How can I override the toString method of an ArrayList in Java?

Question

I would like to have my own implementation of the toString() method for an ArrayList in Java. However, I can't get it working even though I added my toString() like this to the class that contains the ArrayList.

@Override
public String toString() {
    String result = "+";
    for (int i = 0; i < list.size(); i++) {
        result += " " + list.get(i);
    }
    return result;
}

When I call my ArrayList like this list.toString(), I still get the default representation. Am I missing something?

score 14 · Answer 1 · answered Aug 08 '13 at 15:06

14

ArrayList<String> list = new ArrayList<String>()
{
    private static final long serialVersionUID = 1L;

    @Override public String toString()
    {
        return super.toString();
    }
};

answered Aug 08 '13 at 15:06

Сашка724ая

682
5
15

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The downside of this approach is that you must create an anonymous implementation that extends `ArrayList` every time you want/need this =\. – Luiggi Mendoza Aug 08 '13 at 15:10
3

If you really need it one time, it wouldn't be really useful =\. – Luiggi Mendoza Aug 08 '13 at 15:11

score 10 · Answer 2 · edited May 23 '17 at 12:02

You can't override the toString method of ArrayList¹. Instead, you can use a utility class that converts an ArrayList to a String the way you want/need. Another alternative would be using Arrays.deepToString(yourList.toArray()).

Using Java 8, this can be done very easily:

//List<String> list
System.out.println(String.join(",", list));

Or if you have a List<Whatever>:

System.out.println(
    list.stream()
        .map(Whatever::getFieldX())
        .collect(Collectors.joining(", "))
);

I'm still against extending ArrayList or similar, is technically possible but I don't see it as a good option.

¹ you could extend ArrayList and override the toString but generally that's not a great idea. Further info:

Is `ArrayList` final? Can you not `extend ArrayList` and override `toString()`? — Steve P., Aug 08 '13 at 15:07

arshajii · Accepted Answer · 2013-08-08T15:28:40.313

8

You should do something like

public static String listToString(List<?> list) {
    String result = "+";
    for (int i = 0; i < list.size(); i++) {
        result += " " + list.get(i);
    }
    return result;
}

and pass the list in as an argument of listToString(). You can technically extend ArrayList (either with an anonymous class or a concrete one) and implement toString yourself, but that seems unnecessary here.

edited Aug 08 '13 at 15:28

answered Aug 08 '13 at 15:06

arshajii

127,459
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You might also get a performance improvement by using `StringBuilder` instead of `String` here. – Louis Wasserman Aug 08 '13 at 17:20

score 8 · Answer 4 · answered Aug 08 '13 at 15:06

8

You need to write a class extending from ArrayList:

import java.util.ArrayList;

public class MyArrayList extends ArrayList {
    private static final long serialVersionUID = 1L;

    @Override
    public String toString() {
        String result = "+";
        for (int i = 0; i < this.size(); i++) {
            result += " " + this.get(i);
        }
        return result;
    }
}

answered Aug 08 '13 at 15:06

Dennis Kriechel

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It's good to follow the Java naming conventions even for small sample code as other users try to learn from it. Here: the class name should start with a capital letter. It would also be good to include the word "List" in the class name. – Puce Aug 08 '13 at 15:10
@Puce I started with a capital letter, but yes your right the classname wasn't the best i changed it – Dennis Kriechel Aug 08 '13 at 15:11
How do I fix the MyArrayList is not generic error. I did that by adding `public class MyArrayList – Sep 07 '17 at 07:24

Tom Ritch · Answer 5 · 2015-04-15T05:24:06.280

To override toString() for ArrayList Integer array:

class MyArrayList extends ArrayList<Integer> {
    private static final long serialVersionUID = 1L;
    @Override public String toString(  ) {
        String output = "";
        for ( int i = 0 ; i < this.size(); i++ )
            output = Integer.toString( this.get( i ) );
        return output;
        }
    }

ArrayList<Integer> list = 
    new ArrayList<>(Arrays.asList( 5, 4, 2, 9, 1, 7 ) );
System.out.print( "list = " + list );

will generate: list = [5, 4, 2, 9, 1, 7]

score 0 · Answer 6 · answered May 13 '17 at 06:40

You can override the toString method in a new created class but you should make sure the created class extends ArrayList. Then, you can override toString method. This is based on IS relationship. The new created class is a type of ArrayList.

You can also create an ArrayList, which is based on the HAS relationship. In this case, you have to write another method to return String instead of override toString method.

How can I override the toString method of an ArrayList in Java?

6 Answers6

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