Java LinkedList indexOf() method
Syntax and Examples

Introduction

The indexOf() method in Java's LinkedList class is a handy tool for finding the first occurrence of a specified element within the list. Think of it like searching for a specific book on a bookshelf – you want to know its position if it exists.

Syntax

public int indexOf(Object o)

Parameters

Parameter	Description
o	The element to search for within the list. This can be any object. If the list doesn't contain this object, it returns -1.

Return Value

The indexOf() method returns the index of the first occurrence of the specified element in the list. If the element is not found, it returns -1.

Examples

Example 1: Finding an Existing Element

This example demonstrates how to use indexOf() to find a value that's already present in the LinkedList.

import java.util.LinkedList;

public class LinkedListIndexOfExample1 {
    public static void main(String[] args) {
        LinkedList<String> myList = new LinkedList<>();
        myList.add("apple");
        myList.add("banana");
        myList.add("cherry");

        int index = myList.indexOf("banana");
        System.out.println(index);
    }
}

1

Explanation: We create a LinkedList of Strings and add three elements. Then, we call indexOf() to find the index of "banana". Since "banana" is at index 1, that's what gets returned.

Example 2: Finding an Element That Doesn’t Exist

This example shows what happens when you try to find an element that isn't in the list. It demonstrates how `indexOf()` returns -1 in such cases.

import java.util.LinkedList;

public class LinkedListIndexOfExample2 {
    public static void main(String[] args) {
        LinkedList<Integer> myList = new LinkedList<>();
        myList.add(10);
        myList.add(20);
        myList.add(30);

        int index = myList.indexOf(40); // 40 is not in the list
        System.out.println(index);
    }
}

-1

Explanation: We create a LinkedList of Integers and try to find 40, which isn't present in the list. Therefore, indexOf() returns -1.

Example 3: Using indexOf() with Objects

This example illustrates using indexOf() with custom objects. The equals method of the object is used to compare for equality.

import java.util.LinkedList;

class MyObject {
    int value;

    public MyObject(int value) {
        this.value = value;
    }

    @Override
    public boolean equals(Object obj) {
        if (obj instanceof MyObject) {
            return this.value == ((MyObject) obj).value;
        } else {
            return false;
        }
    }
}

public class LinkedListIndexOfExample3 {
    public static void main(String[] args) {
        LinkedList<MyObject> myList = new LinkedList<>();
        myList.add(new MyObject(5));
        myList.add(new MyObject(10));
        myList.add(new MyObject(15));

        MyObject target = new MyObject(10);
        int index = myList.indexOf(target);
        System.out.println(index);
    }
}

1

Explanation: We create a LinkedList of MyObject instances and then use `indexOf()` to find an object with the value 10. The equals method in MyObject is used for comparison, so it correctly identifies the element at index 1.

Example 4: Handling Null Values

This example demonstrates what happens when you attempt to search for a null value within a LinkedList.

import java.util.LinkedList;

public class LinkedListIndexOfExample4 {
    public static void main(String[] args) {
        LinkedList<String> myList = new LinkedList<>();
        myList.add("apple");
        myList.add(null);
        myList.add("cherry");

        int index = myList.indexOf(null);
        System.out.println(index);
    }
}

1

Explanation: The LinkedList contains a null element at index 1, so indexOf(null) returns 1.