Java arraylist find index by value

Find element in ArrayList using indexOf lastIndexOf methods example

Find an element in ArrayList using indexOf lastIndexOf methods example shows how to find element in ArrayList using indexOf and lastIndexOf methods in Java. It also shows how to find an object of a custom class in ArrayList by implementing equals and hashCode methods.

How to find an element in ArrayList using the indexOf lastIndexOf methods in Java?

The ArrayList indexOf method returns the index of an element in the ArrayList object.

This method returns the index of the first occurrence of the specified object in the ArrayList. If the specified object is not found, the indexOf method returns -1.

Similarly, use the lastIndexOf method of the ArrayList class to find an element’s last index.

This method returns the index of the last occurrence of the specified object in the ArrayList. If the specified object is not found in the ArrayList, this method returns -1.

How to find element index in ArrayList of custom class objects?

Consider below given example which uses an ArrayList of custom class objects.

Even if the object was in the ArrayList, the indexOf method could not find it and returned -1. That is because the indexOf and lastIndexOf methods rely on the equals method to compare the objects and since we have not implemented the equals method in the Customer class, it could not find it.

Here is the updated version of Customer class which implements equals and hashCode methods.

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Java ArrayList indexOf() Method example

Java.util.ArrayList class method indexOf(Object o) is used to find out the index of a particular element in a list.

Method indexOf() Signature

public int indexOf(Object o)

This method returns -1 if the specified element is not present in the list.

ArrayList indexOf() Method example

In the following example we have an arraylist of strings and we have added few elements to the arraylist. Here we are using the indexOf() method to find the index of few specified elements in the ArrayList.

import java.util.ArrayList; public class IndexOfExample < public static void main(String[] args) < ArrayListal = new ArrayList(); al.add("AB"); al.add("CD"); al.add("EF"); al.add("GH"); al.add("IJ"); al.add("KL"); al.add("MN"); System.out.println("Index of 'AB': "+al.indexOf("AB")); System.out.println("Index of 'KL': "+al.indexOf("KL")); System.out.println("Index of 'AA': "+al.indexOf("AA")); System.out.println("Index of 'EF': "+al.indexOf("EF")); > >

Index of 'AB': 0 Index of 'KL': 5 Index of 'AA': -1 Index of 'EF': 2

As you can see in the output that the index of element ‘AA’ is returned as -1 because this element does not exist in the ArrayList.

ArrayList indexOf() Method example if duplicate elements are present

In the following example, the element 90 is present twice in ArrayList, at the index 0 and index 3. When we used the indexOf() method to find the index of 90, we got the index of first occurrence of 90, this is because this method returns the index of first occurrence of the specified element. If you want to find out the last occurrence of the specified element use lastIndexOf() method instead.

import java.util.ArrayList; public class JavaExample < public static void main(String[] args) < ArrayListal = new ArrayList(); al.add(90); al.add(15); al.add(20); al.add(90); System.out.println("Index of 90: "+al.indexOf(90)); System.out.println("Index of 20: "+al.indexOf(20)); System.out.println("Index of 15: "+al.indexOf(15)); > >

Output:

Reference:

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Comments

If I want to take the ekement from user and for that element I want to find the index??
Its something like this
String s=s.next();
I want to find the index for s

Yeah we can but why would you want to do that? The purpose of indexOf() method is to find an element in a List, what is the point to find the position of all the elements. You can print an entire ArrayList if you want.

well how come index of aa is -1…….
you can check out my code.. ArrayList list1 = new ArrayList(3);
String x= list1.get(0);
int i= list1.indexOf(x);
System.out.println(i)
which will give you 0…

Your code gave you 0 because you are finding the index of the first element. The index of first element is 0 in ArrayList. The example which I have shared above returns -1 for element ‘AA’ because the element doesn’t exist in the ArrayList. The indexOf() method returns -1 if the element is not present in the List.

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How to get index of object in arraylist java

As we know ArrayList in java works based on the index and each value stores in a unique index. There may be a situation when we want to find the index of an object. Then we can use the indexOf(Object o) and lastIndexOf(Object o). Lets see how to use java arraylist indexof() method.

Here is the table content of the article will we will cover this topic.
1. index(object o) method
2. lastIndexOf(Object o) method

index(object o) method

The index(Object o) method is used to get the index of the specified element. Suppose if you have duplicate elements in ArrayList then it returns the index value of first occurrence of the element. It returns the int value and if the element doesn’t exist in the ArrayList it returns -1.
Sntytax of java arraylist indexof() method

Where, Object represents the type of elements in ArrayList .
o, is the element which you want to find in ArrayList .

import java.util.ArrayList; import java.util.Iterator; public class ArrayListExample < public static void main(String[] args) < ArrayListlistOfNumbers = new ArrayList(); listOfNumbers.add(111); listOfNumbers.add(222); listOfNumbers.add(333); listOfNumbers.add(444); listOfNumbers.add(555); System.out.println("Index value of 111: "+ listOfNumbers.indexOf(111)); System.out.println("Index value of 555: "+ listOfNumbers.indexOf(555)); System.out.println("Index value of 123: "+ listOfNumbers.indexOf(123)); > >

Output:Index value of 111: 0
Index value of 555: 4
Index value of 123: -1

The index value of ArrayList start from 0 and ends to length-1. Here we have 5 elements in ArrayList from index 0 to 4. So when we are getting first element 111 then it returning index value 0 and for 555 it returning 4.

lastIndexOf(Object o) method

The lastIndexOf(Object o) method is also used to find the index of element but it returns the last index of the specified element. It returns the index value of the last occurrence of the element. It returns the int value. If the element doesn’t exist in ArrayList it returns -1.

Where, Object represents the type of elements in ArrayList.
o, is the element which you want to find in ArrayList.

package package1; import java.util.ArrayList; import java.util.Iterator; public class ArrayListExample < public static void main(String[] args) < ArrayListlistOfNumbers = new ArrayList(); listOfNumbers.add(111); listOfNumbers.add(222); listOfNumbers.add(333); listOfNumbers.add(444); listOfNumbers.add(555); listOfNumbers.add(111); listOfNumbers.add(444); listOfNumbers.add(111); System.out.println("Last Index value of 111: "+ listOfNumbers.lastIndexOf(111)); System.out.println("Last Index value of 444: "+ listOfNumbers.lastIndexOf(444)); System.out.println("Last Index value of 123: "+ listOfNumbers.lastIndexOf(123)); > >

Output: Last Index value of 111: 7
Last Index value of 444: 6
Last Index value of 123: -1

As you can see the element 111 is presented at three positions 0, 5, 7. But this method returns only last position of the element.

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