Array, List, Set, and Map

 Here’s a concise overview of key Java collections: Array, List, Set, and Map along with basic usage examples and their documentation.

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✅ 1. Array

An Array is a data structure that stores a fixed-size sequence of elements of the same type.

Documentation:

• Array in Java is an object that holds a fixed number of values of a single type.

• Arrays are zero-indexed, meaning the first element is at index 0.

• Arrays are immutable in size once created (i.e., you can't resize them).

Example:

public class Main {
    public static void main(String[] args) {
        // Declare and initialize an array
        int[] arr = {10, 20, 30, 40, 50};

        // Accessing array elements
        System.out.println("Element at index 2: " + arr[2]);

        // Looping through array
        for (int i = 0; i < arr.length; i++) {
            System.out.println("Element at index " + i + ": " + arr[i]);
        }
    }
}

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✅ 2. List

A List is an ordered collection (sometimes called a sequence). It allows for duplicates and provides positional access to elements.

Key Classes:

• ArrayList (most commonly used)

• LinkedList

• Vector

Documentation:

• List is part of the java.util package.

• Lists maintain the order of insertion and allow duplicate elements.

• You can access elements by their index.

Example (Using ArrayList):

import java.util.ArrayList;

public class Main {
    public static void main(String[] args) {
        // Create a List (ArrayList)
        ArrayList<String> list = new ArrayList<>();

        // Add elements
        list.add("Apple");
        list.add("Banana");
        list.add("Cherry");

        // Accessing elements
        System.out.println("Element at index 1: " + list.get(1));

        // Looping through List
        for (String item : list) {
            System.out.println(item);
        }
    }
}

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✅ 3. Set

A Set is a collection that does not allow duplicate elements. It models the mathematical set abstraction and is part of the Java Collections Framework.

Key Classes:

• HashSet (does not maintain any order)

• LinkedHashSet (maintains insertion order)

• TreeSet (stores elements in sorted order)

Documentation:

• Set is part of the java.util package.

• No duplicates are allowed.

• Elements are unordered in HashSet but sorted in TreeSet.

Example (Using HashSet):

import java.util.HashSet;

public class Main {
    public static void main(String[] args) {
        // Create a Set (HashSet)
        HashSet<String> set = new HashSet<>();

        // Add elements
        set.add("Apple");
        set.add("Banana");
        set.add("Apple");  // Duplicate, won't be added

        // Looping through Set
        for (String item : set) {
            System.out.println(item);
        }
    }
}

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✅ 4. Map

A Map is an object that maps keys to values. A Map cannot contain duplicate keys, and each key can map to only one value.

Key Classes:

• HashMap (does not maintain order)

• LinkedHashMap (maintains insertion order)

• TreeMap (stores keys in sorted order)

Documentation:

• Map is part of the java.util package.

• Each key in a map must be unique. Values can be duplicated.

• Common methods include put(), get(), containsKey(), and remove().

Example (Using HashMap):

import java.util.HashMap;

public class Main {
    public static void main(String[] args) {
        // Create a Map (HashMap)
        HashMap<String, Integer> map = new HashMap<>();

        // Add key-value pairs
        map.put("Apple", 1);
        map.put("Banana", 2);
        map.put("Cherry", 3);

        // Access value using key
        System.out.println("Value for 'Apple': " + map.get("Apple"));

        // Looping through Map
        for (String key : map.keySet()) {
            System.out.println(key + ": " + map.get(key));
        }
    }
}

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Summary of Collections in Java:

Collection Type	Allows Duplicates	Order Maintained	Key-Value Pair
Array	Yes	No	No
List	Yes	Yes	No
Set	No	Depends on Implementation	No
Map	No (Keys)	Depends on Implementation	Yes (Keys and Values)

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📚 Useful Documentation Links:

• Java Array

• Java List

• Java Set

• Java Map

Would you like to see more examples or details on any of these?