ArrayList a1 = new ArrayList<>();
a1.add("A");
a1.add("B");
a1.add("C");
a1.add("D");

// Use an Iterator
Iterator<String> itr = a1.iterator();
System.out.print("Use Iterator: ");
while(itr.hasNext())
    System.out.print( itr.next() +  " ");
System.out.println();

// spliterator
Spliterator<String> spl1 = a1.spliterator();
System.out.print("Use spliterator: ");
while(spl1.tryAdvance( (n) -> System.out.print(n + " ")));                                     // notice the lambda expression
System.out.println();

// Use the new for-loop
System.out.print("Use for-loop: ");
for(String s : a1)
    System.out.print(s +  " ");
System.out.println();

// Get element
System.out.println("Use get() and size(): " + a1.get(0) +  " " + a1.get(a1.size() - 1));       // use get() and size() method

// Remove element
a1.remove(2);                                                                                  // remove 'C' element
System.out.print("Use remove(): ");
for(String s : a1)
    System.out.print(s +  " ");
System.out.println();

Note: output will be
    Use Iterator: A B C D 
    Use for-loop: A B C D
    Use spliterator: A B C D 
    Use get() and size(): A D
    Use remove(): A B D 

HashSet hs1 = new HashSet<>();
hs1.add("Paul");
hs1.add("Will");
hs1.add("Moore");
hs1.add("Graham");

// Use Iterator
Iterator<String> itr = hs1.iterator();
System.out.print("Use Iterator: ");
while(itr.hasNext())                                        // there are no guarantees to ordering
    System.out.print( itr.next() +  " ");
System.out.println();

// Use new for-loop
System.out.print("Use for-loop: ");
for(String s : hs1)                                         // there are no guarantees to ordering
    System.out.print(s +  " ");
System.out.println();

// Get element
// There are no gets methods, you have to loop through the set and use equals

// Remove element
//String will = "Will";                                                 
hs1.remove("will");                                      // you can also pass other objects
System.out.print("Use remove(): ");
for(String s : hs1)
    System.out.print(s +  " ");
System.out.println();

Note: the ouptut will be
    Use Iterator: Moore Graham Will Paul 
    Use for-loop: Moore Graham Will Paul 
    Use remove(): Moore Graham Will Paul 

ArrayDeque ad1 = new ArrayDeque<>();
ad1.push("A");
ad1.push("B");
ad1.push("C");
ad1.push("D");

System.out.print("Pop the stack: ");

while(ad1.peek() != null)
    System.out.print(ad1.pop() + " ");
System.out.println();
System.out.println("Dequeue ad1 has now " + ad1.size() +  " elements");  // we have popped (removed) all the element

Note: the output will be
    Pop the stack: D C B A 
    Dequeue ad1 has now 0 elements

HashMap<String, Integer> hm1 = new HashMap<>();

hm1.put("Paul", 1);
hm1.put("Will", 2);
hm1.put("Moore", 3);
hm1.put("Graham", 4);

// Using for-loop
// you can also use KeySet() to get keys or values() to obtain values
for(Map.Entry<String, Integer> me : hm1.entrySet())
    System.out.println("Key: " + me.getKey() + "\t\tValue: " + me.getValue());
System.out.println();

// using iterators
Iterator<Map.Entry<String, Integer>> itr = hm1.entrySet().iterator();
while(itr.hasNext())
{
    Map.Entry<String, Integer> me = itr.next();
    System.out.println("Key: " + me.getKey() + "\t\tValue: " + me.getValue());
}

// Change a value of a specified key
System.out.println("\nPaul's value : " + hm1.get("Paul"));
hm1.put("Paul", 5);
System.out.println("\nPaul's value : " + hm1.get("Paul"));

package uk.co.datadisk;

import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ConcurrentMap;

class FirstWorker3 implements Runnable {

    private ConcurrentMap<String, Integer> map;

    public FirstWorker3(ConcurrentMap<String, Integer> map) {
        this.map = map;
    }

    @Override
    public void run() {
        try {
            map.put("B",1);
            map.put("H",2);
            map.put("F",3);
            Thread.sleep(1000);
            map.put("A",4);
            Thread.sleep(1000);
            map.put("E",5);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

class SecondWorker3 implements Runnable {

    private ConcurrentMap<String, Integer> map;

    public SecondWorker3(ConcurrentMap<String, Integer> map) {
        this.map = map;
    }

    @Override
    public void run() {
        try {
            Thread.sleep(5000);
            System.out.println(map.get("A"));
            Thread.sleep(1000);
            System.out.println(map.get("E"));
            Thread.sleep(1000);
            System.out.println(map.get("C"));
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

public class ConcurrentMap1 {

    public static void main(String[] args) {

        ConcurrentMap<String, Integer> map = new ConcurrentHashMap<>();

        FirstWorker3 firstWorker = new FirstWorker3(map);
        SecondWorker3 secondWorker = new SecondWorker3(map);

        new Thread(firstWorker).start();
        new Thread(secondWorker).start();

    }
}

TreeMap<String, Integer> tm1 = new TreeMap<>();

tm1.put("Paul", 1);
tm1.put("Will", 2);
tm1.put("Moore", 3);
tm1.put("Graham", 4);

// Using for-loop
// you can also use KeySet() to get keys or values() to obtain values
for(Map.Entry<String, Integer> me : tm1.entrySet())
    System.out.println("Key: " + me.getKey() + "\t\tValue: " + me.getValue());
System.out.println();

// using iterators
Iterator<Map.Entry<String, Integer>> itr = tm1.entrySet().iterator();
while(itr.hasNext())
{
    Map.Entry<String, Integer> me = itr.next();
    System.out.println("Key: " + me.getKey() + "\t\tValue: " + me.getValue());
}

// Change a value of a specified key
System.out.println("\nPaul's value : " + tm1.get("Paul"));
tm1.put("Paul", 5);
System.out.println("\nPaul's value : " + tm1.get("Paul"));

public class Book implements Comparable<Book> {

    private String authorName;
    private String title;
    private int numOfPages;

    public Book(String authorName, String title, int numOfPages) {
        this.authorName = authorName;
        this.title = title;
        this.numOfPages = numOfPages;
    }

    // Getters and Setters ....

    public int getNumOfPages() {
        return numOfPages;
    }

    @Override
    public String toString() {
        return this.authorName + "-" + this.title + "-" + this.numOfPages;
    }

    @Override
    public int compareTo(Book otherBook) {
        // return Integer.compare(this.numOfPages, otherBook.getNumOfPages());    // ASCENDING
        return -Integer.compare(this.numOfPages, otherBook.getNumOfPages());      // DESCENDING
    }
}

// Code to be used in main
List<Book> books = new ArrayList<>();

books.add(new Book("Will Hay","title1",223));
books.add(new Book("Moore Marriot","title2",33));
books.add(new Book("Graham Moffatt","title3",891));
books.add(new Book("Arthur Askey","title4",34));

// Sorting by number of pages
Collections.sort(books);             // will use the compareTo() method in the Book class

System.out.println(books);

// Create a Comparator class
class MyComparator implements Comparator<String> {

    // You can code any ordering you like in the compare method, it returns either 1 (greater than), 0 (equal), -1 (less than)
    public int compare(String aStr, String bStr) {
        return bStr.compareTo(aStr);
    }
}

// Code to be used in main

// Create a TreeSet which is not in order, however natural ordering will be used
        TreeSet<String> ts1 = new TreeSet<>();
        ts1.add("E");
        ts1.add("B");
        ts1.add("D");
        ts1.add("A");
        ts1.add("C");

        System.out.print("Set Values: ");
        for(String s: ts1)
            System.out.print(s + " ");
        System.out.println();

        // Create a TreeSet that will use our own Comparator class
        TreeSet<String> ts2 = new TreeSet<>(new MyComparator());
        ts2.add("E");
        ts2.add("B");
        ts2.add("D");
        ts2.add("A");
        ts2.add("C");

        System.out.print("Set Values: ");
        for(String s: ts2)
            System.out.print(s + " ");
        System.out.println();