ArrayList indexOf

Question

I just started learning about the ArrayList class in Java. I have the following code below testing out the ArrayList class and it's methods:

import java.util.ArrayList;

public class NewArrayList {

    public static void main(String[] args) {
        // TODO Auto-generated method stub
        ArrayList<String> myList = new ArrayList<String>();

        String s = new String();
        myList.add(s);
        String b = new String();
        myList.add(b);

        int theSize = myList.size();
        System.out.println("ArrayList size: " + theSize);

        boolean isTrue = myList.contains(s);
        System.out.println(isTrue);

        int whereIsIt = myList.indexOf(s);
        System.out.println(whereIsIt);
        int whereIsIt2 = myList.indexOf(b);
        System.out.println(whereIsIt2);

    }

}

The indexOf method shows whats the index of an object. So since I added two objects, s and b to the myList ArrayList object reference, it should have 2 objects inside the index. The output of whereIsit and whereIsit2 is both 0. Shouldn't it be 0 1??

Sotirios Delimanolis · Accepted Answer · 2021-12-30 14:01:43Z

10

You're adding two String objects with the same value (empty string) to the list.

So your list looks like

["", ""]

You're then invoking the equivalent of indexOf(""), where indexOf(..) uses the Object#equals(Object) method to compare objects. The first element in the list is equal to "", so that index is returned.

edited Dec 30, 2021 at 14:01

answered May 27, 2014 at 4:48

Sotirios Delimanolis

281k62 gold badges718 silver badges744 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Sajith A.K. · Accepted Answer · 2014-05-27 05:08:45Z

0

Here you just create two objects and that has no value so it will be treated as empty string. So the Indexof will returns the first occurence of the given object.

If you assign different values to s and b then result what you are expecting that will get. Please try with below code.

    String s = "String1";
    myList.add(s);
    String b = "String2";
    myList.add(b);

    int whereIsIt = myList.indexOf(s);
    System.out.println(whereIsIt);
    int whereIsIt2 = myList.indexOf(b);
    System.out.println(whereIsIt2);

answered May 27, 2014 at 5:08

Sajith A.K.

7349 silver badges22 bronze badges

Comments

LMK · Accepted Answer · 2014-05-27 05:13:21Z

0

Please read method description in the docs before using http://docs.oracle.com/javase/7/docs/api/index.html?java/util/ArrayList.html

int     indexOf(Object o)
Returns the index of the first occurrence of the specified element in this list, or -1   if    this list does not contain the element.

and

public int lastIndexOf(Object o)

Returns the index of the last occurrence of the specified element in this list, or -1 if this list does not contain the element. More formally, returns the highest index i such that (o==null ? get(i)==null : o.equals(get(i))), or -1 if there is no such index.

edited May 27, 2014 at 5:13

answered May 27, 2014 at 5:08

LMK

2,9926 gold badges30 silver badges53 bronze badges

Collectives™ on Stack Overflow

ArrayList indexOf

3 Answers 3

Comments

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related