October 10, 2016

How to schedule a periodic task in Java?

To schedule a task periodically in java:

1. Create an object of the Timer class.
    Timer timer = new Timer();
2. Create an object of the TimerTask class and override the run() method of it.
    TimerTask task = new TimerTask() {
    public void run() {
           //call your task here

3. Call the schedule method of the Timer.
    timer.schedule(task, delay, period);
    a) task - task to be scheduled. Pass the task object we have created in step 2.
    b) delay - delay in milliseconds before task is to be executed.
    c) period - time in milliseconds between successive task executions.

December 10, 2013

In Java, how does System.out.println() work?

In Java, the dot operator can only be used to call methods and variables so we know that ‘out’ must be either a method or a variable. Now, how do we categorize ‘out’? Well, ‘out’ could not possibly be a method because of the fact that there are no parentheses – the ‘( )’ – after ‘out’, which means that out is clearly not a method that is being invoked. And, ‘out’ does not accept any arguments because only methods accept arguments – you will never see something like “System.out(2,3).println”. This means ‘out’ must be a variable.

What is “out” in System.out.println()?

We now know that ‘out’ is a variable, so we must now ask ourselves what kind of variable is it? There are two possibilities – it could be a static or an instance variable. Because ‘out’ is being called with the ‘System’ class name itself, and not an instance of a class (an object), then we know that ‘out’ must be a static variable, since only static variables can be called with just the class name itself. So now we know that ‘out’ is a static member variable belonging to the System class.

Is “out” in System.out.println() an instance variable?

Noticing the fact that ‘println()’ is clearly a method, we can further classify the ‘out’ in System.out.println(). We have already reasoned that ‘out’ is a static variable belonging to the class System. But now we can see that ‘out’ must be an instance of a class, because it is invoking the method ‘println()’.
The thought process that one should use to arrive at an answer is purposely illustrated above. Without knowing the exact answer beforehand, you can arrive at an approximate one by applying some basic knowledge of Java. Most interviewers wouldn’t expect you to know how System.out.println() works off the top of your head, but would rather see you use your basic Java knowledge to arrive at an answer that’s close to exact.

When and where is the “out” instantiated in System.out.println?

When the JVM is initialized, the method initializeSystemClass() is called that does exactly what it’s name says – it initializes the System class and sets the out variable. The initializeSystemClass() method actually calls another method to set the out variable – this method is called setOut().

The final answer to how system.out.println() works

The more exact answer to the original question is this: inside the System class is the declaration of ‘out’ that looks like: ‘public static final PrintStream out’, and inside the Prinstream class is a declaration of ‘println()’ that has a method signature that looks like: ‘public void println()’.
Here is what the different pieces of System.out.println() actually look like:
//the System class belongs to java.lang package
class System {
  public static final PrintStream out;

//the Prinstream class belongs to java.io package
class PrintStream{
public void println();

In Java, what’s the difference between method overloading and method overriding?

The difference between overriding and overloading in Java is a common source of confusion – but it is fairly simple to understand. Let’s start the discussion by talking more about method overloading. Overloading in Java can occur when two or more methods in the same class share the same name or even if a child class shares a method with the same name as one of it’s parent classes. But, in order to actually have overloaded methods, the methods not only have to have the same name, but there are other conditions that must be satisfied – read below to see what those conditions are.
Suppose we have a class called TestClass which has 2 methods, and both methods have the same name – let’s say that name is “someMethod” – this would be considered to be method overloading if at least one of these 2 things is true:
1.) The number of parameters is different for the methods   
2.) The parameter types are different.  

How to NOT overload methods:

It’s important to understand that method overloading is NOT something you can accomplish by doing these 2 things:
1.  Changing the return type of the method   
2.  Changing the name of the method parameters, but 
    not changing the parameter types.  
Confused? Well, here are some very helpful examples of where overloading would be both valid and invalid:

Examples of Method Overloading in Java – both valid and invalid:

//compiler error - can't overload based on the   
//type returned -
//(one method returns int, the other returns a float):    

int changeDate(int Year) ;  
float changeDate (int Year);    

//compiler error - can't overload by changing just 
//the name of the parameter (from Month to Year):    

int changeDate(int Month) ;  
float changeDate(int Year);    

//valid case of overloading, since there is an   
//extra parameter in the first method:        

int changeDate(int Year, int Month) ;  
int changeDate(int Year);    

//also a valid case of overloading, since the   
//parameters are of different types:    

int changeDate(float Year) ;  
int changeDate(int Year);  

What about method overriding?
Overriding methods is completely different from overloading methods. If a derived class requires a different definition for an inherited method, then that method can be redefined in the derived class. This would be considered overriding. An overridden method would have the exact same method name, return type, number of parameters, and types of parameters as the method in the parent class, and the only difference would be the definition of the method.
Let’s summarize the differences between overloading and overriding. When overloading, one must change either the type or the number of parameters for a method that belongs to the same class. But, overriding a method means that a method inherited from a base class is what’s being changed.

In Java, what’s the difference between an object and a class?

The terms ‘class’ and ‘object’ are absolutely related to one another, but each term holds its own distinct meaning. The term ‘class’ refers to the actual written piece of code in which the class is defined. A class is a static piece of code that consists of attributes which don’t change during the execution of a program – like the method definitions within a class.

An object is an instance of a class

The term ‘object’, however, refers to an actual instance of a class. Every object must belong to a class. Objects are created and eventually destroyed – so they only live in the program for a limited time. While objects are ‘living’ their properties may also be changed signficantly.
An example will help clarify what we’ve said. Suppose we have a class called ‘Animal’. All Animals have bodies and brains. So, this general ‘template’ of an Animal does not change.
An instance of the Animal class would be a specific animal – like a lion, a cat, or a zebra. These instances of the Animal class would be called objects. Whereas the Animal class is a general concept, the instances of that class – the lions, cats, etc – take that general concept and create a real instance of it. That is why programmers define constructors for their classes – so that when someone wants to create an object of the class, he/she can just pass in the actual properties that he/she wants the object to have – like what kind of animal it is, the name, the weight, etc. So, you can think of a constructor as something that brings the class to life – which is why it is called a constructor, because it constructs a specific instance of a class.

Objects have a lifespan but classes do not

And, as our Animal example clearly shows, every object has a lifespan associated with it – a cat or zebra can not live forever. And, the properties of those objects can change as well while they ‘live’; if we have a ‘size’ variable defined in the class that would of course change as the cat object grows bigger.

Object versus class summary

So, we can say that whereas a class is a general concept (like an Animal), an object is a very specific embodiment of that class, with a limited lifespan (like a lion, cat, or a zebra). Another way of thinking about the difference between a class and an object is that a class provides a template for something more specific that the programmer has to define, which he/she will do when creating an object of that class.

What’s the difference between an interface and an abstract class in Java?

A class must be declared abstract when it has one or more abstract methods. A method is declared abstract when it has a method heading, but no body – which means that an abstract method has no implementation code inside curly braces like normal methods do.

When to use abstract methods in Java?

Why you would want to declare a method as abstract is best illustrated by an example. Take a look at the code below:
/* the Figure class must be declared as abstract 
   because it contains an abstract method  */

public abstract class Figure
 /* because this is an abstract method the 
    body will be blank  */
 public abstract float getArea(); 

public class Circle extends Figure
 private float radius;

 public float getArea()
  return (3.14 * (radius * 2));  

public class Rectangle extends Figure
 private float length, width;

 public float getArea(Figure other)
  return length * width;
In the Figure class above, we have an abstract method called getArea(), and because the Figure class contains an abstract method the entire Figure class itself must be declared abstract. The Figure base class has two classes which derive from it – called Circle and Rectangle. Both the Circle and Rectangle classes provide definitions for the getArea method, as you can see in the code above.
But the real question is why did we declare the getArea method to be abstract in the Figure class? Well, what does the getArea method do? It returns the area of a specific shape. But, because the Figure class isn’t a specific shape (like a Circle or a Rectangle), there’s really no definition we can give the getArea method inside the Figure class. That’s why we declare the method and the Figure class to be abstract. Any classes that derive from the Figure class basically has 2 options: 1. The derived class must provide a definition for the getArea method OR 2. The derived class must be declared abstract itself.

A non abstract class is called a concrete class

You should also know that any non abstract class is called a concrete class. Knowing your terminology defintely pays off in an interview.
Now that we’ve explored the abstract method/class concepts, let’s get into the concept of interfaces and how they differ from abstract classes.

Java interface versus abstract class

An interface differs from an abstract class because an interface is not a class. An interface is essentially a type that can be satisfied by any class that implements the interface.
Any class that implements an interface must satisfy 2 conditions:
  • It must have the phrase "implements Interface_Name" at the beginning of the class definiton.
  • It must implement all of the method headings listed in the interface definition.
This is what an interface called "Dog" would look like:
public interface Dog
 public boolean Barks();

 public boolean isGoldenRetriever();
Now, if a class were to implement this interface, this is what it would look like:
public class SomeClass implements Dog
 public boolean Barks{
 // method definition here

 public boolean isGoldenRetriever{
 // method definition here
Now that we know the basics of interfaces and abstract classes, let’s get to the heart of the question and explore the differences between the two. Here are the three major differences:

Abstract classes and inheritance

1. Abstract classes are meant to beinherited from, and when one class inherits from another it means that there is a strong relationship between the 2 classes. For instance, if we have an abstract base class called "Canine", any deriving classshould be an animal that belongs to the Canine family (like a Dog or a Wolf). The reason we use the word "should" is because it is up to the Java developer to ensure that relationship is maintained.
With an interface on the other hand, the relationship between the interface itself and the class implementing the interface is not necessarily strong. For example, if we have a class called "House", that class could also implement an interface called "AirConditioning". Having air conditioning not really an essential part of a House (although some may argue that point), and the relationship is not as strong as, say, the relationship between a “TownHouse” class and the "House" class or the relationship between an “Apartment” class that derives from a “House” class.
Because a TownHouse is a type of House, that relationship is very strong, and would be more appropriately defined through inheritance instead of interfaces.
So, we can summarize this first point by saying that an abstract class would be more appropriate when there is a strong relationship between the abstract class and the classes that will derive from it. Again, this is because an abstract class is very closely linked to inheritance, which implies a strong relationship. But, with interfaces there need not be a strong relationship between the interface and the classes that implement the interface.

Interfaces are a good substitute for multiple inheritance

2. Java does not allow multiple inheritance – see the discussion on Java Multiple Inheritance if you need a refresher on this. In Java, a class can only derive from one class, whether it’s abstract or not. However, a class can implement multiple interfaces – which could be considered as an alternative to for multiple inheritance. So, one major difference is that a Java class can inherit from only one abstract class, but can implement multiple interfaces.

Abstract classes can have some implementation code

3. An abstract class may provide some methods with definitions – so an abstract class can have non-abstract methods with actual implementation details. An abstract class can also have constructors and instance variables as well. An interface, however, can not provide any method definitions – it can only provide method headings. Any class that implements the interface is responsible for providing the method definition/implementation.

When to use abstract class and interface in Java

Here are some guidelines on when to use an abstract class and when to use interfaces in Java:
  • An abstract class is good if you think you will plan on using inheritance since it provides a common base class implementation to derived classes.
  • An abstract class is also good if you want to be able to declare non-public members. In an interface, all methods must be public.
  • If you think you will need to add methods in the future, then an abstract class is a better choice. Because if you add new method headings to an interface, then all of the classes that already implement that interface will have to be changed to implement the new methods. That can be quite a hassle.
  • Interfaces are a good choice when you think that the API will not change for a while.
  • Interfaces are also good when you want to have something similar to multiple inheritance, since you can implement multiple interfaces.

October 05, 2013

Basic Hello World Program in Java

When we consider a Java program it can be defined as a collection of objects that communicate via invoking each other's methods. Let us now briefly look into what do class, object, methods and instance variables mean.
  • Object - Objects have states and behaviors. Example: A dog has states - color, name, breed as well as behaviors -wagging, barking, eating. An object is an instance of a class.
  • Class - A class can be defined as a template/ blue print that describes the behaviors/states that object of its type support.
  • Methods - A method is basically a behavior. A class can contain many methods. It is in methods where the logics are written, data is manipulated and all the actions are executed.
  • Instance Variables - Each object has its unique set of instance variables. An object's state is created by the values assigned to these instance variables.

First Java Program:

Let us look at a simple code that would print the words Hello World.
public class MyFirstJavaProgram {

    public static void main(String []args) {
       System.out.println("Hello World");

Let's look at how to save the file, compile and run the program. Please follow the steps given below:
  • Open notepad and add the code as above.
  • Save the file as: MyFirstJavaProgram.java.
  • Open a command prompt window and go o the directory where you saved the class. Assume it's C:\.
  • Type ' javac MyFirstJavaProgram.java ' and press enter to compile your code. If there are no errors in your code, the command prompt will take you to the next line (Assumption : The path variable is set).
  • Now, type ' java MyFirstJavaProgram ' to run your program.
  • You will be able to see ' Hello World ' printed on the window.
C : > javac MyFirstJavaProgram.java
C : > java MyFirstJavaProgram 
Hello World

Basic Syntax:

About Java programs, it is very important to keep in mind the following points.
  • Case Sensitivity - Java is case sensitive, which means identifier Hello and hello would have different meaning in Java.
  • Class Names - For all class names the first letter should be in Upper Case.
    If several words are used to form a name of the class, each inner word's first letter should be in Upper Case.
    Example class MyFirstJavaClass
  • Method Names - All method names should start with a Lower Case letter.
    If several words are used to form the name of the method, then each inner word's first letter should be in Upper Case.
    Example public void myMethodName()
  • Program File Name - Name of the program file should exactly match the class name.
    When saving the file, you should save it using the class name (Remember Java is case sensitive) and append '.java' to the end of the name (if the file name and the class name do not match your program will not compile).
    Example : Assume 'MyFirstJavaProgram' is the class name. Then the file should be saved as'MyFirstJavaProgram.java'
  • public static void main(String args[]) - Java program processing starts from the main() method which is a mandatory part of every Java program..

March 25, 2013

How to delete first preceding element

Here is the html from which I want to remove the div element 'div' by clicking the link.

      <a href="javascript:void(0)" class="link">My link</a>

To access the preceding element I will use the .parents() method of jQuery and then call 
the method .remove() to remove the 'div' element.

Here is the code...

$('a.link').click(function() {