DataStructures : ArrayList, HashMap, Set

ArrayList

- Values could be duplicated

- Use to store multiple "elements" together, represented by single variable name

- The "elements" could be of different Data types such as int, double, Objects
- No need to specify size while declaration, size = Dynamic
- Element can be accessed using array "index"
- Array index starts from zero [0]

--------------------------------------------------------------------------------------
Ex. 1: ArrayList 

Source: TestArrayList.java

import java.util.ArrayList;

public class TestArrayList {
    public static void main(String[] args) {


        ArrayList list = new ArrayList();
   
        list.add("Tokyo");
        list.add("Edogawa ku");
        list.add("Minato ku");
        list.add("Toshima ku");


        Customer cust = new Customer();
        list.add(cust); // object can be added to ArrayList
   
        System.out.println(list.get(3));
    }
}

Output: 

Toshima ku

======================================================

HashMap

- Use to store data in key and value pair 

- Key should be unique and of reference data type

- No need to specify size while declaration, size = Dynamic

---------------------------------------------------------------------------------------

Ex. 2: HashMap 

Source: TestHashMap.java

import java.util.HashMap;

public class TestHashMap {
    public static void main(String[] args) {


        HashMap hash = new HashMap();
        //key and value pair (phone/name)
        hash.put("5678", "Yamada");
        hash.put("8934", "Suzuki");
        hash.put("1234", "John");
   
        System.out.println(hash.get("8934"));
    }
}

Output: 

Suzuki


======================================================

TreeSet

- Use to store unique values, no duplicates

- Use to store multiple "elements" together, represented by single variable name

- The "elements" could be of different Data types such as int, double, Objects

- No need to specify size while declaration, size = Dynamic
- Element can be accessed using array "index"
- Array index starts from zero [0]

----------------------------------------------------------------------------------------

Ex. 3: TreeSet 

Source: TestTreeSet.java

import java.util.TreeSet;

public class TestTreeSet {
    public static void main(String[] args) {

        TreeSet tset = new TreeSet();        

        tset.add(1);
        tset.add(2);
        tset.add(3);
        tset.add(3);
   
        System.out.println(tset.toString());    
   }

}

Output: 

[1,2,3]

Application : Calculator (For 3 Numbers)

Requirements:
Create Calculator Application to have following functionality,
- Add 3 numbers
- Subtract 3 numbers
- Divide 3 numbers 
- Multiply 3 numbers


- Take command line input for 3 numbers
- User can enter option based on the menu as shown below,

Menu 
(1=Add, 2=Sub, 3=Div, 4=Mul )


Design:

Implementation:
[CalculatorApp.java]
import java.util.Scanner;

public class CalculatorApp {

    public static void main(String[] args) {


        //read command line arguments from the command line while running this application
        double n1 = Double.parseDouble(args[0]);  //first number
        double n2 = Double.parseDouble(args[1]);  //second number
        double n3 = Double.parseDouble(args[2]);  //third number
        
        //use to get user input for operation
        Scanner scan = new Scanner(System.in);
    
        System.out.println("Menu ");
        System.out.println("(1=Add, 2=Sub, 3=Div, 4=Mul )");

        int option = scan.nextInt();
   
        CalculatorApp appC = new CalculatorApp();

        switch (option) {

        case 1 :
            appC.add(n1, n2, n3);
            break;

        case 2 :
            appC.sub(n1, n2, n3);
            break;

        case 3 :
            appC.div(n1, n2, n3);
            break;
        case 4 :
            appC.mul(n1, n2, n3);
            break;

        default :
            System.out.println("Invalid option.");
        }
   
    } //end of main() method

    //Method to add three numbers and display results
    public void add(double n1, double n2, double n3) {
        System.out.println("Adding...");
        double result = n1 + n2 + n3;
        System.out.println(result);
    }

     //Method to subtract three numbers and display results    
    public void sub(double n1, double n2, double n3) {
        System.out.println("Subtracting...");
        double result = n1 - n2 - n3;
        System.out.println(result);
    }

    //Method to divide three numbers and display results  
    public void div(double n1, double n2, double n3) {
        System.out.println("Dividing...");
        double result = n1 / n2 / n3;
        System.out.println(result);
    }

   //Method to multiply three numbers and display results  
    public void mul(double n1, double n2, double n3) {
        System.out.println("Multiplying...");
        double result = n1 * n2 * n3;
        System.out.println(result);
    }

}


Testing:
=============How to Compile using java command line without Eclipse
javac CalculatorApp.java
The above command will compile CalculatorApp.java to CalculatorApp.class
=============How to Run using Command prompt (such as dos/unix/linux) 
java CalculatorApp 1 2 3
The above command will run CalculatorApp application with 3 arguments (Number1, Number2 and Number3)
=============Output: For Add option [other 3 options (Sub, Div, Mul) ]
Menu 
(1=Add, 2=Sub, 3=Div, 4=Mul )


1
Adding...
6.0

Switch Statement

Source: TestSwitch.java


import java.util.Scanner;

public class TestSwitch {

public static void main(String[] args) {

Scanner scan = new Scanner(System.in);

System.out.println("Enter month number");

int monthNumber = scan.nextInt();

switch (monthNumber) {

case 1: System.out.println("JAN");

break;

case 2 : System.out.println("FEB");

break;

case 3 : System.out.println("MAR");

break;

case 4 : System.out.println("APR");

break;

default :

System.out.println("Invalid Month");

}

}

}

User Interaction (Getting Users' Input)

Source: TestUserInput.java

import java.util.Scanner;

public class TestUserInput {

public static void main(String[] args) {

Scanner scan = new Scanner(System.in);

System.out.println("Enter your Name : ");

String name = scan.next();

System.out.println("Enter your marks : ");

int marks = scan.nextInt();

if (marks > 90) {

System.out.println("Hi " + name + " You got A");

} else if (marks > 70) {

System.out.println("Hi " + name + " You got B");

} else if (marks > 50) {

System.out.println("Hi " + name + " You got C");

} else {

System.out.println("Sorry Dude better luck next time");

}

}

}

Mechanism of Method Execution

When we run TestApp5 application, the JVM start the execution by calling main() method of TestApp5.

Command: 

java TestApp5

Execution Model:

Array

- Use to store multiple "elements" together, represented by single variable name
- Need to specify size while declaration, size = total number of elements
- Element can be accessed using array "index"
- Array index starts from zero [0]


======================================================
Ex. 1: Array of int (primitive data type)

Source: TestArray1.java

public class TestArray1 {

    public static void main(String[] args) {
     
        int[ ] ilist = new int [10];
        ilist[0] = 101;
        ilist[7] = 200;
     
        for(int k = 0; k < ilist.length; k++ ) {
            System.out.println(ilist[k]);
        }
    }
}

Output: 
101
0
0
0
0
0
0
200
0
0
======================================================
Ex. 2: Array of float (primitive data type)

Source: TestArray2.java

public class TestArray2 {


    public static void main(String[] args) {
       
        float[] ilist = new float [10]; // size
        ilist[0] = 101.50;
        ilist[7] = 200.25;
       
        for(int k = 0; k < ilist.length; k++ ) {
            System.out.println(ilist[k]);
        }
    }
}

Output: 
101.50
0.0
0.0
0.0
0.0
0.0
0.0
200.25
0.0
0.0
======================================================
Ex. 3: Array of String (reference data type)

Source: TestArray3.java

public class TestArray3 {

   public static void main(String[] args) {
     
   String[] weekdays = new String[7]; // size
        weekdays[0] = "Monday";
        weekdays[1] = "Tuesday";
        weekdays[2] = "Wednesday";
        weekdays[3] = "Thursday";
        weekdays[4] = "Friday";
        weekdays[5] = "Saturday";
        weekdays[6] = "Sunday";
      
        for(int k = 0; k < weekdays.length; k++ ) {
            System.out.println(weekdays[k]);
        }
    }
}

Output: 
Monday
Tuesday
Wednesday
Thursday
Friday
Saturday
Sunday

Command Line Arguments

With command line arguments it is possible to send configuration information to the application. 

Source: TestArgument.java

public class TestArgument {

         public static void main(String[] args) {        

            int size = Integer.valueOf(args[0]);

            int[] list = new int[size]; 
            System.out.println("Size of the array= " + list.length);           

}

     }

Output: Size of the array=25

Run:

java TestArgument  25

Syntax:

java  ProgramName  argument1  argument2  argumentN