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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