Classes, Objects, and Encapsulation

A class is best described as:

How many copies of an instance variable exist when five objects of a class have been created?

What does the operator new return?

In account.deposit(500), which is the calling object?

Inside an instance method, this refers to:

Which setter is written correctly?

Consider the following Java code. What will happen when you try to compile and run this program?

public class BankAccount {
    double balance;
    void deposit(double amount) {
        balance += amount;
        System.out.println("Balance: " + balance);
    }
}
public class Main {
    public static void main(String[] args) {
        BankAccount.deposit(300);
    }
}

What is the smallest change that makes the previous program work?

What is the output?

public class Item {
    private int count;
    public void add() { count++; }
    public int getCount() { return count; }
    public static void main(String[] args) {
        Item a = new Item();
        Item b = new Item();
        a.add(); a.add(); b.add();
        System.out.print(a.getCount() + " " + b.getCount());
    }
}

If the update method below is called, what can we say about the member variables numOfItems and reportTitle?

public void update(int num, String title) {
    int numOfItems = num;
    reportTitle = title;
}

Which of the following is not a reason to use encapsulation when designing a class?

How is encapsulation achieved in Java?

Which access modifier should instance variables normally have?

A method that only reports the value of a field is called:

By convention, a method returning a boolean property of an object is named:

In a UML class diagram, what does the symbol - before an attribute mean?

In UML, + setName(name : String) : void tells you that:

A driver class is:

Which of the following is true of instance variables?

Complete the following program so that it compiles and prints 1 then 2.

class MyClass {
    int count = 0;
    void incrementCount() {
        ________________;      // Step 1: increment count
        ________________;      // Step 2: print the updated count
    }
    public static void main(String[] args) {
        ________________;      // Step 3: create an object
        ________________;      // Step 4: first call
        ________________;      // Step 5: second call
    }
}

Which set of lines is correct?

What is the output?

public class Box {
    private int size = 1;
    public void grow() { int size = 10; }
    public int getSize() { return size; }
    public static void main(String[] args) {
        Box b = new Box();
        b.grow();
        System.out.print(b.getSize());
    }
}

Which statement about an instance method is correct?

What is a helper method?

Two objects a and b of the same class have different values in their fields. This is possible because:

Which line correctly declares an object variable without creating an object?

Suppose the variable declared in the previous question is an instance variable (a field), so it holds null by default. What happens if a method is called on it before an object is assigned to it?

Which of the following best describes abstraction in OOP?

What does public String toString() conventionally do?

What is the output?

public class Temperature {
    private double celsius;
    public void setCelsius(double c) { if (c >= -273.15) celsius = c; }
    public double getFahrenheit() { return celsius * 9 / 5 + 32; }
    public static void main(String[] args) {
        Temperature t = new Temperature();
        t.setCelsius(-500);
        System.out.print(t.getFahrenheit());
    }
}

The validation in the previous question is an example of:

Which sequence of statements is correct in a driver class?

In the class body, where must instance variables be declared?

Which of the following would you expose as a public method of a BankAccount class?

Given Student a = new Student(); Student b = a;, how many Student objects exist?

Explain the difference between a class and an object to someone who has never programmed, using an example that is not Student or Car.

A colleague makes every field public “because getters and setters are just extra typing”. Give two concrete problems this will cause in a class representing a bank account.

Explain why this is necessary in public void setId(int id) { this.id = id; } but not in public void setId(int newId) { id = newId; }.

Design a Student class with private attributes id (int), name (String), age (int) and grade (double); getters and setters for all four; a method displayInfo() that prints all the details; and a method isPassed() returning true when the grade is at least 50. Then write a separate driver class StudentApp whose main creates two students, modifies one of them with setters, and calls displayInfo() and isPassed() for each, printing Status: Passed or Status: Failed.