Archive for 'Java'

Here are the steps to do extensive debugging of java code using Eclipse IDE with watch variables, expression and Inspect.

Step1: Enable breakpoint

Set the breakpoint at the line of code or point of method entry from where you would like to start debugging the code. Right click on the left margin of the editor next to the line of code and a context menu pops up. Select toggle breakpoint in the context menu as shown below:

Figure1: Toggle Breakpoint



Step2: Configure the breakpoint to stop execution

To start debugging, the execution should stop at the breakpoint specified. For this click on breakpoint properties (refer Figure1)  and do the following:

  1. Check Hit count
  2. Specify value as 1
  3. Select “Suspend thread” option

This will stop the execution when the program hits the breakpoint.



Step3: Switch to debug perspective

In Eclipse, select Window –>Open Perspective –> Debug as shown below:

Step4 switch to debug perspective


Step4: Run in debug mode

Now run the program in debug mode. Select Run –> Debug

Step5 Run in debug mode

Now the program starts running in debug mode and you would see the state of the thread as “running”

Step6 running thread

When the program hits the breakpoint the state of the thread changes from “running” to “suspended

Step7 suspend thread


Step5: Debugging the code with Expressions \Watch variables \ Inspect

Now the code stops at the breakpoint. You could use the watch variables / expression to monitor the current value of the debug variable.

Suppose you set the breakpoint at the variable named “counter” and the program stopped at counter variable as shown below:

step8 stops at code\

Add the expression with the variable name “counter” which allows you to monitor the value of the variable as you execute the program:

Step11 watch variableexpressions

You could also right click on the counter variable and select “Inspect” from the context menu

Step12 right click to inspectstepinto

If it is a method then, select the method name and click on “Step Into Selection“. This will allow you to monitor or debug the method execution line by line.


Step6: Use F6 key to step into the method

To “step into” the next executable line of code in the current method, press the “F6 Key”. This will pass the program control from the current line to the next executable line of code.

Step9 Press F6 key


The code is just a junkyard if it is not designed for reuse. Re-usability is very simple if you understand inheritance, a very powerful principle of object oriented programming. For this reason I call inheritance the Ace of the object-oriented programming world.

Here, I explain inheritance with a simple illustration. From the figure below can you list the properties that are common to all the balloons?


The below properties are common to every balloon:

  1. Color
  2. Size
  3. Price and so on

These properties that are common to all classes of a given type say balloon is stored in a class called the base class or super class or parent class. Inheritance is a powerful object oriented programming feature that allows a class to reuse the properties and methods of an already existing class while adding its own functionality. It’s the process of creating a new class as an extension of an existing class primarily to enhance the code re-usability. The class that extends is called the derived / sub class and the class getting extended is called the super / base class.

Suppose there is a super class called superBalloon. There are different kinds of balloons such as birthday balloons, balloons that represent a theme like sports or Disneyland and so on. For class inheritance java uses the keyword extends. Now let’s write a small java program for the base class “superBalloon”.


package balloonInherit;
public class superBalloon
	 * The superBalloon class defines methods and variables common to all balloons such as
	 * size, color and price
	//commonly used fields in all balloons
	public int size;
        public String color;
	public double price;
	public int number_of_balloons_purchased;
	public double total_cost;
	public void purchase(double price, int number_of_balloons_purchased)
		total_cost = (price * number_of_balloons_purchased);

A derived class is more specific and can have additional functionality of its own apart from that of the base class. Thus a birthday balloon is a specialized version of the balloon class having a new field called decoration cost specific to it’s type to better suit the needs.

We can also override /replace the behavior of the base class in the derived class. Here, in the below example the birthday balloon inherits the superBalloon class and defines a new field called decoration cost specific to it’s kind. We use the super keyword to reuse the already existing properties of the base class.

package balloonInherit;
public class subPartyBalloon extends superBalloon
	 * Since this is a partyBalloon, an additional new field called decoration cost is added
	 * which is specific to this kind of balloon
	public static void main(String args[])
		//Create an object for subPartyBalloon class and invoke the purchase method
                subPartyBalloon bDayBalloon = new subPartyBalloon();
	public void purchase()
		//Additional property specific to the type partyBalloon
		double decoration_cost = 2.00;
		//Reuse the existing properties of the base class using super keyword
		super.size = 20;
		super.number_of_balloons_purchased = 3;
		super.color = "red";
		//These are the prices of the balloon depending on it's size
		switch (super.size)
			case 10:
				super.price = 5.00;
			case 20:
				super.price = 10.00;
			case 30:
				super.price = 15.00;
				System.out.println("Invalid size");
		//Invoke the the purchase methods of the super class to calculate the base_cost
		//Since it is a birthday party balloon the additional cost for decoration is added
		total_cost = total_cost + decoration_cost;
		System.out.format("The total cost of %d %s birthday balloons is %.2f", number_of_balloons_purchased,color,total_cost);

The switch-case control structure decides the price of the balloon based on the input size. Once a matching size is found it assigns value to the price variable and breaks the execution of the switch-case block. The total cost of the birthday balloons is calculated and the output is as below.


The total cost of 3 red birthday balloons is 32.00

Now let’s create a java program for another derived class called subThemeBalloon. A theme based balloon has theme and design as additional fields specific to it’s kind. For example to represent the theme of Disneyland, a balloon is designed with the theme Disneyland and with the shape Mickey mouse.

The subThemeBalloon class reuses the existing properties of the base class superBalloon using the super keyword. It also has three additional new fields viz. theme, design and decoration cost specific to it’s kind to better suit the needs.

package balloonInherit;
public class subThemeBalloon extends superBalloon
	 * A theme based balloon has theme and design as additional fields. For example to represent
	 * the theme of Disneyland a balloon is designed with the theme Disneyland and with
	 * the shape mickey mouse
	public static void main(String[] args)
                //Create an object of subThemeBalloon class and invoke the purchase method
                subThemeBalloon themeBalloon = new subThemeBalloon();
	public void purchase()
		//Declare three additional properties that are specific to the type themeBalloon
		String theme = "Disneyland";
		String design = "Mickey Mouse";
		int decoration_cost = 5;
		//Reuse the existing properties of the base class using super keyword
		super.number_of_balloons_purchased = 20;
		super.price = 15.00;
		//Invoke the purchase method of the base balloon class to calculate base cost using super keyword
		super.purchase(price, number_of_balloons_purchased);
		//Add the decoration cost for the balloon
		total_cost = total_cost + decoration_cost;
		//Print the total cost of the balloon
		System.out.format("The total cost of a %s %s balloon is %.2f", theme, design, total_cost);


The total cost of a Disneyland Mickey Mouse balloon is 305.00


This is all about inheritance. Feel free to post any comments /questions.