Exception Hierarchy.md

Exceptions & Exception Hierarchy in Java

1. What Is an Exception?

An exception is an event that disrupts normal program execution. Java handles exceptions using:

• try
• catch
• finally
• throw
• throws

Exceptions are objects that inherit from the Throwable class.

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2. The Throwable Class

Throwable is the root of Java's exception hierarchy.

Two main subclasses:

1. Exception – recoverable issues
2. Error – serious system-level problems (not recoverable)

Throwable
   ├── Exception
   └── Error

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3. Exception vs Error

Aspect	Exception	Error
Meaning	Problems caused by program or external conditions	Serious issues in JVM/system
Recoverable?	Yes, usually	No
Handled with try-catch?	Yes	Not recommended
Examples	NullPointerException, IOException	OutOfMemoryError, StackOverflowError

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4. Exception Types

Exceptions are mainly categorized into:

1. Checked Exceptions
2. Unchecked Exceptions
3. Errors

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5. Checked Exceptions (Compile-Time)

Checked exceptions must be handled or declared using throws.

Examples:

5.1 IOException

Occurs during file operations.

FileReader fr = new FileReader("abc.txt");

5.2 SQLException

Database access problems.

5.3 ClassNotFoundException

Class not found during runtime loading.

5.4 FileNotFoundException

File missing.

5.5 InterruptedException

Thread interruptions.

5.6 ParseException

Error while parsing formatted text.

Checked exceptions represent external failures that the program should handle gracefully.

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6. Unchecked Exceptions (Runtime – Not Checked at Compile Time)

Subclass of RuntimeException. Do not need to be declared or handled.

Examples:

6.1 NullPointerException (NPE)

String s = null;
s.length();

6.2 ArithmeticException

int x = 10 / 0;

6.3 ArrayIndexOutOfBoundsException

int[] arr = new int[5];
arr[10] = 2;

6.4 NumberFormatException

int x = Integer.parseInt("abc");

6.5 ClassCastException

Object o = new Integer(5);
String s = (String) o;

6.6 IllegalArgumentException

Bad input to method.

6.7 IllegalStateException

Method called at wrong time.

Unchecked exceptions usually represent programming errors.

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7. Error (Unrecoverable)

Errors occur in the JVM or system. You should NOT handle them with try-catch.

Examples:

7.1 OutOfMemoryError

new int[1000000000];

7.2 StackOverflowError

void test() { test(); }

7.3 InternalError

JVM internal issue.

7.4 NoClassDefFoundError

Class missing at runtime.

7.5 AssertionError

Assertion failure.

Errors indicate serious problems the application cannot fix.

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8. Complete Exception Hierarchy (Diagram)

Throwable
│
├── Exception (Recoverable)
│     ├── RuntimeException (Unchecked)
│     │      ├── NullPointerException
│     │      ├── ArithmeticException
│     │      ├── ArrayIndexOutOfBoundsException
│     │      ├── ClassCastException
│     │      ├── NumberFormatException
│     │      └── IllegalArgumentException
│     │
│     ├── IOException (Checked)
│     │      ├── FileNotFoundException
│     │      └── EOFException
│     │
│     ├── SQLException (Checked)
│     ├── ClassNotFoundException (Checked)
│     ├── InterruptedException (Checked)
│     └── InvocationTargetException (Checked)
│
└── Error (Not Recoverable)
       ├── OutOfMemoryError
       ├── StackOverflowError
       ├── InternalError
       ├── AssertionError
       └── NoClassDefFoundError

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9. Most Common Exceptions Explained

NullPointerException

Accessing a method or property on a null reference.

ArithmeticException

Division by zero or illegal arithmetic operation.

ArrayIndexOutOfBoundsException

Index outside array boundary.

ClassCastException

Invalid type casting.

NumberFormatException

String → number conversion fails.

IllegalArgumentException

Invalid argument to a method.

IOException

I/O failures → external resource issues.

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10. Key Differences Summary Table

Category	Checked Exception	Unchecked Exception	Error
Parent	Exception	RuntimeException	Error
Checked at compile time?	Yes	No	No
Must handle using try/throws	Yes	No	No
Recoverable?	Usually	Sometimes	No
Example	IOException	NullPointerException	OutOfMemoryError

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Key Takeaways

• Throwable is the root of all exceptions and errors.

• Exception = problems you should handle.

• RuntimeException = programming mistakes.

• Error = serious system failures → not handled.

• Checked exceptions → must be handled.

• Unchecked exceptions → runtime failures.

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