Checked vs Unchecked Exceptions.md

Checked vs Unchecked Exceptions in Java

1. What Are Exceptions?

An exception is an event that occurs during program execution and disrupts the normal flow of the program.

All exceptions and errors in Java inherit from the class:

java.lang.Throwable

Throwable has two major branches:

Throwable
 ├── Error        (Serious issues, not handled by programmer)
 └── Exception
       ├── Checked Exceptions
       └── Unchecked Exceptions (Runtime Exceptions)

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2. Checked Exceptions

Checked exceptions are checked at compile time. The compiler ensures you handle them using:

• try-catch
• or throws keyword.

If not handled, the program will NOT compile.

Examples:

• IOException
• SQLException
• ClassNotFoundException
• FileNotFoundException
• InterruptedException

Example:

public static void main(String[] args) {
    FileReader fr = new FileReader("data.txt");  // compile-time error if not handled
}

Error: Unhandled exception: FileNotFoundException

Correct Handling:

try {
    FileReader fr = new FileReader("data.txt");
} catch (FileNotFoundException e) {
    System.out.println(e.getMessage());
}

Or:

void readFile() throws FileNotFoundException {
    FileReader fr = new FileReader("data.txt");
}

When to use checked exceptions?

Use them when the caller is expected to recover from the issue (e.g., missing file, database error).

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3. Unchecked Exceptions (Runtime Exceptions)

Unchecked exceptions are not checked at compile time. They occur at runtime and usually indicate programming errors.

These exceptions inherit from:

java.lang.RuntimeException

Examples:

• NullPointerException
• ArrayIndexOutOfBoundsException
• ArithmeticException
• ClassCastException
• NumberFormatException

Example:

public static void main(String[] args) {
    int x = 10 / 0; // ArithmeticException at runtime
}

Compiler DOES NOT force you to handle this.

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#4. Differences Between Checked vs Unchecked Exceptions

Feature	Checked Exceptions	Unchecked Exceptions
Compile-time checking	✔ Yes	❌ No
Must be handled?	✔ Must	❌ Optional
Subclass of	Exception	RuntimeException
Common examples	IOException, SQLException	NPE, AIOOBE, ArithmeticException
Represents	Expected recoverable issues	Logical programming errors
Handling style	Required	Optional
Occurs	External environment issues	Programmer mistakes

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5. Exception Hierarchy Diagram

Throwable
 ├── Error
 │     ├── OutOfMemoryError
 │     └── StackOverflowError
 └── Exception
        ├── Checked Exceptions
        │     ├── IOException
        │     ├── SQLException
        │     └── FileNotFoundException
        └── Unchecked Exceptions (RuntimeException)
              ├── ArithmeticException
              ├── NullPointerException
              ├── ArrayIndexOutOfBoundsException
              └── ClassCastException

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6. Key Examples

6.1 Checked Exception Example

try {
    Thread.sleep(1000);
} catch (InterruptedException e) {
    e.printStackTrace();
}

6.2 Unchecked Exception Example

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

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7. Why Checked Exceptions Exist?

• They force the developer to handle predictable failures

• Useful for operations involving:

  • I/O
  • Database
  • Network
  • Resources

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8. Why Unchecked Exceptions Exist?

• To indicate bugs in code
• Developer is responsible for fixing the logic
• Handling them usually hides problems instead of solving them

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9. Should You Use Checked or Unchecked?

Use checked exceptions when:

• Caller can fix the problem
• External resource failures
• Example: File not found → user can give another file

Use unchecked exceptions when:

• It’s a programming mistake
• Example: Division by zero, accessing null, wrong index

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10. Interview Questions

Q1: Why Java has checked exceptions?

To enforce error handling for recoverable conditions.

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Q2: Why RuntimeException is unchecked?

Because they usually indicate bugs and should not be forced to handle.

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Q3: Is NullPointerException checked or unchecked?

Unchecked.

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Q4: Can we create custom checked & unchecked exceptions?

Yes.

Custom checked:

class MyCheckedException extends Exception {}

Custom unchecked:

class MyUncheckedException extends RuntimeException {}

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Key Takeaways

• Checked exceptions → must be handled (compile-time enforcement).

• Unchecked exceptions → occur at runtime; no compile-time checks.

• Both extend Throwable, but unchecked extend RuntimeException.

• Checked = recoverable; Unchecked = programming errors.

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