Question

Which of the following is true about interfaces:

An interface can have only non abstract methods.

All methods in an interface must be abstract.

A class can only implement one interface.

None of the items listed.

Can not contain constants but can have variables.

What is the rule for a super reference in a constructor?

It must be in the parent class' constructor.

You cannot use super in a constructor.

It must be the last line of the constructor in the child class.

Only one child class can use it.

It must be the first line of the constructor in the child class.

Consider the following class definition.

public class WhatsIt {
    private int length;
    private int width;

    public int getArea () {
        // implementation not shown
    }

    private int getPerimeter () {
        // implementation not shown
    }
}

A child class Thingy that extends WhatsIt would have access to:

width, length, getPerimeter()

getPerimeter()

All of the items listed.

getArea()

width, length, getArea()

Questions 18 - 20 pertain to the following class, Point:

public class Point {
    private double x; 
    private double y;

    public Point() {
         this (0, 0); 
    }

    public Point(double a, double b) {
        /* missing code */
    }
    // ... other methods not shown
}

Which of the following correctly implements the equals method?

public boolean equals(Point p) {
    return (x == p.x && y == p.y ); 
}

public void equals(Point p) {
    System.out.println(x == p.x && y == p.y);
}

public void  equals(Point p) {
    return (x == p.x && y == p.y );
}

public boolean equals(Point p) {
    return (x == Point.x && y == Point.y);
}

public boolean equals(Point p) {
    System.out.println(x == p.x && y == p.y);
}

The default constructor sets x and y to (0, 0) by calling the second constructor. What could be used to replace /* missing code */ so that this works as intended?

a = 0;

b = 0;

this (x, y);

this(0, 0);

x = a;

y = b;

a = x;

b = y;

Which of the following correctly implements a mutator method for Point?

public double getX() {
    return x;
}

public void setCoordinates (double a, double b) {
    x = a;
    y = b;
}

None of the items listed.

public double getX() {
    return a;
}

public void setCoordinates (double a, double b) {
    Point p = new Point(a,b);
}

Answer 1

Q1. The following is true about interfaces:

All methods in an interface must be abstract

This is because the interface will have the abstract methods for applying the multiple inheritances, which means that one class can apply multiple interfaces.

Q2. The rule for a super reference in a constructor is that:

It must be the first line of the constructor in the child class

The super() is called explicitly in the child class constructor, thus enabling the parent constructor to be called from the subclass i.e. child class. The first line will have the parent constructor which will make sure that the parent constructor is called before the child constructor

Q3.
The child class Thingy that extends WhatsIt would have access to:

getArea ()

this is because the width, length, getPerimeter() are private and will just be available inside the WhatsIt class. The getArea is public and thus available for use in child class.

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