Unit 2: Using objects

unit contents:

Introductory materials

The rectangle activity

The bank account activity

Adding security to your bank account application:

Implementing the Role Play activity in Java:

My solution to the Acrobat role play:

constructor and method overloading

String methods

More resources on String objects:

String Activities:

Solutions to String activities:

Referring to an object using "this":

A sample method using the "this" implicit parameter:

Introductory materials

Read, watch, and do the quiz on Runestone Academy - Java Review 2.2
Do the warmup activity on codingbat

The rectangle activity

The rectangle class:

public class Rectangle

    private int width;

    private int height;

    public Rectangle(int w, int h)

        width=w;

        height=h;

    public String shapeInfo()

        return "The height of this shape is: "+height+"\nThe width of this shape is "+width;

    public String drawRectangle()

    String toPrint="";

    for (int i=0;i<height;i++)

            toPrint+="\n";

           for (int j=0;j<width;j++)

               toPrint+="* ";

    return toPrint;

The main class:

public class Main

    public static void Main()

        Rectangle rectangle1=new Rectangle(10,10);

        System.out.println("\nReference for this shape: "+rectangle1);

        System.out.println(rectangle1.shapeInfo());

        System.out.println(rectangle1.drawRectangle());

The bank account activity

Create a class that will simulate the workings of a bank account. A bank account needs to have an owner, starting balance, as well as the ability to inquire about the current balance, to deposit, and to withdraw money.

A new account needs to be created with the owner's name filled out as well as a specific starting balance (think of your constructor here).

Try out creating several accounts and try to deposit, withdraw, and inquire about the balance.

Here is my suggested solution:

BankAccount class:

public class BankAccount

    private String owner;

    private double balance;

    public BankAccount(String ownerName, double StartBalance)

         balance= StartBalance;

         owner=ownerName;

         System.out.println("Congratulations on your new account, "+owner+"!");

         System.out.println("Your initial balance is: "+balance+".");

    public String deposit(double amount)

        balance+=amount;

        return "You have successfully deposited "+amount+" euros.";

    public String withdraw(double amount)

        balance-=amount;

        return "You have successfully withdrawn "+amount+" euros.";

    public double balance()

        return balance;

The Main class:

b;

public class Main

    public static void Main()

        BankAccount acc1=new BankAccount("Matej Kubes",500);

        System.out.println(acc1.deposit(300));

        System.out.println("The new balance is +"+acc1.balance());

        System.out.println(acc1.withdraw(200));

        System.out.println("The new balance is +"+acc1.balance());

Adding security to your bank account application:

Now, you will add user authorization to your Bank Account project. How this will work is that any of the BankAccount class methods will check to see if the user is authorized to deposit, withdraw, or check the balance.

There are multiple ways of implementing this. Do your pen-and-pencil planning first before you start coding. Will you need a new class or will you implement this added functionality within the BankAccount class and/or the Main class? What are the instance variables and methods you will need? Will these methods have parameters or not? Will they return anything or not? Try your best - we will discuss the different possible implementations on Monday.

Here is my solution (not the only correct one):

public class BankAccount

    // instance variables

    private String owner;

    private double balance;

    private boolean activeSession;

    private String password;

    public BankAccount(String ownerName, double StartBalance, String pass)

        // initialise instance variables

         balance= StartBalance;

         owner=ownerName;

         System.out.println("Congratulations on your new account, "+owner+"!");

         System.out.println("Your initial balance is: "+balance+".");

         activeSession=false;

         password=pass;

    public String deposit(double amount)

        if (activeSession)

            {balance+=amount;

            return "You have successfully deposited "+amount+" euros.";

        else

            return("Incorrect credentials.");

    public String withdraw(double amount)

        if (activeSession)

        {balance-=amount;

         return "You have successfully withdrawn "+amount+" euros.";

        else

            return("Incorrect credentials.");

    public double balance()

    { if (activeSession)

        return balance;

      else

        return -0; //This is just so that the method returns something. We will learn to do this more elegantly later.

/**

     * Creating an authorized session

     * @param  username

     * @param  pass

     * @return String

*/

    public String createSession(String username, String pass)

        if (username.equals(owner) && pass.equals(password))

            activeSession=true;

            return "You are successfully logged in.";

        else

            activeSession=false;

            return "Wrong credentials. Access denied.";

    public boolean sessionActive()

        if (activeSession)

            return true;

        else

            return false;

public class Main

    public static void Main()

        BankAccount acc1=new BankAccount("Matej Kubes",500,"pass123");

        System.out.println(acc1.createSession("Matej Kubes","pass123"));

        if (acc1.sessionActive())

            System.out.println(acc1.deposit(300));

            System.out.println("The new balance is +"+acc1.balance());

            System.out.println(acc1.withdraw(200));

            System.out.println("The new balance is +"+acc1.balance());

        else

            System.out.println("Your session is not active.");

Implementing the Role Play activity in Java:

Try to implement the different classes from the role play we played in class on Tuesday. Please, check your school email for the role play cards. Try to implement all of them and then create a main class to test their functionality. For actions you don't know how to implement, just return a message that says, e.g. "clap" or "rectangle", etc.

Think of your instance variables, method parameters (if any), and whether the different methods will be void or will return a value.

My solution to the Acrobat role play:

public class Acrobat

    private int count;

    public Acrobat()

        count= 0;

    public void clap(int times)

        for (int i=0;i<times;i++)

             count++;

             System.out.println("Clap!");

    public void kneeBend(int times)

        for (int i=0;i<times;i++)

             count++;

             System.out.println("bending...");

             System.out.println("Standing up...");

    public int count()

        return count;

public class Main

    public static void Main()

        Acrobat acrobat1=new Acrobat();

        acrobat1.clap(6);

        acrobat1.kneeBend(3);

        System.out.println("The total number of exercises is: "+acrobat1.count());

constructor and method overloading

Constructor overloading occurs when you include two or more constructors with the same name in a class. They must differ either in the number or the data types of its arguments. The compiler will then choose the corresponding constructor to execute according to the parameter provided.

Method overloading, analogue with constructor overloading, occurs when you include two or more methods of the same name in the class body. These methods, however, must differ in the number and/or the data types of the arguments. The compiler will then choose the correct method depending on the arguments provided when calling the method.

Follow up tasks:

Practice method overloading by creating a clap() method that will not take any arguments - it will clap once.
Practice constructor overloading by providing another argument when creating an acrobat - its name. It will, when being created, display "Acrobat NAME has just been created.".

Here is my solution, with another method called "getAcrobatName" added for more functionality, the modified/added lines are in bold italics:

public class Acrobat

    private int count;

    private String acrobatName;

    public Acrobat(String name)

        count= 0;

        acrobatName=name;

      public Acrobat()

        count= 0;

        acrobatName="no name";

    public void clap(int times)

        for (int i=0;i<times;i++)

             count++;

             System.out.println(this.returnAcrobatName()+" Clap!");

    public void clap()

                 count++;

                 System.out.println(this.returnAcrobatName()+" Clap!");

    public void kneeBend(int times)

        for (int i=0;i<times;i++)

             count++;

             System.out.println(this.returnAcrobatName()+" bending...");

             System.out.println(this.returnAcrobatName()+" Standing up...");

    public int count()

        return count;

    public String returnAcrobatName()

        return acrobatName;

public class Main

    public static void Main()

        Acrobat acrobat1=new Acrobat("John");

        Acrobat acrobat2=new Acrobat();

        acrobat1.clap(6);

        acrobat1.kneeBend(3);

        acrobat2.clap();

        acrobat2.kneeBend(2);

        System.out.println("The total number of exercises for "+acrobat1.returnAcrobatName()+" is: "+    acrobat1.count());

        System.out.println("The total number of exercises for "+acrobat2.returnAcrobatName()+" is: "+ acrobat2.count());

String methods

Remember some main points about strings in Java:

Strings are immutable
use the concatenation operator + to join strings
don't use the == operator to test strings for equality

The String object methods included in the AP Java subset are as follows:

string1.equals(string2) testing two strings for equality
toString() returns a string version of any object; this method gets invoked every time you try to print an object. By default, it returns the name and the pointer reference of the object. In order to make it more meaningful for the user, you can override the default method by defining your own version of the toString() method inside your class. An example for this, as seen in the Acrobat class, follows:

    public String toString()

        return "The acrobat's name is "+this.returnAcrobatName()+" and it has done "+this.count()+ "    exercises thus far.";

In this snippet, I used the this keyword, which refers to the object itself that is calling the method. It is also en example of overriding a method which means to provide a class-specific version of a generic method that will be used instead of the default version.

string1.length() returns the length of a string (as an integer)
string1.substring(int startIndex) - returns a new string that is a substring of this string
string1.substring(int startIndex, int endIndex) - returns a new string that is a substring of this string
string1.indexOf(String str) return the index number of the first occurence of string "str" within this string. If the string does not contain the "str" string, it will return -1.
string1.compareTo(String otherString) returns a value <0 if string1 is less than otherString in dictionary order. It will return 0 if the two strings are identical and it will return a value>0 if string1 follows otherString in the dictionary.

More resources on String objects:

Read more on strings in Java on Runestone Academy.
Try to do some String-related challenges on codingbat.

String Activities:

Create a program that will ask for the user's name and display: "Hey NAME, did you know your name is x letters long?"
Create a program that will ask the user for a word and then display it backwards. E.g. "hello" would be displayed as "olleh".
Create a program that checks for password strength - for the sake of simplicity, let's just check to see if the password is at least 8 characters long and contains at least one of the following punctuation symbols: . , ; ! ? (try using two for-loops, having all your punctuation symbols saved in a string of text).
Create a program that check for password strength - this time it will check to see if the password contains the user's first or last name in it. This means that you will have to ask for both the first and last name (separately) and the password and then determine if the password is strong enough. For the sake of simplicity, make it case sensitive - e.g. "Name" is different from "name".
Create an HTML tag creator function - the user will enter the text to display on the page as well as the tag to use and the function will return the HTML notation - the text enveloped in the opening and closing tag. E.g.: toHTML(p,The quick brown fox jumped over the lazy dogs) would result in returning <p>The quick brown fox jumped over the lazy dogs</p> Don't forget to use your escape characters as necessary.
Create a program that will prompt the user to type in a word and then it would check to see if the word is a palindrome or not (for the sake of simplicity, make it case sensitive).
Create a method that will take two strings as inputs and return the following output: "String1 precedes string2 in the dictionary." or the other way around, or "String1 and String 2" are identical - depending on the dictionary order of the two strings input.

Solutions to String activities:

Problem 2:

import java.util.Scanner;

public class Main

    public static void main()

        String name="";

        String reversedName="";

        Scanner keyboard=new Scanner(System.in);

        System.out.println("Type in your name.");

        name=keyboard.next();

        int length=name.length(); // Here I find the length of the string, so that I can set the loop

        for (int i=length-1;i>=0;i--)// counting down,e.g. if a word is 5 letters long: "4,3,2,1,0"

            reversedName=reversedName+name.substring(i, i+1); // adding letter-by-letter to the new str

        System.out.println(reversedName);

Problem 3:

import java.util.Scanner;

public class Main

    public static void Main()

        int passwordStrength=0;

        String password="";

        String punctuation=".,;!?";

        Scanner keyboard=new Scanner(System.in);

        System.out.println("Input the password");

        password=keyboard.next();

        if (password.length()<8)

            System.out.println("Your password is not long enough.");

        else

            for (int i=0;i<punctuation.length();i++) // traversing through the string containing punctuation

                    if (password.indexOf(punctuation.substring(i,i+1))!=-1) //testing to see if this specific punctuation symbol is found in the password

                        passwordStrength++; // counter for the number of punctuation symbols found

            System.out.println("Your password is long enough.");

            System.out.println("You have "+passwordStrength+" punctuation symbol(s) in your password.");

            if (passwordStrength>0)

                System.out.println("You password is strong enough.");

            else

                System.out.println("You password is NOT strong enough.");

Problem 4:

import java.util.Scanner;

public class Main

    public static void Main()

        int passwordStrength=0;

        String firstName="";

        String lastName="";

        String password="";

        Scanner keyboard=new Scanner(System.in);

        System.out.println("Input the First name");

        firstName=keyboard.next();

        System.out.println("Input the Last name");

        lastName=keyboard.next();

        System.out.println("Input the password");

        password=keyboard.next();

            if (firstName.indexOf(password)==-1 && lastName.indexOf(password)==-1  )

            System.out.println("You password is strong enough.");

        else

            System.out.println("You password is NOT strong enough.");

Problem 5:

public class tagCreator

    public tagCreator()

    {//the constructor is empty as the only need for creating a tagCreator object is so that we can call     its toHTML() method

    public String toHTML(String HTMLtag, String textInBetween)

     return "<"+HTMLtag+">"+textInBetween+"</"+HTMLtag+">";

public class Main

    public static void main()

//Here I create a tagCreator object, so that I can call its toHTML() method on the following line.

        tagCreator tc1=new tagCreator();

        System.out.println(tc1.toHTML("p","A quick brown fox jumped over the lazy dogs."));

Problem 6:

import java.util.Scanner;

public class Main

    public static void main()

        String word="";

        Scanner keyboard=new Scanner(System.in);

        System.out.println("Type in the word to check.");

        word=keyboard.next();

        String wordReversed="";

        for (int i=word.length()-1; i>=0;i--)

            // this is where I reverse the word, letter by letter:

            wordReversed+=word.substring(i,i+1);             }

        // if the original word is the same as the reversed, it's a palindrome:

        if (wordReversed.equals(word))

           System.out.println(word+" is a palindrome");

        else

           System.out.println(word+" is NOT a palindrome");

An alternative solution to Problem 6:

import java.util.Scanner;

public class Main

    public static void main()

        String word="";

        Scanner keyboard=new Scanner(System.in);

        System.out.println("Type in the word to check.");

        word=keyboard.next();

        int length=word.length();

        boolean isPalindrome=true;

        for (int i=0; i<length; i++)

                // the line below is just displaying the individual characters being compared (first:last, second:second-to-last, etc.):

                System.out.println(word.substring(i,i+1)+" "+word.substring(length-i-1,length-i));

                if (!word.substring(i,i+1).equals(word.substring(length-i-1,length-i)))

                isPalindrome=false;

        if (isPalindrome)

           System.out.println(word+" is a palindrome");

        else

           System.out.println(word+" is NOT a palindrome");

Referring to an object using "this":

You can refer to an object that called its method from inside this method by using the "this" keyword. Is is called implicit parameter.

Task: Look at the Acrobat class implementation above and see where and how the this keyword was used. Could we, instead of using this use the object name explicitly? (explicit parameter). Would the program work without the "this" keyword.

A sample method using the "this" implicit parameter:

publicVoid printAcrobat()

  System.out.println(this);

This method prints the class type and the memory reference of the object that called the printAcrobat () method.

Wrapper classes

Wrapper classes wrap a primitive value in an object.

Integer - a wrapper class for the int primitive type
Double - a wrapper class for the double primitive type

Integer(int value)

example: Integer (7) creates an Integer class object that has an int primitive type wrapped in it. The value of the int primitive type is 7.

By "wrapping" the primitive object, it provides object functionality - instance variables and methods.

The only instance variable holds the value of the primitive type that is being "wrapped".
Integer class methods:
- compareTo(another Integer object) returns 0 if the two Integer objects equal, a negative integer if it is less than the other Integer, a positive value if it's greater than another Integer.
- intValue() returns the int value of an Integer object
- equals(object) returns true if the Integer object equals another object

Math class

Importing the class - it should not be necessary, as it it part of the java.lang library:

import java.lang.Math;

Math class methods:

abs(int/double x) - returns the absolute value of int/double x
pow(double base,double exponent) - returns base to the power of exponent.
sqrt(double x) - returns the square root of x
random() - returns a random number between 0 and 1.

Tasks:

Create a dice roller with the user having an opportunity to guess the number (1-6). The suggested structure for this is as follows:
- The Dice class:
  - implements the roll() method (use the Math class, not the Random class), returns an integer between 1 and 6.
- The Main class:
  - creates a keyboard for user input (use the Scanner class)
  - creates a dice to roll with (creating an instance of the Dice class)
  - gets the number guess from the user using the keyboard (use the correct method of your keyboard object)
  - rolls the dice (no parameter needed to call the method)
  - compares the user's guess with the number rolled
  - displays the feedback to the user
- An advanced version of the program would keep asking the user for their guess until they got it right and it would also keep track of the number of times it took the user to guess the number.
Create a calculator that will calculate the n-th power of a base, as input by the user. The user input should be in the following form: "base^exponent", so "2^3" would translate as two to the power of three. You will need to use String methods as well as one Math method for this. Creating a keyboard for user input will be necessary as well. Think about whether you will need to create two classes or just the Main class.

Solutions:

Dice roller:

public class Dice

    public Dice()

    public int roll()

        int face=(int) Math.ceil(Math.random()*6);

        return face;

import java.util.Scanner;

public class Main

    public static void Main()

        Scanner keyboard=new Scanner(System.in);

        Dice dice=new Dice();

        System.out.println("What is your guess? a number between 1 and 6");

        int guess=keyboard.nextInt();

        int rolledNumber=dice.roll();

        if (guess==rolledNumber)

            System.out.println("You got it.");

        else

            System.out.println("Not quite. The number that was rolled was "+rolledNumber);

Problem 2: n-th power:

public class power

     public double power(int parsedBase, int parsedExponent)

        int base=parsedBase;

        int exponent=parsedExponent;

        return Math.pow(base,exponent);

import java.util.Scanner;

public class Main

    public static void Main()

        Scanner keyboard=new Scanner(System.in);

        power power1=new power();

        System.out.println("Type BASE^EXPONENT");

        String command=keyboard.next();

        int base =Integer.parseInt(command.substring(0,command.indexOf("^")));

        int exponent =Integer.parseInt(command.substring(command.indexOf("^")+1));

        double answer=power1.power(base,exponent);

        System.out.println(command+" is "+answer);

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