Day-76 Understanding Exception Handling in Java with Eclipse IDE

Setting Up Java Project in Eclipse

To start working with Java in Eclipse, follow these steps:

1.File → New → Java Project
Provide a project name and click Finish.

2.Java Version
Use Java 21 (or your preferred version).

3.Create Package & Class

• Navigate to src → New → Package
• Enter the package name and save
• Then go to File → New → Class
• Enter the class name

4.Main Method Shortcut
Use Ctrl + Space inside the class to generate the main method quickly.

5.Generate Getters and Setters
Go to Source → Generate Getters and Setters to quickly manage class properties.

Java Exception Handling Basics

1. try and catch Blocks

try { // risky code } catch (Exception e) { // handle error } 

You can catch specific exceptions like NullPointerException or use a general Exception class to catch all exceptions.

Example of a NullPointerException:

Home h = null; h.string(); // This will throw NullPointerException 

2. Calling Methods on Null Objects

When you call methods on objects that haven’t been initialized, you encounter a NullPointerException. For example:

Demo demo = null; demo.display(); // Throws NullPointerException 

Smart Exception Handling Tips

• Use Superclass in Catch Block: Instead of catching every exception separately, catch their parent class:

 catch (Exception e) { e.printStackTrace(); } 

• Understanding Exception Class:
  In Java, every exception is a subclass of the Throwable class. Use this hierarchy to your advantage.

• Using throws Keyword:
  If a method does not handle the exception, it must declare it using throws.
  

 public void readFile() throws IOException { // code } 

• Can We Throw Objects? You can throw only objects that are instances of Throwable or its subclasses:

 throw new IOException("File not found"); 

• User-Defined Exceptions: You can create your own exceptions by extending the Exception class:

 class MyException extends Exception { public MyException(String message) { super(message); } } 

• Finally Block: Used for cleanup code that runs regardless of exception occurrence:

 finally { // code that runs always } 

Understanding Java Internals

• Object is the Parent Class:
  All Java classes by default inherit from the Object class.

• throw vs throws:

  • throw is used to actually throw an exception.
  • throws is used in method signature to declare exceptions.

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