Wednesday, 16 December 2015

Armstrong number - Java Program

Java program to check if a given number is an Armstrong number or not can be termed as a fresher level interview question. An  Armstrong number is a number that is equal to the sum of the digits in a number raised to the power of number of digits in the number.

As Example if we take 371, it is an Armstrong number as the number of digits here is 3, so

371 = 33 + 73 + 13 = 27 + 343 + 1 = 371

Another Example is 9474, here the number of digits is 4, so

9474 = 94 + 44 + 74 + 44 = 6561 + 256 + 2401 + 256 = 9474

And obviously 0 and 1 are also Armstrong number.

So let's write a Java program to check whether a given number is an Armstrong number or not.

package org.netjs.prgrm;

import java.util.Scanner;

public class ArmstrongNumber {

 public static void main(String[] args) {
      System.out.println("Please enter a number : ");
      Scanner scanIn = new Scanner(System.in);
      int scanInput = scanIn.nextInt();
      boolean isArmstrong = checkForArmstrongNo(scanInput);
      if(isArmstrong){
       System.out.println(scanInput + "  is an Armstrong number");
      }else{
       System.out.println(scanInput + " is not an Armstrong number"); 
      }
      scanIn.close();
 }
 
 private static boolean checkForArmstrongNo(int number){
  // convert number to String
  String temp = number + "";
  int numLength = temp.length();
  int numCopy = number;
  int sum = 0;
  while(numCopy != 0 ){
   int remainder = numCopy % 10;
   // using Math.pow to get digit raised to the power
   // total number of digits
   sum = sum + (int)Math.pow(remainder, numLength);
   numCopy = numCopy/10;
  }
  System.out.println("sum is " + sum );
  return (sum == number) ? true : false;
 }
}

Some outputs -

Please enter a number : 
125
sum is 134
125 is not an Armstrong number

Please enter a number : 
371
sum is 371
371  is an Armstrong number

Please enter a number : 
54748
sum is 54748
54748  is an Armstrong number

Explanation

Here the input is taken from the user, that number is converted to String just to get the length of the number. Logic here is to get one digit of the number at a time, starting from the last digit, get the value of that number power raised to the number of the digits and then divide the number by 10 to reduce the number by one digit. You can also add some conditions like checking for negative number, integer range etc.

That's all for this topic Armstrong number - Java Program. If you have any doubt or any suggestions to make please drop a comment. Thanks!


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>>>Go to Java Programs page

Monday, 14 December 2015

Java Multi-threading interview questions

  1. What is thread in Java?

    According to JavaDoc A thread is a thread of execution in a program. The Java Virtual Machine allows an application to have multiple threads of execution running concurrently.


  2. How do we create thread in Java?

    In Java there are two ways to create thread.

    • By implementing the Runnable interface.
    • By extending the Thread class.

    Example code snippet using Runnable interface

    class MyThread implements Runnable{
        MyThread(){
            Thread t = new Thread(this, "My Thread");
            t.start();
        }
    

    Example code snippet using Thread class

    class TestThread extends Thread{
    
    and then
    TestThread t = new TestThread();
    // starting the thread
    t.start();
    
    After the new thread is created, it will not start running until you call the start( ) method.

    Read more about How to create thread in Java here.

  3. What is the difference between thread and process?

    • A process has a self-contained execution environment, Threads exist within a process - every process has at least one.
    • Process are heavyweight tasks whereas threads are referred as lightweight processes as creating a new thread requires fewer resources than creating a new process.
    • Each Process has its own separate address spaces, threads with in the same process share the process' resources, including memory and open files. This means that it's very easy to share data amongst threads, but it's also easy for the threads to bump on each other, which can lead to unpredictable scenarios.
    • Inter process communication is expensive whereas inter thread communication is inexpensive and in Java can be achieved easily using wait and notify.
    • Context switching from one process to another is expensive; context switching between threads is generally less expensive than in processes.
    • Threads are easier to create than processes as separate address space is not required for a thread.

    Read more about difference between thread and process in Java here.

  4. What are the different thread states?

    • New - When a thread is created either by extending Thread class or implementing Runnable interface it is in "New State".
    • Runnable - When we call start() method on the thread object that causes the thread to begin execution and it's the Java Virtual Machine that calls the run method of the thread.
    • Blocked - When a resource is shared among various threads then a thread may go into blocked state as the resource may be used by another thread.
    • Waiting - A thread that is waiting indefinitely for another thread to perform a particular action is in the waiting state.
    • Timed_Waiting - A thread that is waiting for another thread to perform an action for up to a specified waiting time is in timed_waiting state.

    Read more about different thread states in Java here.

  5. What are the thread priorities and how to set thread priority?

    When a Java thread is created, it inherits its priority from the thread that created it. Thread's priority can be modified at any time after its creation using the setPriority() method which is a member of Thread class.

     public final void setPriority(int newPriority)
     
    Here newPriority specifies the new priority setting for the calling thread. The priority level ranges from 1 (least important) to 10 (most important) and the default priority level is 5.
    In Thread class, three constants are provided to define min, max and default priority for a thread.
    public final static int MIN_PRIORITY = 1;
    public final static int NORM_PRIORITY = 5;
    public final static int MAX_PRIORITY = 10;
    

    Read more about thread priorities in Java here.

  6. What does isAlive() method do in Java threading?

    isAlive() method is the member of the Thread class and its general form is -

    public final boolean isAlive()
    
    isAlive() method tests if the thread it is called upon is alive. A thread is alive if it has been started and has not yet died. The isAlive( ) method returns true if the thread upon which it is called is still running, otherwise it returns false.

    If we have a thread -
    Thread t1 = new Thread(new MyRunnableClass(), "t1");
    
    Then we can use isAlive to check if it is still running or not.
    System.out.println("t1 Alive - " + t1.isAlive());
    
    Read more about isAlive() method here.

  7. What is join() method?

    Join() method is used when you want to wait for the thread to finish. Its general form is -

    public final void join() throws InterruptedException
    
    This method waits until the thread on which it is called terminates.

    Read more about join() method here.

  8. Can we call run() method directly instead of start()?

    When we call start() method on the thread that causes this thread to begin execution and it's the Java Virtual Machine that calls the run method of this thread.
    If we directly call run method it will be treated as a normal overridden method of the thread class (or runnable interface). This run method will be executed with in the context of the current thread not in a new thread.

    Read more about Can we call run() method directly here.

  9. Can we start the same thread twice in Java?

    No. A thread can only be started once and any attempt to start the same thread twice will throw IllegalThreadStateException.

    Read more about Can we start the same thread twice here.

  10. What are wait, notify, notifyall method used for in Java multi-threading?

    Java provides inter-thread communication using the wait(), notify() and notifyAll() methods of the Object class.

    • wait method - tells the current thread (thread which is executing code inside a synchronized method or block) to give up monitor and go to sleep, until another thread invokes the notify() or notifyAll() method for this object.
    • notify method - Wakes up a single thread that is waiting on this object's monitor. If more than one threads are waiting on this object, one of them is chosen to be awakened. The choice is arbitrary and occurs at the discretion of the implementation.
    • notifyAll method - wakes up all the threads that called wait( ) on the same object.

    Read more about wait, notify, notifyall here.

  11. What is the difference between notify() and notifyAll()?

    • Notify method wakes up a single thread that is waiting to acquire a lock on the object. If more than one threads are waiting on this object, one of them is chosen to be awakened.
    • NotifyAll method wakes up all the threads that called wait on the same object. Note that only one of these threads will get a lock to the object.

  12. Why wait(), notify() and notifyAll() methods are in object class?

    wait() and notify() work at the monitor level, thread which is currently holding the monitor is asked to give up that monitor through wait and through notify (or notifyAll) thread which are waiting on the object's monitor are notified that thread can wake up. Important point to note here is that monitor is assigned to an object not to a particular thread. That's one reason why these methods are in Object class.

    wait, notify and notifyAll() are used for inter-thread communication. But threads themselves have no knowledge of each others status. It is the shared object among the threads that acts as a communicator among the threads. Threads lock an object, wait on an object and notify an object.

    Read more about Why wait(), notify() and notifyAll() methods are in object class here.

  13. When notify method is called does the thread which comes out of waiting start executing instantly?

    No, thread which comes out of waiting because of the notify() method will not be able to proceed until the current thread relinquishes the lock on this object. The awakened thread just changes to the runnable state and it is ready to be scheduled again. The awakened thread will compete in the usual manner with any other threads that might be actively competing to synchronize on this object.

    Read more about wait, notify, notifyall here.

  14. Why wait, notify and notifyAll must be called inside a synchronized method or block?

    wait method tells the current thread (thread which is executing code inside a synchronized method or block) to give up monitor. Object's lock is acquired by a thread only when it is executing in a synchronized context. So it makes sense to use wait() method, which asks thread to release the lock only in synchronized context.

    Same way; when object's notify() or notifyAll() method is called, single thread (in case of notify) or all of the threads (in case of notifyAll), waiting for the object's lock change state to runnable and contend for the object's lock, and the thread that gets the lock starts execution. Here again, notify and notifyAll can inform other threads, that the object's lock can be acquired now, only if these methods are called from the synchronized object.

    Read more about Why wait, notify and notifyAll must be called inside a synchronized method or block here.

  15. What is spurious wakeup?

    Once wait is called on an object the thread that is currently executing with in the synchronized context waits until notify or notfyAll method is called. But there is a possibility that a waiting thread resumes again even when notify() or notifyAll() are not called (this will rarely occur in practice). This is known as spurious wakeup.
    To guard against it the recommendation is that; call to wait() method should be with in a loop that checks the condition on which the thread is waiting.


  16. What does yield method do?

    Yield - A hint to the scheduler that the current thread is willing to yield its current use of a processor. The scheduler is free to ignore this hint. The executing thread is suspended and the CPU is given to some other runnable thread. This thread will wait until the CPU becomes available again.Technically, in process scheduler's terminology, the executing thread is put back into the ready queue of the processor and waits for its next turn.
    yield is a static method of the Thread class. When called it will work on the currently executing thread, not on any particular thread.

    Read more about yield method here.

  17. What is the difference between sleep and wait?

    • The very first difference is that sleep is defined as a static method in Thread class and operates on the currently executing thread. Whereas wait() method is in Object class and operates on the thread which is currently holding the lock on the object on which the wait method is called.
    • Since wait method is supposed to release the lock on an object so it has to be called from a synchronized context (with in a synchronized method or synchronized block). If not called from a synchronized context will result in a IllegalMonitorStateException being thrown. With sleep method there is no such mandatory condition it doesn't need to be called from a synchronized context.
    • Wait method will release the lock it holds on an object before going to waiting state which gives another thread a chance to enter the synchronized block. Sleep method if called from a synchronized block or method will not release the lock so another won't get a chance to enter the synchronized block.

    Read more about difference between sleep and wait here.

  18. Why Thread.sleep() and yield() are static?

    yield and sleep methods are used to work on the currently executing thread. Thus these methods are static so that you don't call it on any other thread.


  19. What is synchronization in Java multi-threading?

    In a multithreaded environment when more than one thread are trying to access a shared resource we need to have some way to ensure that the resource will be used by only one thread at a time. The process by which it is ensured is called synchronization.

    Read more about synchronization in Java multi-threading here.

  20. What is a synchronized block?

    It is not always needed to synchronize the whole method, let's say we have a 100 line code method, out of which critical section (shared resource) is only 7 lines then it makes sense to synchronize only those 7 lines rather than synchronizing the whole method. That is known as synchronized block or statement.

    class Message{
        public void displayMsg(String msg){
            System.out.println("Inside displayMsg method " +    Thread.currentThread().getName());
            synchronized(this){
                System.out.print("**" + msg);        
                try {
                    Thread.sleep(3);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                System.out.println("*");
            }
        }
    }
    
    
    Read more about synchronization in Java multi-threading here.

  21. What is race condition in multi-threading?

    Race condition occurs in a multi-threaded environment when more than one thread try to access a shared resource (modify, write) at the same time. Note that it is safe if multiple threads are trying to read a shared resource as long as they are not trying to change it. Since multiple threads try to race each other to finish executing a method thus the name race condition.

    Read more about race condition in Java multi-threading here.

  22. What is a deadlock in Multi-threading?

    Deadlock describes a situation where two or more threads are blocked forever, waiting for each other. To describe it in a simple manner let's assume there are two threads Thread-1 and Thread-2 and two objects obj1 and obj2. Thread-1 already holds a lock on obj1 and for further processing it needs a lock on obj2. At the same time Thread-2 holds a lock on obj2 and wants a lock on obj1. In that kind of scenario both threads will be waiting for each other forever to release lock they are already holding thus creating a deadlock.

    Read more about deadlock in Multi-threading here.

  23. Write a program to create deadlock in Java.

    A deadlock can be created by having nested synchronized blocks where lock on objects are acquired in a reverse manner. i.e. if there are two objects obj1 and obj2 then first thread tries to acquire lock in sequence obj1 and then obj2. In the same time second thread tries to acquire lock in sequence obj2 first and then obj1.

    class Test{
        private final String name;
        public Test(String name){
            this.name = name;
        }
        public String getName() {
            return this.name;
        }
    }
    
    class ThreadA implements Runnable{
        private Test test1;
        private Test test2;
        ThreadA(Test test1, Test test2){
            this.test1 = test1;
            this.test2 = test2;
        }
        @Override
        public void run() {
            synchronized(test1){
                System.out.println("" + test1.getName());
                synchronized(test2){
                    System.out.println("Reached here");
                }
            }
            
        }
        
    }
    
    class ThreadB implements Runnable{
    
        private Test test1;
        private Test test2;
        ThreadB(Test test1, Test test2){
            this.test1 = test1;
            this.test2 = test2;
        }
        @Override
        public void run() {
            synchronized(test2){
                System.out.println("" + test2.getName());
                synchronized(test1){
                    System.out.println("Reached here");
                }
            }   
        }
    }
    public class DeadLockDemo1{
    
        public static void main(String[] args) {
            Test test1 = new Test("Test-1");
            Test test2 = new Test("Test-2");
            Thread t1 = new Thread(new ThreadA(test1, test2));
            Thread t2 = new Thread(new ThreadB(test1, test2));
            t1.start();
            
            t2.start();
        }
    }
    

    Thread t1 will start execution of run method in ThreadA and acquire lock on object test1 and then try to acquire lock on object test2. Meanwhile Thread t2 will start execution of run method in ThreadB and acquire lock on object test2 and then try to acquire lock on object test1. So both threads are trying to acquire a lock which is already held by another thread. Thus causing a deadlock.

    Read more about deadlock in Multi-threading here.

  24. What is a thread local variable?

    In Java there is a class called ThreadLocal which provides another way of thread-safety apart from synchronization. Usually when we have multiple threads sharing an object we need to synchronize the critical section of the code in order to make it thread safe.
    ThreadLocal class provides thread-local variables where each thread that accesses one (via its get or set method) has its own, independently initialized copy of the variable. Since each and every threadhas its own copy of the object so explicit synchronization is not needed to provide thread safety.

    Read more about thread local variable here.

  25. What is volatile in Java?

    When a variable is declared as volatile, the compiler (even runtime) seeing the keyword volatile knows that this variable is shared. So the volatile variables are not cached in registers and read operation on a volatile variable always returns the most recent write by any thread.

    Read more about volatile here.

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Sunday, 13 December 2015

ArrayList in Java

ArrayList is one of the most used collection and most of its usefulness comes from the fact that it grows dynamically. You don't have to anticipate, how many elements you are going to store in the arraylist, in advance like in the case of array. As and when elements are added list keeps growing.

Though internally it is not really some "elastic" array which keeps growing, it is as simple as having an array with an initial capacity (default is array of length 10). When that limit is crossed another array is created which is 1.5 times the original array and the elements from the old array are copied to the new array.

In this post I'll list out some of the salient features of the arraylist.

Hierarchy of the ArrayList

To know about the hierarchy of the ArrayList we need to know about 2 interfaces and 2 abstract classes.

  • Collection Interface - Collection interface is the core of the Collection Framework. It must be implemented by any class that defines a collection.
  • List interface - List interface extends Collection interface. Apart from extending all the methods of the Collection interface, List interface defines some methods of its own.
  • AbstractCollection - Abstract class which implements most of the methods of the Collection interface.
  • AbstractList - Abstract class which extends AbstractCollection and implements most of the List interface.

ArrayList extends AbstractList and implements List interface too. Apart from List interface, ArrayList also implements RandomAccess, Cloneable, java.io.Serializable interfaces.

Adding elements to ArrayList

List provides a method add(E e) which appends specified element to the end of the list. Using add(E e) method will mean keep adding elements sequentially to the list.

Apart from that there is another add method - add(int index, E element)
This method inserts the specified element at the specified position in this list.

Third method
addAll(int index, Collection<? extends E> c)

Inserts all of the elements in the specified collection into this list, starting at the specified position.

Allows duplicates

In list there is no such restriction that duplicates elements can not be added. List does allow duplicate elements to be added.

public class LoopListDemo {

    public static void main(String[] args) {
        // Using Diamond operator, so with ArrayList 
        // don't need to provide String, this option is available from 
        // Java 7 onwards
        List<String> cityList = new ArrayList<>();
        cityList.add("Delhi");
        cityList.add("Mumbai");
        cityList.add("Bangalore");
        cityList.add("Mumbai");
        cityList.add("Mumbai");
        
        
        // Using for-each loop 
        System.out.println("With for-each loop - Java 5");
        for(String name : cityList){
            System.out.println("City Name - " + name);
        }
    }
}

Output

With for-each loop - Java 5
City Name - Delhi
City Name - Mumbai
City Name - Bangalore
City Name - Mumbai
City Name - Mumbai

Here it can be seen that Mumbai is added 3 times and when I am looping the list and displaying the elements in the list it is showing Mumbai 3 times.

Allows any number of nulls

In ArrayList any number of nulls can be added. Let's see it with an example.

public class LoopListDemo {
    public static void main(String[] args) {
        // Using Diamond operator, so with ArrayList 
        // don't need to provide String, this option is available from 
        // Java 7 onwards
        List<String> cityList = new ArrayList<>();
        cityList.add("Delhi");
        cityList.add("Mumbai");
        cityList.add("Bangalore");
        cityList.add("Mumbai");
        cityList.add(null);
        cityList.add("Mumbai");
        cityList.add(null);
        
        // Using for-each loop 
        System.out.println("With for-each loop - Java 5");
        for(String name : cityList){
            System.out.println("City Name - " + name);
            //cityList.remove(2);
        } 
    }
}

Output

With for-each loop - Java 5
City Name - Delhi
City Name - Mumbai
City Name - Bangalore
City Name - Mumbai
City Name - null
City Name - Mumbai
City Name - null

It can be seen here that two null elements are added in the AraryList.

Removing elements from the list

ArrayList provides several methods to remove elements from the List. Since ArrayList internally uses array to store elements, one point to note here is that when an element is removed from the List internally the remaining elements are shifted to fill the gap created in the underlying array.

  • clear() - Removes all of the elements from this list.
  • remove(int index) - Removes the element at the specified position in this list.
  • remove(Object o) - Removes the first occurrence of the specified element from this list, if it is present.
  • removeAll(Collection<?> c) - Removes from this list all of its elements that are contained in the specified collection.
  • removeIf(Predicate<? super E> filter) - Removes all of the elements of this collection that satisfy the given predicate.

Note that removeIf is added in Java 8

Example code using removeIf

public class LoopListDemo {
    public static void main(String[] args) {
        // Using Diamond operator, so with ArrayList 
        // don't need to provide String, this option is available from 
        // Java 7 onwards
        List<String> cityList = new ArrayList<>();
        cityList.add("Delhi");
        cityList.add("Mumbai");
        cityList.add("Bangalore");
        cityList.add("Mumbai");
        cityList.add("Mumbai");
        
        cityList.removeIf((String name )->name.equalsIgnoreCase("Bangalore"));
        // Using for-each loop 
        System.out.println("With for-each loop - Java 5");
        for(String name : cityList){
            System.out.println("City Name - " + name);
            //cityList.remove(2);
        }
    }
}

Output

With for-each loop - Java 5
City Name - Delhi
City Name - Mumbai
City Name - Mumbai
City Name - Mumbai

It can be seen that Bangalore is removed from the list based on the condition provided with removeIf. Note that parameter for the removeIf is of type Predicate which is a functional interface, so it's method can be implemented using lambda expression.

ArrayList is not synchronized

ArrayList is not synchronized. That means sharing an instance of arrayList among many threads where those threads are modifying the collection (adding or removing the values) may result in unpredictable behaviour. If we need to synchronize an ArrayList you can use synchronizedList method provided by Collections class, which returns a synchronized (thread-safe) list backed by the specified list.

Iterator for an ArrayList

ArrayList provides iterator to traverse the list in a sequential manner. Since ArrayList implements List interface so it provides ListIterator too which is different from the iterator in a way that it provides iteration in both directions.

One point to note here is that both iterator and listiterator are fail fast, fail-fast iterator fails if the underlying collection is structurally modified at any time after the iterator is created, thus the iterator will throw a ConcurrentModificationException if the underlying collection is structurally modified in any way except through the iterator's own remove or add (if applicable as in list-iterator) methods.

Performance of ArrayList

  • Adding an element - If you are adding at the end using add(E e) method it is O(1). Even in the case of adding at the last ArrayList may give O(n) performance in the worst case. That will happen if you add more elements than the capacity of the underlying array, as in that case a new array (1.5 times the last size) is created, and the old array is copied to the new one. If you are using add(int index, E element) then it is O(n - index) and it'll become O(n) if every time element is added at the beginning of the list.
  • Retrieving an element - Since ArrayList internally uses an array to store elements so get(int index) means going to that index directly in the array. So, for ArrayList get(int index) is O(1).
  • Removing an element - If you are removing using the remove(int index) method then, in case of ArrayList getting to that index is fast but removing will mean shuffling the remaining elements to fill the gap created by the removed element with in the underlying array. It ranges from O(1) for removing the last element to O(n). Thus it can be said remove(int index) operation is O(n - index) for the arraylist.

That's all for this topic ArrayList in Java. If you have any doubt or any suggestions to make please drop a comment. Thanks!


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How to convert Array to ArrayList in Java

There may be a situation when you want to convert an array to an ArrayList in order to use some of the methods provided by ArrayList in Java.

As example using contains() method provided by List to check if an element is there in the list.

If you have this need of converting an array to an ArrayList then Java does provide a class Arrays which has a method for doing the same.

Arrays class contains various methods for manipulating arrays (such as sorting and searching). This class also contains a static factory that allows arrays to be viewed as lists. The method to convert Array to ArrayList is asList() whose general form is

public static <T> List<T> asList(T... a)

Using Arrays.asList() to convert array to ArrayList

In the following Java code I have an array of cities which I am converting into an ArrayList.

public class CovertToArrayList {
    public static void main(String[] args) {
         String cityArray[] = {"Delhi", "Mumbai", "Bangalore", "Hyderabad", "Chennai"};
         //Converting array to List
         List<String> cityList = Arrays.asList(cityArray);
         for(String name : cityList){
            System.out.println("City : " + name);
         } 
    }
}

There are few points to note about Arrays.asList() method

  • This method returns a List that is a view onto the array. You can say it is more of a wrapper around an array. That makes it more efficient than any other option as array elements are not actually copied to the List.
  • The list returned by this method is a fixed-size list.
  • Since list returned is just a view onto the array, if you change any element in the list that change is reflected in the array too.
  • The ArrayList class used to create the list while using this method Arrays.asList is actually a static class within the Arrays class and many of the methods of the original ArrayList are not supported by this ArrayList. As example add() method is not there in this ArrayList so any attempt to add new element to the returned list will throw "UnsupportedOperationException"

Example code

public class CovertToArrayList {

    public static void main(String[] args) {
         String cityArray[] = {"Delhi", "Mumbai", "Bangalore", "Hyderabad", "Chennai"};
         //Converting array to List
         List<String> cityList = Arrays.asList(cityArray);
         // Attempt to add new element in the list
         // This will throw exception
         cityList.add("Pune");
         for(String name : cityList){
            System.out.println("City : " + name);
         }
        
    }

}

Output

Exception in thread "main" java.lang.UnsupportedOperationException
 at java.util.AbstractList.add(Unknown Source)
 at java.util.AbstractList.add(Unknown Source)
 at org.netjs.prog.CovertToArrayList.main(CovertToArrayList.java:16)
  • You can also initialize a List using Arrays.asList() method like this
    List<String> cityList = Arrays.asList("Delhi", "Mumbai", "Bangalore", 
    "Hyderabad", "Chennai");
    
  • As stated above no new element can be added to the List returned by Arrays.asList() method. If you have to add new elements to the List then there are other options.

    • Wrapping the list returned by Arrays.asList to a new ArrayList.
      public class CovertToArrayList {
          public static void main(String[] args) {
              String cityArray[] = {"Delhi", "Mumbai", "Bangalore", 
      "Hyderabad", "Chennai"};
              //Converting array to List
              ArrayList<String> cityList = new ArrayList<String>(Arrays.asList(cityArray));
              
              cityList.add("Pune");
              for(String name : cityList){
                 System.out.println("City : " + name);
              }     
          }
      }
      

      Output

      City : Delhi
      City : Mumbai
      City : Bangalore
      City : Hyderabad
      City : Chennai
      City : Pune
      

      It can be seen that now new elements can be added to the list.

    • Using Collections.addAll method. Here again a new List is created using the array, by passing both as parameters.
      public class CovertToArrayList {
          public static void main(String[] args) {
               String cityArray[] = {"Delhi", "Mumbai", "Bangalore", "Hyderabad", "Chennai"};
               List<String> cityList= new ArrayList<String>();
               Collections.addAll(cityList, cityArray);
               cityList.add("Pune");
               for(String name : cityList){
                  System.out.println("City : " + name);
               }        
          }
      }
      
    • Using Java 8 Stream - If you are using Stream API in Java 8 then you have another option by using collect() method.

      public class CovertToArrayList {
          public static void main(String[] args) {
               String cityArray[] = {"Delhi", "Mumbai", "Bangalore", "Hyderabad", "Chennai"};
               // Using stream
               List<String> cityList = Stream.of(cityArray).collect(Collectors.toList());
               cityList.add("Pune");
               for(String name : cityList){
                  System.out.println("City : " + name);
               }        
          }
      }
      

      Output

      City : Delhi
      City : Mumbai
      City : Bangalore
      City : Hyderabad
      City : Chennai
      City : Pune
      

    That's all for this topic How to convert Array to ArrayList in Java. If you have any doubt or any suggestions to make please drop a comment. Thanks!


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    Thursday, 10 December 2015

    Difference between Array and ArrayList in Java

    Difference between Array and ArrayList is one question you may come across in a Java technical interview. Though performance wise both Array and ArrayList may give almost similar performance but functionality wise they both are used in quite different scenarios.

    This question may be asked while asking some Java Collections interview questions in order to test the knowledge of the candidate about the ArrayList being dynamic and the performance difference if any.

    Array Vs ArrayList

    • First and the most important difference is Array is static and can't be resized once declared.
      Whereas ArrayList is dynamic and that is why also known as dynamic array. ArrayList also uses array of Object internally, but it has the logic to keep growing the size of the array as and when previous size is not able to fit in the number of elements stored in the ArrayList.
    • Refer How ArrayList works internally in Java to know more about the internal implementation of ArrayList.
    • Array can store both primitive types as well as Objects whereas ArrayList can store only objects. Though Autoboxing and Unboxing has blurred that difference, but the important point to note here is that in case of ArrayList any primitive data type is still wrapped and stored as an Object.

      For example, if you want an array of primitive type int -

      int[] intArray = new int[3];
      

      Or, if you have a class Employee and you want an array of size 5 to hold 5 Employee objects then -

      Employee[] employees = new Employee[5];
      

      In case of ArrayList if you want to store integers then you have to do this, note the use of wrapper class Integer -

      List<Integer> myList = new ArrayList<Integer>();
      
    • Difference number 2 also indicates one more difference, which is about "type safety". Since Array knows the type of the data which it can hold so it will give compiler error "Type Mismatch" or "ArrayStoreException" if it is not able to resolve it at run time.
      As Exp.
      Object[] names = new String[3];
      names[0] = 12;
      
      This will throw ArrayStoreException.
      Where as this
      String[] names = new String[3];
      names[0] = 12;
      
      Will throw compile time error "Type Mismatch".

      In case of ArrayList, generics brought the much needed type safety which, as shown above, is not required for Array.

      If a list, which should store only Integers, is needed it should be defined as -

      List<Integer> myList = new ArrayList<Integer>();
      

    • Performance wise both Array and ArrayList are almost similar as ArrayList also uses array of Objects internally. But there is some overhead in case of ArrayList in case of resizing the array.
    • Array has length variable which gives the length of the array. Note that length attribute denotes the length of the array at the time of declaration.
      As exp. If an array is declared like this -
      String[] names = new String[3];
      names[0] = "Java";
      
      Then length var will always be 3 even if array names has only one value.

      In case of ArrayList, size() method is used and it will give the size as the current number of elements in the ArrayList.

    That's all for this topic Difference between Array and ArrayList in Java. If you have any doubt or any suggestions to make please drop a comment. Thanks!


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    Tuesday, 8 December 2015

    Difference between ArrayList and Vector in Java

    Vector, just like ArrayList, is also a growable dynamic array. As of Java v1.2, this class was retrofitted to implement the List interface, making it a member of the Java Collections Framework.

    With JDK 5 it was also retrofitted for generics and implements Iterable interface too which means it can also use enhanced for loop.

    So, in many ways Vector is just like ArrayList apart from some differences and this post is about those differences between the ArrayList and Vector.

    ArrayList Vs Vector in Java

    • ArrayList is not synchronized whereas Vector is Synchronized. Which means that the methods in the Vector class are Synchronized methods making Vector thread-safe and ready to use as-is in a multi-threaded environment.
      ArrayList, if needed in a multi-threaded environment, has to be synchronized externally using Collections.synchronizedList method which returns a synchronized (thread-safe) list backed by the specified list.
    • Refer How and why to synchronize ArrayList in Java to know how to synchronize an ArrayList.

    • A Vector, by default, doubles the size of its array when it needs to expand the array, while the ArrayList increases its array size by 50 percent.
    • As mentioned though Vector is retrofitted to implement the List interface it still has legacy methods which are not there in ArrayList. As Exp. methods like addElement(), removeElement(), capacity() which are there in Vector class but not in ArrayList.
    • Performance wise Vector is comparatively slower than the ArrayList because it is synchronized. That means only one thread can access method of Vector at the time and there is an overhead of acquiring lock on the object too.
    • For traversing an ArrayList as well as Vector, Iterator or ListIterator can be used. That Iterator/ListIterator is fail-fast and throws ConcurrentModificationException if any other Thread modifies the map structurally by adding or removing any element except Iterator's own remove() method.
      For Vector enumeration can also be used for traversing which is not fail-fast.

    That's all for this topic Difference between ArrayList and Vector in Java. If you have any doubt or any suggestions to make please drop a comment. Thanks!


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