: (B). The parameters hide the instance variables, so each assignment copies a parameter to itself. The instance variables keep their defaults, null and 0. The compiled program printed null 0.

(A) is what the programmer intended, not what happens.
(C) wrongly sets one of the two variables.
(D) is false: the code compiles without a warning.

Question 4 (multiple choice · core)

Scorer has an instance variable private int total = 0;, a method add(int points) that does total += points;, a method getTotal() that returns total, and the method below.

public int bonus(int total)
{
    total = total + 5;
    return total;
}

What is printed?

Scorer s = new Scorer();
s.add(7);
System.out.println(s.bonus(20) + " " + s.getTotal());

(A) 25 7
(B) 25 32
(C) 12 12
(D) 5 7

Answer and explanation

Answer: (A). In bonus, the parameter total hides the instance variable. The method returns 20 + 5 = 25 and the instance variable stays 7. The compiled program printed 25 7.

(B) assumes bonus added 25 to the instance variable (7 + 25 = 32).
(C) mixes up the arguments and instance variable.
(D) forgets to use the argument 20.

Question 5 (multiple choice · core)

public int sumTo(int n)
{
    int total = 0;
    for (int i = 1; i <= n; i++)
    {
        total += i;
    }
    return i;
}

What happens when this code is compiled?

(A) It compiles and returns n.
(B) It compiles and returns the sum.
(C) It does not compile, because i is not in scope at the return statement.
(D) It does not compile, because total is declared before the loop.

Answer and explanation

Answer: (C). i is declared in the for header, so it exists only in the loop. The compiler reports cannot find symbol. (The intended return total; would work.)

(A) and (B) imagine that i is still available.
(D) is wrong: declaring total before the loop is correct.

Question 6 (multiple choice · core)

Scorer also has a method

public int localOnly()
{
    int total = 100;
    total++;
    return total;
}

After Scorer s = new Scorer(); s.add(7); what is printed by System.out.println(s.localOnly() + " " + s.getTotal());?

(A) 101 101
(B) 101 7
(C) 8 8
(D) 100 7

Answer and explanation

Answer: (B). The local total hides the instance variable. The local is incremented to 101 and returned; the instance variable stays 7. The compiled program printed 101 7.

(A) assumes the local variable is the instance variable.
(C) increments the instance variable instead.
(D) forgets the total++.

Question 7 (constructed response · core)

public class Calc
{
    private int x = 2;

    public int calc(int y)
    {
        int z = x + y;
        if (z > 5)
        {
            int w = z * 2;
            z = w;
        }
        return z;
    }
}

(a) For each of x, y, z and w, state where in the class it can be used.

(b) State the values returned by calc(1) and calc(4), showing the values of z.

Answer and explanation

(a) x (instance variable): anywhere in the class. y (parameter): only inside calc. z (local): inside calc from its declaration to the end of the method. w (local): only inside the if block, from its declaration.

(b) calc(1): z = 2 + 1 = 3; 3 > 5 is false, so the if is skipped; returns 3. calc(4): z = 2 + 4 = 6; 6 > 5 is true; w = 12; z = 12; returns 12. Compiled and run: 3 12.

(c) w is declared inside the if block and does not exist after its closing brace, so the compiler reports cannot find symbol.

Suggested Marlbridge rubric (6 marks).

Mark	Requirement
1	Part (a): `x` in scope in the whole class
1	Part (a): `y` and `z` limited to the method
1	Part (a): `w` limited to the `if` block
1	Part (b): `calc(1)` is 3 with `z` shown
1	Part (b): `calc(4)` is 12 with `w` and `z` shown
1	Part (c): `w` is out of scope after the block

Question 8 (constructed response · core)

A programmer’s constructor produces objects whose name is always null.

public class Pet
{
    private String name;
    private int age;

    public Pet(String name, int age)
    {
        name = name;
        age = age;
    }
}

(a) Explain exactly why the instance variables are not set.

(b) Rewrite the constructor so that it works, without using any keyword you have not met.

Answer and explanation

(a) In the constructor body, name and age refer to the parameters, which hide the instance variables of the same names. Each statement assigns a parameter to itself, so the instance variables keep their defaults.

(b)

public Pet(String petName, int petAge)
{
    name = petName;
    age = petAge;
}

(c) Before: name is null and age is 0. After: name is “Rex” and age is 4. (The unfixed output null 0 and the fixed output Rex 4 were confirmed by running both versions.)

Suggested Marlbridge rubric (4 marks).

Mark	Requirement
1	Part (a): parameters hide the instance variables
1	Part (a): each statement assigns a parameter to itself
1	Part (b): different parameter names, with the assignments written correctly
1	Part (c): correct before and after values

Question 9 (constructed response · stretch)

A programmer writes localOnly() as shown in Question 6 and complains that “getTotal() still returns 7”.

(a) Explain why.

(b) Rewrite localOnly so that it adds 94 to the instance variable and returns the new instance value.

Answer and explanation

(a) int total = 100; declares a new local variable that hides the instance variable. All of total++ and return total; use the local, so the instance variable is never changed.

(b)

public int localOnly()
{
    total += 94;
    return total;
}

With total at 7, this makes it 101 and returns 101. (There is no declaration of a second total.)

(c) It ceases to exist: a local variable lasts only until the end of its block.

Suggested Marlbridge rubric (4 marks).

Mark	Requirement
1	Part (a): a local variable hides the instance variable
1	Part (a): the instance variable is never modified
1	Part (b): no redeclaration; updates the instance variable and returns it
1	Part (c): the local ceases to exist when its block ends

How did you do?

Questions 1 to 6 right and the rubrics mostly met: move on to the this Keyword study guide.
Missed Questions 1, 2 or 5: reread “Local variables” in the study guide.
Missed Questions 3, 4 or 6: reread “When two variables share a name” and Worked example 1.
Missed the long questions: redo Worked example 2, then tick the topic checklist.