Difference between Error and Exception in Java

1. Introduction

In Java, Error and Exception are both subclasses of Throwable that represent issues that can arise during the execution of a program. However, they are used for different types of issues. 

Errors are typically used to indicate serious problems that a reasonable application should not try to catch, mostly related to the environment in which the application is running. 

Exceptions, on the other hand, are used for conditions that a program might want to catch and provide a way to recover.

2. Key Points

1. Errors indicate serious problems and are not intended to be caught and handled by the application.

2. Exceptions are conditions that a program might want to catch and are either checked or unchecked.

3. Checked exceptions are subject to the Catch or Specify Requirement and are checked at compile-time.

4. Unchecked exceptions, also known as runtime exceptions, are not checked at compile time.

3. Differences

Error	Exception
Indicates serious problems that an application should not attempt to handle.	Indicates conditions that a program might want to catch or handle.
Not meant to be caught and handled by the application.	Can be caught and handled to allow the application to recover or gracefully shut down.
Often related to the environment in which the application is running.	Usually, due to application logic issues or external circumstances, the application can anticipate.

4. Example

// Example of Error
try {
    // Assume that the Java Virtual Machine runs out of memory
    throw new OutOfMemoryError("Out of Memory Error thrown");
} catch (Throwable t) {
    System.out.println(t.getMessage());
}

// Example of Exception
try {
    // Trying to access an index that does not exist
    int[] numbers = {1, 2, 3};
    System.out.println(numbers[5]);
} catch (Exception e) {
    System.out.println(e.getMessage());
}

Output:

Out of Memory Error thrown
Index 5 out of bounds for length 3

Explanation:

1. An OutOfMemoryError is thrown manually to simulate an environment issue where the JVM runs out of memory.

2. An ArrayIndexOutOfBoundsException, which is an unchecked exception, occurs when attempting to access an array index that does not exist.

5. When to use?

- Do not try to catch Errors in your application, as they are indications of serious problems that an application should not attempt to handle.

- Catch and handle Exceptions in your application to recover from common, often recoverable, issues or to perform clean-up operations when a recoverable exception occurs.