List Interface in Java

The List interface in Java extends the Collection interface and represents an ordered collection of elements that allows duplicates. It provides methods to access, insert, delete, and manipulate elements by their index.

1. Key Implementations of List

Class	Description
ArrayList	Dynamic array (fast random access, slower insertions/deletions in the middle).
LinkedList	Doubly-linked list (fast insertions/deletions, slower random access).
Vector	Legacy synchronized version of ArrayList (thread-safe but slower).
Stack	Extends Vector (LIFO structure with push(), pop(), peek()).

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2. List Interface Methods (Detailed)

📌 Basic Operations

Method	Description	Example
boolean add(E e)	Appends an element to the end of the list.	list.add("Java");
void add(int index, E e)	Inserts an element at a specified position.	list.add(1, "Python");
E get(int index)	Returns the element at the specified position.	String lang = list.get(0);
E set(int index, E e)	Replaces the element at the given index.	list.set(1, "C++");
E remove(int index)	Removes and returns the element at the index.	String removed = list.remove(0);
boolean remove(Object o)	Removes the first occurrence of the object.	list.remove("Java");
int size()	Returns the number of elements.	int len = list.size();
boolean isEmpty()	Checks if the list is empty.	if (list.isEmpty()) {...}
void clear()	Removes all elements.	list.clear();
boolean contains(Object o)	Checks if the element exists.	if (list.contains("Java")) {...}

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📌 Bulk Operations (Working with Multiple Elements)

Method	Description	Example
boolean addAll(Collection<? extends E> c)	Adds all elements from another collection.	list.addAll(otherList);
boolean addAll(int index, Collection<? extends E> c)	Inserts all elements at a given position.	list.addAll(1, otherList);
boolean removeAll(Collection<?> c)	Removes all elements present in the given collection.	list.removeAll(toRemove);
boolean retainAll(Collection<?> c)	Keeps only elements present in the given collection.	list.retainAll(toKeep);

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📌 Search & Index Operations

Method	Description	Example
int indexOf(Object o)	Returns the first index of the element (or -1 if not found).	int idx = list.indexOf("Java");
int lastIndexOf(Object o)	Returns the last index of the element (or -1).	int lastIdx = list.lastIndexOf("Java");

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📌 Sublist & List Iteration

Method	Description	Example
List<E> subList(int fromIndex, int toIndex)	Returns a view of the portion of the list.	List<String> sub = list.subList(1, 3);
Iterator<E> iterator()	Returns an iterator for sequential access.	Iterator<String> it = list.iterator();
ListIterator<E> listIterator()	Returns a bidirectional iterator (supports hasPrevious(), previous()).	ListIterator<String> li = list.listIterator();

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📌 Conversion & Array Operations

Method	Description	Example
Object[] toArray()	Converts the list to an array.	Object[] arr = list.toArray();
<T> T[] toArray(T[] a)	Converts the list to a typed array.	String[] arr = list.toArray(new String[0]);

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3. Example: Using List Methods

import java.util.*;

public class ListExample {
    public static void main(String[] args) {
        List<String> languages = new ArrayList<>();

        // 1. Adding elements
        languages.add("Java");
        languages.add("Python");
        languages.add(1, "C++");  // Insert at index 1

        System.out.println(languages);  // [Java, C++, Python]

        // 2. Accessing elements
        String first = languages.get(0);  // "Java"
        System.out.println("First element: " + first);

        // 3. Updating elements
        languages.set(1, "JavaScript");
        System.out.println(languages);  // [Java, JavaScript, Python]

        // 4. Removing elements
        languages.remove(2);  // Removes "Python"
        languages.remove("Java");  // Removes "Java"
        System.out.println(languages);  // [JavaScript]

        // 5. Checking size and emptiness
        System.out.println("Size: " + languages.size());  // 1
        System.out.println("Is empty? " + languages.isEmpty());  // false

        // 6. Sublist example
        languages.add("Kotlin");
        languages.add("Go");
        List<String> sub = languages.subList(0, 2);
        System.out.println("Sublist: " + sub);  // [JavaScript, Kotlin]

        // 7. Iterating using ListIterator
        ListIterator<String> it = languages.listIterator();
        while (it.hasNext()) {
            System.out.println(it.next());
        }
    }
}

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4. Key Points to Remember

✔ List maintains insertion order (unlike Set).

✔ Allows duplicates (unlike Set).

✔ Supports index-based operations (get, set, add, remove).

✔ Common implementations: ArrayList (fast access), LinkedList (fast insertions/deletions).

✔ ListIterator allows bidirectional traversal.

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5. When to Use List?

✅ When order matters (e.g., maintaining a sequence).

✅ When duplicates are allowed.

✅ When frequent access by index is needed (ArrayList).

✅ When frequent insertions/deletions are needed (LinkedList).

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Summary Table of Key Methods

Method	Action
add(E e)	Appends an element
add(int index, E e)	Inserts at a position
get(int index)	Retrieves by index
set(int index, E e)	Replaces an element
remove(int index)	Removes by index
remove(Object o)	Removes first occurrence
indexOf(Object o)	Finds first index
subList(from, to)	Gets a sublist
listIterator()	Bidirectional traversal

This covers all major methods of the List interface in Java. 🚀 Let me know if you need further clarifications!