4.0 Table of Contents

4.1 Session 3 Review

4.1.1 Solution to Assignment 3

View Solution

4.1.2 Frequent Error #1

Array Index vs Array Value

int[] values = {99,100,101};
System.out.println(values[0]); // 99

values:  99 100 101
indices: 0  1   2

4.1.3 Frequent Error #2

Curly braces {} after if/else/for/while statements

what does this print?

for (int i = 0; i < 10; i++)
    System.out.println(i);
    System.out.println("Done");

4.1.4 Frequent Error #3

Variable initialization

int getMinValue(int[] values){
    int min = 0;
    for (int i = 0; i < values.length; i++){
        if (values[i] < min)
            min = values[i];
    }
    return min;
}
  • What if vals = {0,1,2}?
  • set min = values[0] or min = Integer.MAX_VALUE instead

4.1.5 Frequent Error #4

Variable Initialization - Part 2

int minIdx = getMinValue(values);
int secondIdx = 0;
for (int i = 0; i < values.length; i++){
    if (values[i] < values[minIdx])
        secondIdx = i;
}
  • What if vals = {0,1,2}?
  • see solutions

4.1.6 Frequent Error #5

Defining a method inside a method

public static void main(String[] args){
    public static void findMin(int[] values){
        // ...
    }
}

4.1.7 Debugging Tip #1

Use System.out.println() to print out values of variables

for (int i = 0; i < values.length; i++){
    System.out.println("i: " + i);
    System.out.println("values[i]: " + values[i]);
    if (values[i] < values[minIdx])
        secondIdx = i;
}

4.1.8 Debugging Tip #2

Format your code

This is rather hard to read:

for (int i = 0; i < values.length; i++){if (values[i] < values[minIdx])
{  secondIdx = i;}return secondIdx;
}

4.2 Object Oriented Programming

  • it is representative of the real world
Baby:
    name
    gender
    age
    weight
    height
  • it is a blueprint for creating objects
  • objects group together data and methods
  • data: attributes
  • methods: behaviors
Baby:
    String name
    boolean isBoy
    int age
    double weight
    double height

4.3 Why use classes?

  • why not just use variables?
// baby alex
String nameAlex;
double weightAlex;
// baby david
String nameDavid;
double weightDavid;
  • what if another baby is also named Alex?
  • what there are 100 babies?
Baby1:
    Name
    Weight
    gender
    ...

Baby2:
    Name
    Weight
    gender
    ...

Baby3:
    Name
    Weight
    gender
    ...

Baby4:
    Name
    Weight
    gender
    ...

...

Baby100:
    Name
    Weight
    gender
    ...

Then we can place all the babies into an array:


Nursery:
    Baby1
    Baby2
    Baby3
    Baby4
    ...
    Baby100

We can also add nurses to the nursery:

Nursery:
    Baby1
    Baby2
    Baby3
    Baby4
    ...
    Baby100

    Nurse1
    Nurse2
    Nurse3
    Nurse4
    ...
    Nurse100

We can also place the nursery into a hospital:

Hospital:
    Nursery
        Nurse[]
        Baby[]
    EmergencyRoom
        Nurse[]
        Patient[]

4.4 Defining a Class

4.4.1 Overview

Class Definition

public class Baby{
    String name;
    boolean isBoy;
    double weight;
    double height;
    int age;

    void cry(){
        System.out.println("Waaaaaah!");
    }

    void eat(){
        weight += 0.1;
    }

    void updateAge(){
        age++;
    }
}

Class Instantiation

Baby alex = new Baby();

4.4.2 Note on Creating Classes

  • class names are capitalized
  • 1 class per file
  • have a main method means the class is executable

4.4.3 Class Fields

  • fields are variables that belong to a class
public class Baby{
    TYPE var_name;
    TYPE var_name = some_value;
}
  • TYPE can be any data type
public class Baby{
    String name;
    boolean isBoy;
    double weight;
    double height;
    int age = 0;
}
  • add a sibilings field to the Baby class
public class Baby{
    String name;
    boolean isBoy;
    double weight;
    double height;
    int age = 0;
    Baby[] siblings;
}

4.4.4 Instantiate a Baby

Baby alex = new Baby();
  • new keyword creates a new object
  • to assign values to the fields, create constructors to initialize the fields

4.4.5 Constructors

  • a constructor is a special method that is called when an object is created
  • it is used to initialize the fields of the object
public class CLASSNAME{
    CLASSNAME(){
        // initialize fields
    }

    CLASSNAME([ARGUMENTS]){
        // initialize fields
    }
}

CLASSNAME obj1 = new CLASSNAME();
CLASSNAME obj2 = new CLASSNAME([ARGUMENTS]);
  • constructors have the same name as the class
  • no return type because it never returns anything
  • it is usually used to initialize the fields of the object
  • all classes must have at least one constructor
  • if no constructor is defined, then the compiler will create a default constructor
CLASSNAME(){}

public class Baby{
    String name;
    boolean isBoy;

    Baby(String myname, boolean boy){
        name = myname;
        isBoy = boy;
    }
}

4.4.6 Methods

  • they are used to perform operations on the fields of the class
  • they are defined inside the class
public class Baby{
    String name;
    void sayName(){
        System.out.println("My name is " + name);
    }
}

public class Baby{
    double weight;

    void eat(double foodWeight){
        if (foodWeight > 0 && foodWeight < weight)
            weight += foodWeight;
    }
}

4.5 Using Classes

4.5.1 Classes and Instances

// class Definition
public class Baby {...}

// class Instances
Baby alex = new Baby("Alex", true);
Baby anna = new Baby("Anna", false);

4.5.2 Accessing Fields

Object.field

Baby alex = new Baby("Alex", true);

// access fields
System.out.println(alex.name);
System.out.println(alex.isBoy);

4.5.3 Calling Methods

Object.method([ARGUEMENTS])

Baby alex = new Baby("Alex", true);
alex.sayName();
alex.eat(0.1);

4.6 References vs Values

4.6.1 Primitive vs Reference

  • primitive types are basic java types
  • int, double, boolean, char, etc.

  • the actual value is stored in the variable

  • Reference types are objects and arrays
  • String, Baby, int[], etc.

4.6.2 Storing Primitive Values

  • in Java, variables are like fixed size cups
  • primitives are small enough to fit in the cup

4.6.3 Storing Reference Values

  • reference types are too big to fit in the cup
  • hence they are stored somewhere else
  • the variable stores the address of the object

4.6.4 References

  • the object’s address/location is called a reference
  • == compares the references

would alex1 == alex2 be true or false?

Baby alex1 = new Baby("Alex", true);
Baby alex2 = new Baby("Alex", true);

They would not be the same

alex1: reference1
alex2: reference2

reference1 stored at 0x1234:
    name: "Alex"
    isBoy: true

reference2 stored at 0x5678:
    name: "Alex"
    isBoy: true
  • use = updates the reference
Baby alex1 = new Baby("Alex", true);
Baby alex2 = new Baby("Alex", true);
alex1 = alex2;

alex1: reference2
alex2: reference2

What is i, j, and k?

void doSomething(int x, int[] y, Baby b){
    x=99;
    y[0]=99;
    b.name = "99";
}
...

int i = 0;
int[] j = {0};
Baby k = new Baby("0", true);
doSomething(i,j,k);

4.7 Static Types and Methods

4.7.1 Static

  • Applies to fields and methods
  • means that the field/method
    • belongs to the class
    • not unique to each instance

4.7.2 Static Fields

public class Baby{
    static int numBabies = 0;
    String name;

    Baby(String myname){
        name = myname;
        numBabies++;
    }
}

...
Baby alex = new Baby("Alex");
Baby anna = new Baby("Anna");

What is the value of alex.numBabies and anna.numBabies?

  • 2 and 2
  • numBabies is a static field
  • there is only one copy of it
  • the value is shared by all instances of the class

4.7.3 Static Methods

public class Baby{
    static void cry(Baby thisBaby){
        System.out.println(thisBaby.name + " cries");
    }
}

public class Baby{
    void cry(){
        System.out.println(name + " cries");
    }
}
  • Non-static methods can reference static methods, but not the other way around
  • Static methods can only reference static methods

Why?

public class Baby {
    String name = "alex";
    static void whoami(){
        System.out.println(name);
    }
}
  • whoami cannot access name because name is not static, therefore it unique to each instance of the class, and whoami does not know which instance of name to access

4.7.4 Why is main static?

  • main is static because it is the entry point of the program
  • it is the first method that is called
  • there is no instance of the class yet
  • hence it must be static

4.8 Assignment 4 - Library

Problem

You are in charge of creating an electronic library system for a small town. The town has two libraries, each with a collection of books for borrow. Customers can print the list of available books, borrow a book, and return a book.

two classes are provided for you: Book and Library. You must implement the missing methods to make the classes work.

Step 1: Implement the Book class

First a class needs to model books. Copy and paste the skeleton code for Book.java into a file called Book.java. This class defines methods to get the title of a book, find out if it is available, borrow the book, and return the book. The skeleton the methods are provided, but you must implement the methods. The main method can be used to test the code. When you run Book.java, the output should be the following:

Title (should be The Da Vinci Code): The Da Vinci Code
Rented? (should be false): false
Rented? (should be true): true
Rented? (should be false): false

Hint: Look at the main method to see how the methods are used, then fill in the code for each method.

Step 2: Implement the Library class

Next a class needs to model a library to manage the books. All libraries have the same hours: 9AM to 5PM everyday. However, they have different addresses and book collections (arrays of Book objects).

Copy and paste the skeleton code for Library.java into a file called Library.java. The main method procided creates two libraries, then performs some operations on the books. However, the methods and variables are not implemented. Define and implement those. Read the main method and look and compile erros to figure out what methods are missing.

Note

  • some methods will need to be static, and some needs to be instance methods
  • careful with comparing String objects as they are reference types and can only be compared using .equals() (String1.equals(String2))
  • Try to get a small part of the code working at a time, then move on to the next part. You can start by commenting the entire main method, then uncommenting one line at a time. Run the program and get the first lines working before uncommenting the next line. You can usually comment/uncomment lines of code by selecting the lines and pressing Ctrl + / (Windows) or Cmd + / (Mac). You cancomment a block of code in Eclipse by selecting the code, then choosing Source → Toggle Comment. Do the same again to uncomment it.
  • You must not modify the main method.

The output of Library.java when you run it should be the following:

Library hours:
Libraries are open daily from 9am to 5pm.

Library addresses:
10 Main St.
228 Liberty St.

Borrowing The Lord of the Rings:
You successfully borrowed The Lord of the Rings
Sorry, this book is already borrowed.
Sorry, this book is not in our catalog.

Books available in the first library:
The Da Vinci Code
Le Petit Prince
A Tale of Two Cities

Books available in the second library:
No book in catalog

Returning The Lord of the Rings:
You successfully returned The Lord of the Rings

Books available in the first library:
The Da Vinci Code
Le Petit Prince
A Tale of Two Cities
The Lord of the Rings

Book.java

public class Book {
    String title;
    boolean borrowed;

    // Creates a new Book
    public Book(String bookTitle) {
        // Implement this method
    }

    // Marks the book as rented
    public void borrowed() {
        // Implement this method
    }

    // Marks the book as not rented
    public void returned() {
        // Implement this method
    }

    // Returns true if the book is rented, false otherwise
    public boolean isBorrowed() {
        // Implement this method
    }

    // Returns the title of the book
    public String getTitle() {
        // Implement this method
    }

    public static void main(String[] arguments) {
        // Small test of the Book class
        Book example = new Book("The Da Vinci Code");
        System.out.println("Title (should be The Da Vinci Code): " + example.getTitle());
        System.out.println("Borrowed? (should be false): " + example.isBorrowed());
        example.borrowed();
        System.out.println("Borrowed? (should be true): " + example.isBorrowed());
        example.returned();
        System.out.println("Borrowed? (should be false): " + example.isBorrowed());
    }
}

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Library.java

public class Library {
    // Add the missing implementation to this class

    public static void main(String[] args) {
        // Create two libraries
        Library firstLibrary = new Library("10 Main St.");
        Library secondLibrary = new Library("228 Liberty St.");

        // Add four books to the first library
        firstLibrary.addBook(new Book("The Da Vinci Code"));
        firstLibrary.addBook(new Book("Le Petit Prince"));
        firstLibrary.addBook(new Book("A Tale of Two Cities"));
        firstLibrary.addBook(new Book("The Lord of the Rings"));

        // Print opening hours and the addresses
        System.out.println("Library hours:");
        printOpeningHours();
        System.out.println();
        System.out.println("Library addresses:");
        firstLibrary.printAddress();
        secondLibrary.printAddress();
        System.out.println();

        // Try to borrow The Lords of the Rings from both libraries
        System.out.println("Borrowing The Lord of the Rings:");
        firstLibrary.borrowBook("The Lord of the Rings");
        firstLibrary.borrowBook("The Lord of the Rings");
        secondLibrary.borrowBook("The Lord of the Rings");
        System.out.println();

        // Print the titles of all available books from both libraries
        System.out.println("Books available in the first library:");
        firstLibrary.printAvailableBooks();
        System.out.println();
        System.out.println("Books available in the second library:");
        secondLibrary.printAvailableBooks();
        System.out.println();

        // Return The Lords of the Rings to the first library
        System.out.println("Returning The Lord of the Rings:");
        firstLibrary.returnBook("The Lord of the Rings");
        System.out.println();

        // Print the titles of available from the first library
        System.out.println("Books available in the first library:");
        firstLibrary.printAvailableBooks();
    }
}

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