java-exception.md

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The Overview of Java Exception

In Java, an exception is an event that occurs during the execution of a program that disrupts the normal flow of instructions. When an exception occurs, the normal flow of the program is interrupted, and the control is transferred to the appropriate exception handler. This helps in detecting and handling errors or exceptional conditions in the program.

In Java, all exceptions are objects that belong to a class derived from the java.lang.Exception class or the java.lang.RuntimeException class. There are three categories of exceptions in Java: checked exceptions, unchecked exceptions, and errors.

Checked exceptions are the exceptions that are checked at compile-time, and the programmer must handle them or declare them to be thrown in the method signature using the throws keyword.

Unchecked exceptions are the exceptions that are not checked at compile-time, and the programmer is not required to handle them or declare them to be thrown in the method signature.

Errors are the exceptional conditions that are not recoverable by the application, and the programmer is not expected to handle them.

Java provides several built-in exception classes, such as ArithmeticException, NullPointerException, ArrayIndexOutOfBoundsException, ClassCastException, FileNotFoundException, IOException, SQLException, etc., that can be used to handle specific types of exceptions. Additionally, programmers can also define their own exception classes by extending the Exception or RuntimeException classes.

Java Exception FAQ

1. What is an exception in Java?

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

2. What is the purpose of the try-catch block?

The try-catch block is used to handle exceptions in Java. The code that may throw an exception is placed in the try block, and the code that handles the exception is placed in the catch block.

3. What is the difference between checked and unchecked exceptions?

Checked exceptions are those that are checked at compile-time, while unchecked exceptions are not checked at compile-time. Checked exceptions must be either caught or declared in the method signature, while unchecked exceptions can be caught or left uncaught.

4. What is the difference between a runtime exception and an error in Java?

Runtime exceptions are exceptions that occur at runtime and are not checked at compile-time. Errors, on the other hand, are more serious problems that can occur during the execution of a program and cannot be handled by the program itself.

5. What is the difference between throw and throws in Java?

Throw is used to manually throw an exception, while throws is used to declare that a method may throw one or more exceptions.

6. What is the difference between final, finally, and finalize in Java?

Final is a keyword used to declare a variable or method that cannot be modified. Finally is a block of code that is always executed after a try-catch block, regardless of whether an exception is thrown or not. Finalize is a method called by the garbage collector to clean up resources before an object is destroyed.

7. What is a NullPointerException in Java?

A NullPointerException is an unchecked exception that occurs when an attempt is made to use a null reference where an object is required.

8. What is an OutOfMemoryError in Java?

An OutOfMemoryError is a type of error that occurs when the Java Virtual Machine (JVM) cannot allocate enough memory for an object.

9. What is the difference between the printStackTrace() and getMessage() methods of the Throwable class in Java?

The printStackTrace() method prints the stack trace of the exception to the standard error stream, while the getMessage() method returns the error message associated with the exception.

10. How can you create a custom exception in Java?

To create a custom exception in Java, you can extend the Exception or RuntimeException class and define your own exception class with the desired properties and methods.