# Notes

# Collections

A Collection is a group of individual objects represented as a single unit.

Java provides Collection Framework which defines several classes and interfaces to represent a group of objects as a single unit.
Java 提供了 Collection Framework,它定义了几个类和接口,来将一组对象表示为单个单元。

Collection 集合
Root interface with basic methods like add() , remove() , contains() , isEmpty() , addAll() , ... etc.
Doesn't allow duplicates.
Can contain duplicates and elements are ordered.
Queue 队列
Typically order elements in FIFO order.
通常以 FIFO 顺序对元素进行排序

# Some cool applicabilities using Collections

auto copyrotate (shift elements by distance)
frequency counter

# SORTS 排序

// Java program to demonstrate working of Collections.sort() 
import java.util.*; 
public class CollectionTest { 
    public static void main(String[] args) { 
        // Create a list of strings 
        ArrayList<String> trendList = new ArrayList<String>(); 
        trendList.add("data science"); 
        trendList.add("digital forensics"); 
        trendList.add("cloud computing"); 
        /* Collections.sort method is sorting the 
        elements of ArrayList in ascending order. */
        // Let us print the sorted list 
        System.out.println("List after the use of" + 
                            " Collection.sort() :\n" + trendList); 


List after the use of Collection.sort() :
[cloud computing, data science, digital forensics]
// Java program to demonstrate working of Collections.reverse() 
public static void main(String[] args) { 
        // ::
        // Let us print the sorted list 
        System.out.println("List after the use of" + 
                            " Collection.sort() :\n" + trendList);


List after the use of Collection.reverse() :
[digital forensics, data science, cloud computing]

# AUTOCOPY (n copies)

// Create copies of some objects. 
List list = Collections.nCopies(4, "Java Power"); 
// Displaying the list returned 
System.out.println("The list returned is :");
Iterator itr = list.iterator();
while (itr.hasNext()) { 
    System.out.print(itr.next() + " ");


The list returned is :
Java Power Java Power Java Power Java Power 


// count the frequency of the words "data science" 
System.out.println("The frequency of the word data science is: " +
                    Collections.frequency(trendList, "data science"));


The frequency of the word data science is: 1


  • Interface allows multiple sorting sequences (ex. Name, id, and price, etc.)

  • Uses compare() method to sort elements
    compare() takes 2 elements as arguments

    public int compare(Object obj1, Object obj2)
  • Comparator is found in java.util package.

# Action plan

  • Create a class to implement the interface
  • Use Arrays.sort to sort the class objects
class Dog {
	int size, weight;
	public Dog(int s, int w){
		size = s;
		weight = w; 
class DogSizeComparator implements Comparator<Dog> {
	public int compare(Dog o1, Dog o2) {
		return o1.size - o2.size;
class DogWeightComparator implements Comparator<Dog> {
 	public int compare(Dog o1, Dog o2) {
		return o1.weight - o2.weight;
public class ArraySort {
 	public static void main(String[] args) {
		Dog d1 = new Dog(2, 50);
		Dog d2 = new Dog(1, 30);
		Dog d3 = new Dog(3, 40);
		Dog[] dogArray = {d1, d2, d3};
		Arrays.sort(dogArray, new DogSizeComparator());	
		Arrays.sort(dogArray, new DogWeightComparator());	
 	public static void printDogs(Dog[] dogs){
		for(Dog d: dogs)
			System.out.print("size="+d.size + " weight=" + 
                  d.weight + " ");


size=2 weight=50 size=1 weight=30 size=3 weight=40 
size=1 weight=30 size=2 weight=50 size=3 weight=40 
size=1 weight=30 size=3 weight=40 size=2 weight=50 

# Queue Interface

The Queue interface is available in java.util package and extends the Collection interface.
Queue 接口在 java.util 包中可用,并且扩展了 Collection 接口。

The queue collection is used to hold the elements about to be processed and provides various operations like the insertion, removal etc.

It is an ordered list of objects with its use limited to insert elements at the end of the list and deleting elements from the start of list i.e. it follows the FIFO or the First-In-First-Out principle.
它是对象的有序列表,其使用仅限于在列表末尾插入元素,并从头开始删除元素,即遵循 FIFO 或先进先出原则。

Queue example: Implements LinkedList
import java.util.LinkedList; 
import java.util.Queue; 
public class QueueExample  { 
  public static void main(String[] args)  {
    Queue<Integer> q = new LinkedList<>(); 
    // Adds elements {0, 1, 2, 3, 4} to queue 
    for (int i=0; i<5; i++) 
    // Display contents of the queue. 
    System.out.println("Elements of queue-"+q); 
    // To remove the head of queue. 
    int removedele = q.remove(); 
    System.out.println("removed element-" + removedele); 
    // To view the head of queue 
    int head = q.peek(); 
    System.out.println("head of queue-" + head); 
    //Show queue size 
    int size = q.size(); 
    System.out.println("Size of queue-" + size); 

# Map Interface

general key/value
map interface
most widely
used Map
HashMap does not allow duplicate keys however it allows to have duplicate values.
Implementation example: Map generic type interface
import java.util.Collection;
import java.util.HashMap;
import java.util.Map;
import java.util.Set;
Map<String, String> states = new HashMap<String, String>(); 
// store values in map
states.put("ca", "California"); 
states.put("az", "Arizona"); 
states.put("mn", "Minnesota"); 
states.put("nj", "New Jersey");
// retrieve values from map
states.get("ca"); // returns "California"
states.get("nj"); // returns "New Jersey"
// Pull out live Collection of all the values.
Collection<String> values = states.values();
System.out.println(values); // [Minnesota, New Jersey,… Arizona] 
// Use keySet to pull out live set of the keys -- use to print key->value for the whole map. 
// The order of the keys is random for a HashSet.
Set<String> keys = states.keySet();
for (String key:keys) {  
	System.out.println(key + "->" + states.get(key)); 


nj->New Jersey
ca->The Golden State