LinkedList indexOf() method - Syntax and Examples

Learn about the Java LinkedList indexOf() method. This tutorial explains its syntax, parameters, return value, and provides practical examples to demonstrate how it works.

This reference explains LinkedList indexOf() method - Syntax and Examples through its syntax, examples and boundaries. Use the linked course lessons when the surrounding Java concepts are unfamiliar.

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.

public int indexOf(Object o)

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.

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.

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.

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.

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.

Check the example’s types, inputs and documented behavior before copying it into an application. Some APIs require external libraries, files or platform services; consult their setup notes.

Related explanations: Java Data Types - Primitive and Non-Primitive, Java Class - Blueprint of Objects, Exceptions in Java Introduction.

Supporting documentation: Official Java learning guide.