Collections in Java

Lists

package com.example.collections;

import java.util.*;

class JavaCollections {
    public static void main(String[] args) {

        // Lists - ArrayList

        List<String> strings = List.of("Java", "is", "cool");
        String message = String.join(" ", strings);
        System.out.println(message);
        // message returned is: "Java is cool"

        // ArrayList
        
        ArrayList<String> words = new ArrayList<String>();
        words.add("Name");
        words.add("is");
        words.add("Bond");
        words.add("James Bond");
        words.add("Bond");
        
        for (String word : words) {
            System.out.println(word);
        }

        System.out.println(words.get(3)); // James Bond
        System.out.println(words.size()); // 5
        System.out.println(words.contains("Bond")); // true
        System.out.println(words.indexOf("Bond")); // 2
        System.out.println(words.remove(3)); // James Bond
        System.out.println(words.size()); // 4
        System.out.println(words.remove("Bond")); // true
        System.out.println(words.size()); // 3
        // Join words with a space
        System.out.println(String.join(" ", words)); // Name is Bond

    }
    
}

Sorting Containers

package com.example.collections;

import java.util.*;

class JavaCollections {
    public static void main(String[] args) {

        // Sorting Lists

        Collections.reverse(words);
        Collections.reverse(words);

        Collections.sort(words);
        System.out.println(String.join(" ", words)); // Bond Name is

        Collections.sort(words, Collections.reverseOrder());
        System.out.println(String.join(" ", words)); // is Name Bond

        // custom comparator
        Collections.sort(words, (String a, String b) -> {
            // return a.length() - b.length();
            return a.charAt(a.length() - 1) - b.charAt(b.length() - 1);
        });

        Collections.sort(words);
        int position = Collections.binarySearch(words,  "abc");
        System.out.println(position); // 0


        System.out.println(String.join(" ", words)); // is Name Bond


    }
    
}

Maps

package com.example.collections;

import java.util.*;

class JavaCollections {
    public static void main(String[] args) {


        // Maps - HashMap, TreeMap

        // Unordered Map - HashMap
        HashMap<String, String> map = new HashMap<String, String>();
        map.put("James Bond", "007");

        if (map.get("James Bond") != null) {
            System.out.println(map.get("James Bond")); // 007
        }
        System.out.println(map.get("James Bondy")); // null

        // Ordered Map - TreeMap

        Map<String, String> map2 = new TreeMap<String, String>();
        map2.put("008", "James Bondy");
        map2.put("007", "James Bond");

        for (String key : map2.keySet()) {
            System.out.println(key + " " + map2.get(key));
        }

        
    }
    
}

Sets

package com.example.collections;

import java.util.*;

class JavaCollections {
    public static void main(String[] args) {

        // Sets

        // Unordered Set - HashSet
        Set<String> strings2 =
            new HashSet<>(List.of("Java", "is", "very", "cool"));

        System.out.println(strings2.contains("java"));

        String message2 = String.join("-", strings2);
        System.out.println(message2);
        // message returned is: "Java-is-very-cool"


        // Ordered Set - TreeSet
        SortedSet<String> strings3 = new TreeSet<>();
        strings3.add("java");
        strings3.add("is");
        strings3.add("very");
        strings3.add("cool");


        String message3 = String.join("-", strings3);
        System.out.println(message3);

    }
    
}

Queues

package com.example.collections;

import java.util.*;

class JavaCollections {
    public static void main(String[] args) {

        // Queues

        // PriorityQueue

        PriorityQueue<String> queue = new PriorityQueue<>();

        queue.add("James Bond");
        queue.add("007");
        queue.add("Smart and cool");

        System.out.println(queue.peek()); // 007 - returns the head of the queue, does not remove it
        queue.remove(); // removes the head of the queue
        System.out.println(queue.poll()); // James Bond - removes the head of the queue

        // Deque

        Deque<String> deque = new ArrayDeque<>();
        deque.addLast(message3);
        System.out.println(deque.peekLast());
        System.out.println(deque.pollLast());

        
    }
    
}