How to search in a List of Java object?

A List is a sequence of elements of similar data types, and like an array, its elements are stored at specific indices, which can be accessed and searchable through the index (i.e., starting at index 0).

We can search through each element in the list and retrieve its index (or position) if it is found in the list. We can search in a List in the following ways -

• Using indexOf() Method of List Interface
• Using lastIndexOf() Method
• Using Comparison Approach

Searching in a List using indexOf() Method

The indexOf() method of the List interface searches and returns the first occurrence of the specified element in the list, or -1 if the list does not contain the specified element. Following is the syntax of the indexOf() method:

int indexOf(Object o)

Here, o is an element that needs to be searched.

Example

In the following example, we use the indexOf() method to search a specified element 30 in the List {10, 20, 30, 40}:

import java.util.ArrayList;
import java.util.List;

public class searchElement {
   public static void main(String[] args) {
   
      //creating a list
      List<Integer> list = new ArrayList<>();
      
      //adding element to it
      list.add(10);
      list.add(20);
      list.add(30);
      list.add(40);
      
      System.out.println("The list is: " + list);
      
      int ele = 30;
      System.out.println("The item needs to be searched: " + ele);
      
      //using indexOf() method
      int result = list.indexOf(ele);
      if(result != -1){
         System.out.println("Yes! '" + ele + "' is present in the list.");
      }
   }
}

The above program produces the following output:

The list is: [10, 20, 30, 40]
The item needs to be searched: 30
Yes! '30' is present in the list.

Using lastIndexOf() Method

The lastIndexOf() method of the List interface searches and returns an index of the last occurrence of the specified element in this list, or -1 if this list does not contain the specified element. Following is the syntax of the lastIndexOf() method:

int lastIndexOf(Object o)

Here, o is an element that needs to be searched.

Example

The following program uses the lastIndexOf() method to search and return the index of the last occurrence of the specified element in the list {1, 2, 3, 4, 3, 2, 1}:

import java.util.ArrayList;
import java.util.List;

public class searchElement {
   public static void main(String[] args) {
   
      //creating a list
      List<Integer> list = new ArrayList<>();
      
      //adding element to it
      list.add(1);
      list.add(2);
      list.add(3);
      list.add(4);
      list.add(3);
      list.add(2);
      list.add(1);
      
      System.out.println("The list is: " + list);
      
      int ele = 2;
      System.out.println("The item needs to searched: " + ele);
      
      //using lastIndexOf() method
      int result = list.lastIndexOf(ele);
      if(result != -1){
         System.out.println("Yes!, the element " + ele + " is present in the List.");
      }
   }
}

Following is the output of the above program:

The list is: [1, 2, 3, 4, 3, 2, 1]
The item needs to searched: 2
Yes!, the element 2 is present in the List

Using the Comparison Approach

This is another way of searching in a List. We will iterate through the list and compare each element with the given element (using == operator). If the element is found equal to any of the list elements, the index of that element will be displayed.

Example

In this example, we loop through the list to compare the given element with each item in the list. If they are equal, the index (position) of the element in the list will be displayed:

import java.util.ArrayList;
import java.util.List;

public class searchElement {
   public static void main(String[] args) {
   
      //creating a list
      List<String> vowels = new ArrayList<>();
      
      //adding element to it
      vowels.add("A");
      vowels.add("E");
      vowels.add("I");
      vowels.add("O");
      vowels.add("U");
      
      System.out.println("The list is: " + vowels);
      
      String ele = "O";
      System.out.println("The item needs to searched: " + ele);
      
      for(int i = 0; i<vowels.size(); i++){
         if(vowels.get(i) == ele){
            System.out.println("The element '" + ele +"' is found at index " + i);
            break;
         }
      }
   }
}

Below is the output of the above program:

The list is: [A, E, I, O, U]
The item needs to searched: O
The element 'O' is found at index 3