Thread: ArrayList.remove() problem. (solved)

Hi.

I created a simple test list while studying ArrayList. For some reason (some obvious i guess) i cant remove objects from list.
There is tiny class Person with some values, which are added to arraylist. Everything else works, except when i try to remove first occurance of "Pekka". Nothing happens and .remove returns false.

import java.util.*;
 
public class MyArrayList2 {
	public static class Person {
		private String name;
		private int id;
 
		public Person(String name, int id) {
			this.name = name;
			this.id = id;
		}
 
		public String getName() {
			return this.name;
		}
		public String toString() {
			return name;
		}
	}
 
 
	public static void main(String[] args) {
		// Create a list from class Person
		ArrayList<Person> henkilot = new ArrayList<Person>();
 
		// Adding few Persons to list
		henkilot.add(new Person("Maija", henkilot.size() + 1));
		henkilot.add(new Person("Pekka", henkilot.size() + 1));
		henkilot.add(new Person("Marko", henkilot.size() + 1));
		henkilot.add(new Person("Pekka", henkilot.size() + 1));
		henkilot.add(new Person("Pekka", henkilot.size() + 1));
 
		// Trying to remove first occurance with name "Pekka"
		System.out.println(henkilot.remove("Pekka"));
 
		// Printing out the list
		System.out.println(henkilot.toString());
 
	}
}

Output is:
false
[Maija, Pekka, Marko, Pekka, Pekka]

Java is:
java version "1.7.0_55"
Java(TM) SE Runtime Environment (build 1.7.0_55-b13)
Java HotSpot(TM) Server VM (build 24.55-b03, mixed mode)

The remove() method is looking for a String object: "Pekka" in the ArrayList, but the ArrayList contains Person objects so it doesn't find anything to remove.
If you want to find a Person object that contains the String: "Pekka" you will need to iterate through the list testing each Person object to see if it contains that String and then remove the object that has the String.

overwrite the equals method to check for equality with strings.

@Cornix Do you have some test code that shows what you are saying?

I was thinking about something like this:

	public static class Person {
		private String name;
		private int id;
 
		public Person(String name, int id) {
			this.name = name;
			this.id = id;
		}
 
		public String getName() {
			return this.name;
		}
		public String toString() {
			return name;
		}
		public boolean equals(Object obj) {
			if (obj instanceof String) {
				return obj.equals(toString());
			}
			return obj == this;
		}
	}

This should enable the OP to remove persons from the list by calling the remove method with a string as argument as long as the string matches the name of the person.


Edit: Just checked the documentation for the list interface again, this does not work. The list will use the equals method from the object passed as argument to the remove method and not the implementation of the objects contained in the list. Bummer.
I guess this is a question of personal preference, but I would have preferred it to use the equals method of the contained objects.

Just checked the documentation for the list interface again, this does not work.

That should be BOLD so it stands out.

A few days late, but...

Edit: Just checked the documentation for the list interface again, this does not work. The list will use the equals method from the object passed as argument to the remove method and not the implementation of the objects contained in the list. Bummer.
I guess this is a question of personal preference, but I would have preferred it to use the equals method of the contained objects.

Check the contract for the equals method of the Object class...

It is symmetric: for any non-null reference values x and y, x.equals(y) should return true if and only if y.equals(x) returns true.