Definition

Any exception which is must to handle or catch it while writing the program is called checked exception.

Any exception which is not mandatory to handle or catch it while writing the program or executing the program is called unchecked exception.

Example

FileNotFoundException

FileReader file = new FileReader("file.txt");

IOException

InputStream input = new FileInputStream("input.txt");

NullPointerException

Employee e1 = null;

System.out.println(e.name);

ArrayIndexOutOfBoundsException 

int a[] = new int[10];

System.out.println(a[20]);  

Hierarchy

They are all subclasses of Exception.

They are all subclasses of RuntimeException.

Ignorance

Must be formally acknowledged in the program one way or another.

Unchecked exceptions can be ignored completely in the code if desired.

Compilation

If your code fails to either handle a checked exception or declare that it is thrown, your code won't compile.

If your code fails to either handle an Unchecked exception or declare that it is thrown, your code will compile.

Handling verification

Checked Exception in Java is those Exceptions whose handling is verified during Compile time.

Unchecked Exception in Java is those Exceptions whose handling is not verified during Compile time.

JVM

JVM enforces developer to handle or catch it:

JVM does not enforce developer  to handle or catch it:

Propagation

By default Unchecked Exceptions are forwarded in calling chain (propagated).

By default, Checked Exceptions are not forwarded in calling chain (propagated).

Caught/ uncaught

They are also called as caught exceptions

They are also called as uncaught exceptions.

Throws clause

The throws clause on a method header must be included for checked exceptions that are not caught and handled in the method.

The throws clause on a method header is not mandatory to be included for unchecked exceptions that are not caught and handled in the method.

Scenario

CheckedException represent scenario with higher failure rate

UnCheckedException are mostly programming mistakes.

Overriding

• If the superclass method does not declare any exception 

If the superclass method does not declare any exception, subclass overridden method cannot declare the checked exception but it can declare unchecked exception.

• If the superclass method declares  exception 

If the superclass method declares an exception, subclass overridden method can declare same, subclass exception or no exception but cannot declare parent exception.

Use

• You try reading a file but someone deletes it between the time you check if it exists and the time the read operation begins. By declaring a checked exception, you are telling the caller to anticipate this failure so that giving the user an opportunity to try another filename. 

• You are uploading a file, but while uploading if network connection is lost. By declaring a checked exception, you are telling the caller to anticipate this failure so that giving the user an opportunity to try another attempt or resume from the point where stopped. 

• User input validation: User is expected to enter a digit, but he enters alphabet 

• Business logic or rules validation: User is not supposed to violet the rule of dividing a number by zero.