Exception Handling

Exception Handling is a mechanism to handle runtime errors, ensuring the normal flow of the application can be maintained.

Throwable

The superclass of all errors and exceptions in the Java language. Only objects that are instances of this class (or one of its subclasses) are thrown by the JVM.

Exception

An event that disrupts the normal flow of the program. It is mainly caused by the application itself and can be handled.

Error

Indicates a serious problem that a reasonable application should not try to catch (e.g., OutOfMemoryError). It is usually unrecoverable.

Checked Exceptions

Exceptions checked at Compile Time. The compiler forces you to handle them (try-catch) or declare them (throws).

Example: IOException, SQLException

Unchecked Exceptions

Exceptions checked at Run Time. The compiler does not check these. They usually result from programming logic errors.

Example: NullPointerException, ArrayIndexOutOfBoundsException

Java Exception Keywords

Keyword	Description
try	Specifies a block of code to test for errors. Must be followed by `catch` or `finally`.
catch	Executes if an exception occurs in the try block. Handles the exception.
finally	Executes code after try/catch, regardless of result. Used for cleanup (closing files, etc).
throw	Used to explicitly throw an exception.
throws	Used in method signatures to declare that a method might throw an exception.

Rules for Exception Handling with Method Overriding

Rule 1: Parent declares NO Exception

Child can declare Unchecked exceptions.
Child cannot declare Checked exceptions.

Rule 2: Parent declares Unchecked Exception

Child can declare any Unchecked exception.
Child cannot declare Checked exceptions.

Rule 3: Parent declares Checked Exception

Child can declare the same exception.
Child can declare a subclass of that exception.
Child can declare no exception.

Rule 4: Mixed Exceptions

Rules follow the hierarchy constraint: Child cannot throw a broader Checked exception than the Parent.

Difference Between Error and Exception

Error	Exception
Impossible to recover (Unrecoverable).	Can be recovered using `try-catch`.
Always Unchecked (java.lang.Error).	Can be Checked or Unchecked.
Happens at Runtime.	Happens at Compile-time or Runtime.
Caused by environment (JVM, Hardware).	Caused by the application code/logic.

Common Checked Exceptions

ClassNotFoundException	Class definition not found.
InterruptedException	Thread interrupted while waiting.
IOException	Failed I/O operation.
SQLException	Database access error.
FileNotFoundException	File does not exist.

Common Unchecked Exceptions

ArithmeticException	Math error (e.g., divide by zero).
NullPointerException	Accessing object that is null.
ArrayIndexOutOfBounds	Invalid array index.
NumberFormatException	Invalid string-to-number conversion.
ClassCastException	Invalid type casting.

Data Types String Class

Java Training