ArrayList.remove(int index) not working with non-anonymous class object

Question

Hi All,

ArrayList.remove(int index) is working with the anonymous instance of ActionListener class :-

DeleteModule.java :-

import java.awt.GridLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.util.ArrayList;

import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JOptionPane;
import javax.swing.JTextField;



class MyFrame extends JFrame{

    private ArrayList<String> list = new ArrayList<String>() ; 
    private JButton btn = new JButton("Enter index to delete : ") ;
    private JTextField fld = new JTextField() ;

    MyFrame(){
        populateList() ;

        setLayout(new GridLayout(1, 2)) ;
        setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE) ;
        setSize(400, 60) ;

        btn.addActionListener(new ActionListener() {

            @Override
            public void actionPerformed(ActionEvent e) {
                list.remove( Integer.parseInt( fld.getText() ) ) ;
                JOptionPane.showConfirmDialog(null, list, "Total Elements : " + list.size(), JOptionPane.DEFAULT_OPTION, JOptionPane.PLAIN_MESSAGE) ;
            }
        });

        add(btn) ;
        add(fld) ;

        setTitle("Total Elements : " + list.size()) ;

        setVisible(true) ;
    }

    private void populateList(){
        for(int i = 1 ; i <= 5 ; ++i){
            list.add("Key " + i) ;
        }
    }
}

public class DeleteModule {

    public static void main(String[] args) {
        new MyFrame() ;
    }
}

But when I am integrating this with the original module (below is the modified reduced source code with non-anonymous instance of DeleteFromPoolListener.class), it's returning false for deletion. I really don't know why it's not working.

Demo.java

import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.GridLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.util.ArrayList;
import java.util.Vector;

import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JOptionPane;
import javax.swing.JPanel;
import javax.swing.JTextArea;
import javax.swing.JTextField;

class MyFrame extends JFrame{

    private ArrayList<Pair> list = new ArrayList<Pair>() ;

    private JButton deleteBtn = new JButton("Delete Pair at index : ") ;
    private JTextField deleteIndexText = new JTextField() ;

    MyFrame(){
        populateList() ;
        setTitle("List size : " + list.size()) ;
        setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE) ;

        JPanel lowerPanel = new JPanel() ;
        lowerPanel.setLayout(new GridLayout(2, 1)) ;

        deleteBtn.addActionListener(new DeleteFromPoolListener()) ; lowerPanel.add(deleteBtn) ;
        lowerPanel.add(deleteIndexText) ;
        add(lowerPanel) ;
        pack() ;
        setVisible(true) ;
    }

    private void populateList(){
        for(int i = 1 ; i <= 5 ; ++i){
            list.add( new Pair( "Key " + i, "Value " + i ) ) ;
        }
    }


    class DeleteFromPoolListener implements ActionListener{

        @Override
        public void actionPerformed(ActionEvent e) {

            Integer i = null ;
            try{
                i = Integer.parseInt(deleteIndexText.getText().trim()) ;
                boolean b = list.remove(i) ;

                JOptionPane.showConfirmDialog(null, list + "\n\n" + "Deleted : " + b + " from index " + i, "List size : " + list.size(), JOptionPane.DEFAULT_OPTION, JOptionPane.PLAIN_MESSAGE) ;
                setTitle("Total Pairs : " + list.size()) ;
                deleteIndexText.setText("") ;
            }
            catch(NumberFormatException nfe){
                JOptionPane.showConfirmDialog(null, "Enter numeric value in range.", "Error", JOptionPane.DEFAULT_OPTION, JOptionPane.PLAIN_MESSAGE) ;
                deleteIndexText.setText("") ;
            }       
        }
    }
} 



public class Demo {

    public static void main(String[] args) {
        new MyFrame() ;
    }
}

class Pair{

    private String key ;
    private String value ;

    Pair(String k, String v){
        key = k ;
        value = v ;
    }

    public String toString() {
        return "[" + value + "]" ;
    }

    public String getKey() {
        return key;
    }

    public void setKey(String key) {
        this.key = key;
    }

    public String getValue() {
        return value;
    }

    public void setValue(String value) {
        this.value = value;
    }
}

I am really confused why it's not working... :(

Answer 1

The ArrayList class has two remove methods:

• one which takes an Object and attempts to remove that object from the list, and
• one which takes an int index and removes the object in the list at that index.

You declared your variable i as an Integer, so it is also an Object. Therefore you will be invoking the remove(Object) method. Nothing happens when you attempt to remove i since your list has Pairs in it, not Integers.

What you need to do is to declare your variable i as an int, give it a default value of 0 (you can't assign null to an int), and change the line

boolean b = list.remove(i) ;

to

boolean b = (list.remove(i) != null) ;

because remove(int) returns the object removed from the list, instead of a boolean indicating whether it removed anything.

Answer 2

The problem occurs because you are calling list.remove(Object) instead of list.remove(int). Since i is an Integer (or an Object) and not a primitive type, the JVM thinks that you are trying to remove the object rather than the actual index; thus return false because there are no Integer object in your list.

Simply replace the code with this one:

int i;
try{
   i = Integer.parseInt(deleteIndexText.getText().trim()) ;
   boolean b = (list.remove(i) != null);

   JOptionPane.showConfirmDialog(null, list + "\n\n" + "Deleted : " + b + " from index " + i, "List size : " + list.size(), JOptionPane.DEFAULT_OPTION, JOptionPane.PLAIN_MESSAGE) ;
   setTitle("Total Pairs : " + list.size()) ;
   deleteIndexText.setText("") ;
}
catch(NumberFormatException nfe){
   JOptionPane.showConfirmDialog(null, "Enter numeric value in range.", "Error", JOptionPane.DEFAULT_OPTION, JOptionPane.PLAIN_MESSAGE) ;
   deleteIndexText.setText("") ;
}       

Yes, Java can peform

Integer i = 2, j = new Integer(10);
i = i + j;
System.out.println(i.toString());

just fine, but int and Integer are not the same types :)

Answer 3

In Demo.java you should use:

 boolean b = (list.remove(i.intValue()) != null)

instead of

 boolean b = list.remove(i) ;

The reason is the following. ArrayList has 2 remove functions: one that takes as parameter an int (not Integer!) and one that takes as a parameter an Object.

In your first example you used the first remove (as you converted to int using intValue()). In the second you were trying to remove the Object represented by the Integer (which, obviously wasn't in the collection).