AP Computer Science A Unit 5: Writing Classes

Which of the following best explains the purpose of the this keyword in the following constructor?

public class Point {
    private double x;
    private double y;
    
    public Point(double x, double y) {
        this.x = x;
        this.y = y;
    }
}

What is the output of the following code?

public class MathHelper {
    private int x;
    
    public MathHelper(int x) {
        this.x = x;
    }
    
    public int triple() {
        return x * 3;
    }
    
    public static int square(int n) {
        return n * n;
    }
}

// In main:
MathHelper m = new MathHelper(4);
System.out.println(m.triple());
System.out.println(MathHelper.square(5));

Which of the following correctly defines a constructor for a Dog class?

public class Dog {
    private String name;
    // Which option is a valid constructor?
}

What is the output of the following code?

public class Student {
    private String name;
    private int grade;
    
    public Student(String name, int grade) {
        this.name = name;
        this.grade = grade;
    }
    
    public void setGrade(int grade) {
        if (grade >= 0 && grade <= 100) {
            this.grade = grade;
        }
    }
    
    public int getGrade() { return grade; }
}

// In main:
Student s = new Student("Ali", 85);
s.setGrade(110);
System.out.println(s.getGrade());

Given the following class, which statement correctly accesses the name variable from OUTSIDE the class?

public class Person {
    private String name;
    
    public Person(String n) {
        name = n;
    }
    
    public String getName() {
        return name;
    }
}

Given the following class definition, which of the following correctly modifies the balance from a test class BankTester?

public class BankAccount {
    private double balance;
    
    public BankAccount(double initial) {
        balance = initial;
    }
    
    public void deposit(double amount) {
        if (amount > 0) balance += amount;
    }
    
    public double getBalance() { return balance; }
}

What is printed by the following code?

public class Temp {
    private double celsius;
    
    public Temp(double c) { celsius = c; }
    
    public double toFahrenheit() {
        return celsius * 9 / 5 + 32;
    }
    
    public String toString() {
        return celsius + "C";
    }
}

// In main:
Temp t = new Temp(100);
System.out.println(t);
System.out.println(t.toFahrenheit());

What is the output of the following code?

public class Box {
    private int value;
    
    public Box(int v) {
        value = v;
    }
    
    public void doubleValue() {
        value = value * 2;
    }
    
    public int getValue() { return value; }
}

// In main:
Box b1 = new Box(5);
Box b2 = b1;
b2.doubleValue();
System.out.println(b1.getValue());

What is the output of the following code?

public class Counter {
    private int count;
    
    public Counter(int start) {
        count = start;
    }
    
    public Counter add(int n) {
        count += n;
        return this;
    }
    
    public int getCount() { return count; }
}

// In main:
Counter c = new Counter(0);
System.out.println(c.add(3).add(4).getCount());

What is the output of the following code?

public class Counter {
    private int count = 0;
    
    public void increment() {
        count++;
    }
    
    public int getCount() {
        return count;
    }
}

// In main:
Counter c = new Counter();
c.increment();
c.increment();
c.increment();
System.out.println(c.getCount());

What is the output of the following code?

public class Thing {
    private int x;
    
    public Thing() {
        x = 10;
    }
    
    public Thing(int x) {
        this.x = x;
    }
    
    public int getX() { return x; }
}

// In main:
Thing a = new Thing();
Thing b = new Thing(7);
System.out.println(a.getX() + b.getX());

What is the output of the following code?

public class Accumulator {
    private static int total = 0;
    private int value;
    
    public Accumulator(int v) {
        value = v;
        total += v;
    }
    
    public static int getTotal() { return total; }
}

// In main:
Accumulator a1 = new Accumulator(10);
Accumulator a2 = new Accumulator(20);
Accumulator a3 = new Accumulator(5);
System.out.println(Accumulator.getTotal());