Basics Of Exceptions

1. What Is an Exception?

An exception is an event that occurs during program execution and disrupts the normal flow of the program.
When an error occurs, Java creates an exception object containing information about the problem.
Creating and sending this object is called throwing an exception.

int result = 10 / 0;

This causes Java to throw an exception because division by zero is not valid for integers.

2. How Java Handles an Exception

When an exception is thrown:

Exception occurs
      ↓
Exception object is created
      ↓
Java searches for a handler
      ↓
Current method
      ↓
Calling method
      ↓
Further up the call stack
      ↓
Handler found?
   ↙       ↘
 Yes        No
 ↓           ↓
catch       Thread terminates

Java searches the call stack for an appropriate exception handler.
If a suitable catch block is found, the exception is caught.
If no handler is found, the thread terminates.
If the terminated thread is the main thread, the program ends.

3. Catch or Specify Requirement

For checked exceptions, Java requires you to either:

Catch the exception

try {
    // code that may throw an exception
} catch (Exception e) {
    // handle exception
}

Or specify the exception

void readFile() throws IOException {
    // code that may throw IOException
}

So:

Catch it or declare it with throws.

If you do neither for a checked exception, the code will not compile.

4. Three Types of Exceptions

Java's exception hierarchy can be simplified as:

Throwable
├── Error
└── Exception
    ├── RuntimeException
    └── Other checked exceptions

A. Checked Exceptions

Represent conditions that an application should generally anticipate and recover from.
Must satisfy the Catch or Specify Requirement.
Examples:

IOException
FileNotFoundException
SQLException

Example:

void readFile() throws IOException {
    // ...
}

B. Runtime Exceptions

Represent programming errors or incorrect API usage.
They occur during runtime.
They do not have to be caught or declared.
Extend RuntimeException.

Examples:

NullPointerException
ArrayIndexOutOfBoundsException
IllegalArgumentException
ArithmeticException

Example:

String name = null;
System.out.println(name.length()); // NullPointerException

C. Errors

Represent serious problems that applications generally should not try to recover from.
Extend Error.
They are not subject to the Catch or Specify Requirement.

Examples:

OutOfMemoryError
StackOverflowError

5. Checked vs. Unchecked Exceptions

Type	Must catch/declare?	Typical meaning
Checked Exception	Yes	Expected/recoverable condition
`RuntimeException`	No	Programming/API error
`Error`	No	Serious system/JVM problem

RuntimeException and Error are collectively called unchecked exceptions.

Throwable
│
├── Error ──────────────── Unchecked
│
└── Exception
    │
    ├── RuntimeException ─ Unchecked
    │
    └── Other Exceptions ─ Checked

6. Why Does Java Have Checked Exceptions?

Checked exceptions force developers to explicitly think about certain failures.

For example:

void readFile() throws IOException

tells whoever calls the method:

"This method may fail because of an I/O problem. You need to deal with it."

This helps make error handling explicit and improves program reliability.

7. Important Terms

Term	Meaning
Exception	Event that disrupts normal execution
Throw	Create/send an exception
Catch	Handle an exception
Exception handler	Code that handles an exception
Call stack	Chain of methods that led to the current execution
Checked exception	Must be caught or declared
Unchecked exception	Does not have to be caught or declared
`throws`	Declares that a method may throw an exception
`try`	Contains code that may throw an exception
`catch`	Handles a thrown exception

8. Key Idea to Remember

The most important distinction is:

Checked Exception
        ↓
Must catch OR declare

RuntimeException / Error
        ↓
No catch-or-specify requirement

So, in simple terms:

Exceptions are Java's mechanism for reporting and handling problems during program execution. Checked exceptions must be handled or declared, while runtime exceptions and errors are unchecked.