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?

Answer 1

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

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

Answer 2

You can't override the toString method of ArrayList ¹ . 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:

• Extending a java ArrayList
• Can you extend ArrayList in Java?
• To extend ArrayList, or to not extend ArrayList

Answer 3

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;
    }
}

Answer 4

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.

Answer 5

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]

Answer 6

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.

Answer 7

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?