Calling Instance Methods: Practice Questions (Computer Science A 1.14)

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. Questions use this class: area() returns width times height, perimeter() returns twice the sum of width and height, grow(int amount) adds amount to both width and height, and describe() returns text such as "3 x 4".

class Rectangle {
    private int width;
    private int height;

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

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

    public int perimeter() {
        return 2 * (width + height);
    }

    public void grow(int amount) {
        width = width + amount;
        height = height + amount;
    }

    public String describe() {
        return width + " x " + height;
    }
}

Question 1 (multiple choice · foundation)

r is a Rectangle variable that refers to an object. Which statement calls the instance method area on it?

(A) Rectangle.area();
(B) area(r);
(C) r(area);
(D) r.area();

Answer and explanation

Answer: (D). An instance method is called using the object name, the dot operator and the method name.

(A) uses the class name, which works only for class methods.
(B) is the form for a method that takes the object as an argument, and area takes no parameters.
(C) is not valid call syntax.

Question 2 (multiple choice · foundation)

What is printed by the following code segment?

Rectangle a = new Rectangle(2, 5);
Rectangle b = new Rectangle(3, 3);
System.out.println(a.area() + b.area());

(A) 10
(B) 19
(C) 9
(D) 109

Answer and explanation

Answer: (B). a.area() is 2 * 5 = 10 and b.area() is 3 * 3 = 9. Both calls return int values, which are added: 19.

(A) uses only the first object.
(C) uses only the second object.
(D) joins the digits as text; both values are int, so they are added.

Question 3 (multiple choice · core)

What is printed by the following code segment?

Rectangle r = new Rectangle(1, 2);
r.grow(2);
System.out.println(r.describe());

(A) 1 x 2
(B) 2 x 4
(C) 3 x 4
(D) 3 x 2

Answer and explanation

Answer: (C). grow(2) adds 2 to both sides: width 1 becomes 3 and height 2 becomes 4. describe() returns "3 x 4".

(A) ignores the grow call.
(B) doubles the sides instead of adding 2.
(D) adds 2 to the width only.

Question 4 (multiple choice · core)

Which statement would cause a compile-time error?

(A) int c = r.grow(3);
(B) int a = r.area();
(C) r.grow(3);
(D) String s = r.describe();

Answer and explanation

Answer: (A). grow is a void method, so there is no value to store in c.

(B) stores the returned int.
(C) calls a void method as a statement, which is correct.
(D) stores the returned String.

Question 5 (multiple choice · core)

What is printed by the following code segment?

Rectangle r = new Rectangle(1, 1);
Rectangle s = r;
s.grow(4);
System.out.println(r.area());

(A) 1
(B) 4
(C) 16
(D) 25

Answer and explanation

Answer: (D). s = r; copies the reference, so r and s refer to the same object. s.grow(4) changes that object to 5 by 5, and r.area() returns 25.

(A) assumes that r is a separate object, unaffected by s.
(B) takes the amount of growth as the area.
(C) squares 4, ignoring the starting size of 1.

Question 6 (multiple choice · core)

What happens when the following code segment is compiled and run?

Rectangle t = null;
System.out.println(t.area());

(A) It prints 0
(B) It does not compile
(C) It compiles and then throws a NullPointerException when run
(D) It compiles and throws an ArithmeticException when run

Answer and explanation

Answer: (C). The compiler checks only that Rectangle has an area() method; it does not know that t will refer to no object. At run time, an instance method call on a null reference results in a NullPointerException.

(A) invents an object with a default area.
(B) is wrong: the code is valid Java.
(D) is the exception for integer division by zero, which does not apply here.

Question 7 (multiple choice · stretch)

What is printed by the following code segment?

String word = "programming";
System.out.println(word.length() + word.indexOf("g"));

(A) 14
(B) 113
(C) 3
(D) 11

Answer and explanation

Answer: (A). The word has 11 characters, and the first "g" is at index 3 (p0 r1 o2 g3). Both calls return int values, so they are added: 11 + 3 = 14.

(B) joins the digits as if the values were text.
(C) and (D) use only one of the two calls.

Question 8 (tracing · core)

Trace the following code segment and give the output.

Rectangle p = new Rectangle(2, 3);
Rectangle q = new Rectangle(4, 4);
p.grow(1);
q.grow(2);
int total = p.area() + q.perimeter();
System.out.println(p.describe() + " " + q.describe() + " " + total);

Worked solution

After statement	p	q	total
`Rectangle p = new Rectangle(2, 3);`	2 x 3	-	-
`Rectangle q = new Rectangle(4, 4);`	2 x 3	4 x 4	-
`p.grow(1);`	3 x 4	4 x 4	-
`q.grow(2);`	3 x 4	6 x 6	-
`int total = p.area() + q.perimeter();`	3 x 4	6 x 6	36

p.area() is 12 and q.perimeter() is 2 * (6 + 6) = 24, so total is 36. The output is 3 x 4 6 x 6 36.

Suggested Marlbridge rubric (4 marks):

Mark	Awarded for
1	`p` becomes 3 x 4
1	`q` becomes 6 x 6
1	`total` is 36 (12 + 24)
1	Output line `3 x 4 6 x 6 36`

Question 9 (writing calls · core)

Write code (inside a main method) that:

(a) creates a Rectangle plot that is 6 by 4;

(b) makes it 3 larger on each side using one method call;

(d) prints plot.describe() and fence. State the output.

Worked solution

Rectangle plot = new Rectangle(6, 4);
plot.grow(3);
int fence = plot.perimeter();
System.out.println(plot.describe() + " " + fence);

After grow(3) the rectangle is 9 by 7, so describe() returns "9 x 7", and the perimeter is 2 * (9 + 7) = 32. The output is 9 x 7 32.

Suggested Marlbridge rubric (5 marks):

Mark	Awarded for
1	Creates the object with `new Rectangle(6, 4)`
1	Calls `grow(3)` on the object as a statement
1	Stores `plot.perimeter()` in an `int` variable
1	Prints both values
1	Output `9 x 7 32`

Question 10 (null and a fix · stretch)

The following code segment compiles but stops with an exception.

Rectangle shed;
shed = null;
int a = shed.area();
System.out.println(a);

(a) Name the exception and say when it occurs.

(b) Explain why the compiler does not report this.

Worked solution

(a) A NullPointerException, at run time, when shed.area() is executed.

(b) The compiler only checks that the type Rectangle has a method area(). It does not track which object, or no object, a variable refers to at run time.

(c)

Rectangle shed = new Rectangle(3, 4);
int a = shed.area();
System.out.println(a);

The program prints 12.

Suggested Marlbridge rubric (4 marks):

Mark	Awarded for
1	Names `NullPointerException` and says it occurs at run time
1	Says the variable refers to no object when the method is called
1	Explains that the compiler checks the method exists, not the object
1	A corrected version that creates the object before the call

How did you do?

Questions 1 and 4: re-read “Asking an object to do something” in the study guide.
Questions 2, 3, 5 and 8: keep a table of what each object holds after each statement.
Questions 6 and 10: revisit “Calling a method on null”.
Questions 7 and 9: write the call, then check the return type.

Then try the checklist.