Object Oriented Programming

Workshop 2: Classes and Objects

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Classes and Objects

Table of Contents

  1. Describe the difference between the terms class and objects.
    • class: this is a abstract data type with attributes and methods
    • object: this is an instance of a class
  2. Label the different parts of the following class:
public class Book {                             // <- class definition: singular, capitalised
    private String title;                       // <- class attributes
    private String author;                      // <- default value is null if not specified
    private String borrowedBy = null;           // <- default values for attributes
    private boolean borrowed = false;
    private int borrowDuration;                 // <- default value is 0

    public Book(String author, String title) {  // <- constructor
        this.author= author;
        this.title = title;
    }

    public void borrow(String owner, int duration) { // <- class method
        borrowed = true;
        borrowedBy = owner;
        borrowDuration = duration;
    }
}
  1. What is the purpose of a constructor, and how do we use them?
    • A constructor is used to create and initialise an object
    • e.g. to initialise a new book: Book book = new Book("James", 14);
  2. What does the keyword this mean? Why do we use it?
    • this refers to the calling object
    • used to refer to attributes/methods of the calling object, e.g. in constructors so that you can use the same name for the constructor argument and the attribute
    • sometimes people use _ as a prefix to the argument name so that you don’t need to use this
  3. What does null mean in Java?
    • it’s a constant that can be assigned to any data type in Java, indicating the variable has no real value
    • can be used to initialise variables where there is no obvious/useful choice
    • null is not an object: for comparison you use normal operators == !=, not equals method
    • attempting to invoke a method on a null object will throw a Null Pointer Exception
  4. For the following questions, the class definition for IntegerHolder is: 
    class IntegerHolder {
 int value;
 public IntegerHolder(int value) {
     this.value = value;
 }
}

    Determine the output for each code snippet. a.

    public static void increment(int input) {
 input = input + 1;
}
public static void main(String[] args) {
 int a = 0;
 increment(a);
 System.out.println(a);  // prints "0" as no value is returned, and no reference to a is passed, int is passed by value
}

    b.

    public static void triple(IntegerHolder integerHolder) {
 integerHolder.value = integerHolder.value * 3;
}
public static void main(String[] args) {
 int a = 25; 
 IntegerHolder myHolder = new IntegerHolder(a);
 triple(myHolder);
 System.out.println(myHolder.value); // prints "75"
 System.out.println(a);  // prints "25"
}
  5. What are getters and setters in Java? Why are they needed?
    • getters/setters are used to mutate state of an object
    • access control: ensures you are modifying object per prescribed behaviour: produces a more secure/predictable result
    • you define a clean interface with which to interact/act upon an object
    • hides implementation details
  6. What are two special methods that every class in Java has? What do they do? (Hint: not getters/setters)
    • equals(): allows you to make equality comparison between two objects
    • toString(): allows you to print a string representation of an object
    • clone(): produce a copy of an object
  7. Static attributes and methods
    • shared between all instances of a class
    • c.f. global variables in C
    • easy to write confusing/difficult to maintain code
    • occassionally they are the write thing to do
    • for variables in a method (not attributes!) you do not use private keyword
    • non-static attributes/methods end up on heap (dynamic memory)
    • static attributes/methods end up in static memory (similar to stack)
    • useful for e.g. counting number of instances of a given class
    • System.out.println("Hello"); // out is a static attribute of System
    • Math.sqrt(2.0); // sqrt() is a static method of Math
    • be aware compiler will say “Did you want this to be a static attribute?” when you try to reference a non-static attribute without an instance reference

Design a chair class

Complex number

public class ComplexNumber {
    private double real;
    private double imaginary;

    public ComplexNumber(double real, double imaginary) {
        this.real = real;
        this.imaginary = imaginary;
    }

    public double getReal() {
        return real;
    }

    public double getImaginary() {
        return imaginary;
    }

    public void setReal(double real) {
        this.real = real;
    }

    public void setImaginary(double imaginary) {
        this.imaginary = imaginary;
    }

    public double getModulus() {
        return Math.sqrt(Math.pow(real, 2) + Math.pow(imaginary, 2));
    }

    public boolean equals(ComplexNumber c) {
        return Double.compare(this.real, c.real) == 0 && Double.compare(this.imaginary, c.imaginary) == 0;    
    }
}

Can a class have multiple parent classes?

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