CODEWITHSUNDEEP
javaarraylist
user2781663
user2781663

Reputation: 1

ArrayList constructor and use in methods

Hi I am a novice n just learning java. I was studying ArrayList n came accross this code for example {CODE1}. I would like to use the same code but add a ArrayListDemo constructor n create methods such as displayList and removeElement. I tried to find such examples but i did not understand them.

This is the code that i tried {CODE2} With my modifications please tell me where m going wrong.

***CODE1 {Example Code}****

import java.util.ArrayList;

public class AraryListDemo {

  public static void main(String[] args) {

    ArrayList al = new ArrayList();
    System.out.print("Initial size of al :  " + al.size());
    System.out.print("\n");

    //add.elements to the array list
    al.add("C");
    al.add("A");
    al.add("E");
    al.add("B");
    al.add("D");
    al.add("F");
    al.add(1,"A2");//inserts objects "A2" into array at index 1

    System.out.print("size of al after additions " + al.size());
    System.out.print("\n");

    //display the array list
    System.out.print("contents of al: " +  al );
    System.out.print("\n");

    //Remove elements from the array list
    al.remove("F");
    al.remove(2);

    System.out.print("size of after deletions : " + al.size());
    System.out.print("\n");
    System.out.print("contents of al:" + al);

  }

}

********CODE 2 {My Modifications}*************

class ArrayListDemo

{
ArrayList<String> al;//variable declared




ArrayListDemo() throws IOException//try constructor for this
{
    al = new ArrayList<String>();
    BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
    System.out.println("\n Enter Student Names");
    for(int i=0;i<=5;i++)// will dispaly 
    {
        al.add(br.readLine());
    }



}
void dispList(ArrayList <String> al)
{
    System.out.println("\n Display Student Names");
    for(String str : al)
    {
        System.out.println("\t Name :  "+str+"\n");
    }
}
}
class DisplayArrayList
{
public static void main(String []args) throws IOException
{

    ArrayList <String> al = new ArrayList <String>();
    ArrayListDemo e = new ArrayListDemo();
    e.dispList(al);

}
}

Upvotes: 0

Views: 3498

Answers (4)

Ashwani
Ashwani

Reputation: 3481

Your code runs as following :-

  • ArrayList <String> al = new ArrayList <String>(); // Initialise an ArrayList of type string

  • ArrayListDemo e = new ArrayListDemo(); // Initialised class ArrayListDemo

  • class constructor reads data from user input and add to ArrayList a1 by br.readLine()

  • e.dispList(al); iterates the ArrayList instance a1 and print its output.

Upvotes: 0

Muhammad Kashif Nazar
Muhammad Kashif Nazar

Reputation: 23885

The easiest (but not a prefered) solution to make your effort work is to pass the array to the displist() method that was filled by the constructor.

public static void main(String []args) throws IOException
{
    ArrayListDemo e = new ArrayListDemo();
    e.dispList(e.al);
}

Upvotes: 0

chrylis -cautiouslyoptimistic-
chrylis -cautiouslyoptimistic-

Reputation: 77187

It's not clear what exactly you're asking, but I note that you have a problem with your declarations (plural) of al: You have one ArrayList named al in your main, and you have another one that belongs to ArrayListDemo. You're reading values into the second one and then printing out the (empty) first one.

You really don't need a separate class with a constructor here. You can just have two static methods readList(List<String> al) and dispList(List<String> al). If you really do want to have a separate class, pick one place to store the List (either in main or in the class).

As a note, it's generally a good idea to use the most general type for variables and method parameters that you can. You're declaring an ArrayList, which is fine, but if you make your variable and parameters Lists, your code is more flexible.

Upvotes: 0

sanbhat
sanbhat

Reputation: 17622

ArrayList <String> al = new ArrayList <String>();
ArrayListDemo e = new ArrayListDemo();
e.dispList(al);

In the above code, you are creating a new ArrayList al, and passing the same to dispList() method, which doesn't iterate, because the al has no elements.

I guess you wanted to iterate through the elements which you created within ArrayListDemo. So you may want to write dispList() method as below, which will now use ArrayList defined within the class 

void dispList() //method parameter "al" is removed now and, al is the al of ArrayListDemo
{
    System.out.println("\n Display Student Names");
    for(String str : al) //here al refers to ArrayList defined within the class
    {
        System.out.println("\t Name :  "+str+"\n");
    }
}

Upvotes: 2

Related Questions