Java & DS Algorithms: August 2017

Thursday, August 31, 2017

HashSet vs. TreeSet vs. LinkedHashSet

HashSet is Implemented using a hash table. Elements are not ordered. The add, remove, and contains methods have constant time complexity O(1).

TreeSet is implemented using a tree structure(red-black tree in algorithm book). The elements in a set are sorted, but the add, remove, and contains methods has time complexity of O(log (n)). It offers several methods to deal with the ordered set like first(), last(), headSet(), tailSet(), etc.

LinkedHashSet is between HashSet and TreeSet. It is implemented as a hash table with a linked list running through it, so it provides the order of insertion. The time complexity of basic methods is O(1).

TreeSet Example

TreeSet<Integer> tree = new TreeSet<Integer>();
tree.add(12);
tree.add(63);
tree.add(34);
tree.add(45);
 
Iterator<Integer> iterator = tree.iterator();
System.out.print("Tree set data: ");
while (iterator.hasNext()) {
    System.out.print(iterator.next() + " ");
}

Output is sorted as follows:

Tree set data: 12 34 45 63

Now let's define a Dog class as follows:

class Dog {
 int size;
 
 public Dog(int s) {
  size = s;
 }
 
 public String toString() {
  return size + "";
 }
}

Let's add some dogs to TreeSet like the following:

import java.util.Iterator;
import java.util.TreeSet;
 
public class TestTreeSet {
 public static void main(String[] args) {
  TreeSet<Dog> dset = new TreeSet<Dog>();
  dset.add(new Dog(2));
  dset.add(new Dog(1));
  dset.add(new Dog(3));
 
  Iterator<Dog> iterator = dset.iterator();
 
  while (iterator.hasNext()) {
   System.out.print(iterator.next() + " ");
  }
 }
}

Compile ok, but run-time error occurs:

Exception in thread "main" java.lang.ClassCastException: collection.Dog cannot be cast to java.lang.Comparable
 at java.util.TreeMap.put(Unknown Source)
 at java.util.TreeSet.add(Unknown Source)
 at collection.TestTreeSet.main(TestTreeSet.java:22)

Because TreeSet is sorted, the Dog object need to implement java.lang.Comparable's compareTo() method like the following:

class Dog implements Comparable<Dog>{
 int size;
 
 public Dog(int s) {
  size = s;
 }
 
 public String toString() {
  return size + "";
 }
 
 @Override
 public int compareTo(Dog o) {
         return size - o.size;
 }
}

References

https://www.programcreek.com/2013/03/hashset-vs-treeset-vs-linkedhashset/

The output is:

1 2 3

ArrayList vs. LinkedList vs. Vector

COLLECTION HIERARCHY

Performance of ArrayList vs. LinkedList

The time complexity comparison is as follows:

The difference of their performance is obvious. LinkedList is faster in add and remove, but slower in get. Based on the complexity table and testing results, we can figure out when to use ArrayList or LinkedList. In brief, LinkedList should be preferred if:

there are no large number of random access of element
there are a large number of add/remove operations

References

https://www.programcreek.com/2013/03/arraylist-vs-linkedlist-vs-vector/

Arrays.asList() in java

Modifying a List returned by Arrays.asList(array)

asList() convert a fixed array into a fixed size list on which no modification can take place. If try to modify, it gives UnsupportedOperationException

public class ListModified {

public static void main(String[] args) {

Integer[] values = { 1, 3, 7 };

* Returns a fixed-size list backed by

the specified array.

List<Integer> list = Arrays.asList(values);

System.out.println("List size is " + list.size()

+ " List is " + list);

* When trying to modify the list, It gives

* UnsupportedOperationException

list.add(10);

System.out.println(list);

}

Summary

When we use Arrays.asList the size of the returned list is fixed because the list returned is not java.util.ArrayList, but a private static class defined inside java.util.Arrays. So if we add or remove elements from the returned list, an UnsupportedOperationException will be thrown.

Even if I try to remove an element from the list , it would give us UnsupportedOperationException

Sunday, August 27, 2017

Compare Objects using Comparator

Compare Students based on there score. If score is equal , then sort those students based on there name.

import java.util.Arrays;

import java.util.Comparator;

class Student {

String name;

int score;

public Student(String name, int score) {

super();

this.name = name;

this.score = score;

}

public String getName() {

return name;

}

public void setName(String name) {

this.name = name;

}

public int getScore() {

return score;

}

public void setScore(int score) {

this.score = score;

}

public class SortStudents implements Comparator<Student> {

public static void main(String[] args) {

Student[] students = new Student[5];

students[0] = new Student("Paras", 100);

students[1] = new Student("Varun", 200);

students[2] = new Student("Rahul", 300);

students[3] = new Student("Parul", 400);

students[4] = new Student("Shubham", 100);

Arrays.sort(students, new SortStudents());

printArray(students);

}

static void printArray(Student[] students){

for(Student s: students){

System.out.println("Name: "+ s.getName()

+" Score: "+ s.getScore());

}

/*Sort students based on score and if score equals

*then sort using names

**/

public int compare(Student s1, Student s2) {

if(s1.getScore()==s2.getScore())

return s2.getName().compareTo(s1.getName());

else

return s2.getScore()-s1.getScore();

}

Output

Name: Parul Score: 400

Name: Rahul Score: 300

Name: Varun Score: 200

Name: Shubham Score: 100

Name: Paras Score: 100

Monday, August 21, 2017

Implementation of Own BlockingQueue using LinkedList

In previous article BlockingQueue , we've seen famous Producer-Consumer problem being solved by using by default BlockingQueue provided by JDK.

What if we need to implement our own BlockingQueue ?
Below code snippet is self-explanatory and gives clear picture what needs to be done for own BlockingQueue implementation.

package com.concurrent.jdk5;

import java.util.LinkedList;

import java.util.List;

/* Own Implementation of BlockingQueue using LinkedList */

public class OwnBlockingQueue {

public List<Message> queue;

public int limit;

public OwnBlockingQueue(int limit) {

this.limit = limit;

queue = new LinkedList<>();

}

public synchronized void put(Message message) throws InterruptedException {

while (this.queue.size() == this.limit) {

System.out.println(Thread.currentThread().getName() +"Waiting");

wait();

}

if (this.queue.size() == 0) {

System.out.println(Thread.currentThread()
.getName()+"Notified to all waiting threads");

notifyAll();

}

this.queue.add(message);

}

public synchronized Message take() throws InterruptedException {

while (this.queue.size() == 0) {

System.out.println(Thread.currentThread()
.getName() +"Waiting");

wait();

}

if (this.queue.size() == this.limit) {

System.out.println(Thread.currentThread()
.getName() +"Notified to all waiting threads");

notifyAll();

}

return this.queue.remove(0);

}

public static void main(String[] args) throws InterruptedException {

OwnBlockingQueue blockingQueue = new OwnBlockingQueue(10);

/* PRODUCER THREAD */

Thread producer = new Thread(new Runnable() {

@Override

public void run() {

for (int i = 0; i < 100; i++) {

Message message = new Message("" + i);

try {

blockingQueue.put(message);

Thread.sleep(10);

sysout("Produced message"+mes);

} catch (InterruptedException e) {

e.printStackTrace();

}

/* Putting last Message */

Message message = new Message("EXIT");

try {

blockingQueue.put(message);

} catch (InterruptedException e) {

e.printStackTrace();

}

System.out.println("Produced LAST Message "+message.getMessage());

}

});

/* CONSUMER THREAD */

Thread consumer = new Thread(new Runnable() {

public void run() {

Message message;

try {

while (!((message = blockingQueue.take()).getMessage().equals("EXIT"))){

Thread.sleep(10);

System.out.println("Message Consumed " + message.getMessage());

}

} catch (InterruptedException e) {

e.printStackTrace();

}

System.out.println("All Messages Produced , Consumed and Finished");

}

});

producer.setName("producer");

consumer.setName("consumer");

producer.start();

consumer.start();

producer.join();

consumer.join();

}

Output

consumerWaiting