Calling Class Methods: Practice Questions (Computer Science A 1.10)

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. Unless stated, the class Tools below is the class used in the questions.

public class Tools {
    public static int twice(int n) {
        return n * 2;
    }

    public static int plus(int a, int b) {
        return a + b;
    }

    public static void show(String label, int value) {
        System.out.println(label + ": " + value);
    }
}

Question 1 (multiple choice · foundation)

Which keyword in a method header shows that the method is a class method?

• (A) static
• (B) public
• (C) void
• (D) class

Question 2 (multiple choice · foundation)

A class Calc defines public static int cube(int n). Which statement correctly calls it from a different class, with argument 4?

• (A) cube(4);
• (B) Calc.cube(4);
• (C) Calc(4).cube;
• (D) new Calc().cube(4);

Question 3 (multiple choice · core)

What is printed by the following statement?

System.out.println(Tools.plus(Tools.twice(3), Tools.twice(4)));

• (A) 14
• (B) 7
• (C) 10
• (D) 24

Question 4 (multiple choice · core)

Which statement in a different class would cause a compile-time error?

• (A) int r = Tools.twice(7);
• (B) Tools.show("n", 7);
• (C) int r = Tools.twice(2.5);
• (D) int r = Tools.plus(1, 2) + Tools.twice(3);

Question 5 (multiple choice · core)

The class Tools is extended with a method demo. The call to twice appears inside Tools, in a method that is also in Tools.

public class Tools {
    public static int twice(int n) {
        return n * 2;
    }

    public static int demo() {
        int r = twice(6);
        return r;
    }
}

Which statement is true?

• (A) It does not compile, because the class name is missing
• (B) It compiles, because the class name is optional inside the defining class
• (C) It compiles only if twice is declared before the method containing the call
• (D) It compiles only if an object of Tools has been created

Question 6 (multiple choice · core)

What is printed by the following statement?

System.out.println(Math.sqrt(Math.abs(-16)) + Math.abs(-3));

• (A) 7
• (B) 19.0
• (C) 13.0
• (D) 7.0

Question 7 (writing and calling · core)

(a) Write a class Geometry with a class method perimeter that takes two int parameters, width and height, and returns the perimeter of a rectangle as an int.

(b) Write a main method in a class Main that calls it for a 7 by 4 rectangle and prints the result. State the output.

class Geometry {
    public static int perimeter(int width, int height) {
        return 2 * (width + height);
    }
}

public class Main {
    public static void main(String[] args) {
        System.out.println(Geometry.perimeter(7, 4));
    }
}

22

Question 8 (finding errors · core)

Each line below is in a class other than Tools. Say whether it compiles. If it does not, give the reason and a corrected line.

(a) int r = plus(2, 3); (b) Tools.show(7, "total"); (c) String s = Tools.twice(4); (d) int r = Tools.plus(Tools.twice(2), 3);

Question 9 (tracing · stretch)

State exactly what the following program prints.

public class Q9 {
    public static int step(int n) {
        System.out.println("step " + n);
        return n + 1;
    }

    public static void main(String[] args) {
        int a = step(1);
        int b = step(a) * step(a + 1);
        System.out.println(a + " " + b);
    }
}

step 1
step 2
step 3
2 12

How did you do?

• Questions 1 and 2: re-read “Methods that belong to the class” in the study guide.
• Questions 3, 6 and 9: work nested calls from the inside out, one line per call.
• Questions 4, 5 and 8: use the four checks in “Why a call does not compile”.
• Question 7: write another class with two methods and call them from main.

Then try the checklist.