Java Exceptions - Java

What is Java Exceptions?

An exception (or exceptional event) is a trouble that arises during the execution of a program. When an Exception takes place the normal flow of this system is disrupted and this system/application terminates abnormally, which isn't always endorsed, therefore, those exceptions are to be treated.

An exception can arise for plenty special reasons. Following are some scenarios where an exception takes place.

  • A user has entered an invalid data.
  • A document that wishes to be opened cannot be observed.
  • A network connection has been misplaced within the center of communications or the JVM has run out of memory.
  • Some of these exceptions are due to user error, others by using programmer error, and others by physical resources which have failed in some manner.
  • based on those, we've got three categories of Exceptions. You need to recognize them to know how exception managing works in Java.
  • Checked exceptions − A checked exception is an exception that takes place at the compile time, those are also called as collect time exceptions. those exceptions can't surely be ignored at the time of compilation, the programmer must contend with (deal with) these exceptions.

as an example, in case you use FileReader elegance for your application to read records from a file, if the record laid out in its constructor does not exist, then a FileNotFoundException happens, and the compiler activates the programmer to address the exception.

Example

If you try to compile the above program, you will get the following exceptions.

Output


Note −
Since the methods read() and close() of FileReader class throws IOException, you can observe that the compiler notifies to handle IOException, along with FileNotFoundException.

  • Unchecked exceptions − An unchecked exception is an exception that occurs at the time of execution. These are also called as Runtime Exceptions. These include programming bugs, such as logic errors or improper use of an API. Runtime exceptions are ignored at the time of compilation.

For example, if you have declared an array of size 5 in your program, and trying to call the 6th element of the array then an ArrayIndexOutOfBoundsExceptionexception occurs.

Example

If you compile and execute the above program, you will get the following exception.

Output

  • Errors − These are not exceptions at all, but problems that arise beyond the control of the user or the programmer. Errors are typically ignored in your code because you can rarely do anything about an error. For example, if a stack overflow occurs, an error will arise. They are also ignored at the time of compilation.

Exception Hierarchy

All exception classes are subtypes of the java.lang.Exception class. The exception class is a subclass of the Throwable class. apart from the exception class, there's some other subclass known as error that is derived from the Throw able class.

Mistakes are atypical conditions that take place in case of severe failures; those aren't dealt with through the Java packages. Mistakes are generated to signify errors generated by using the runtime environment. Instance: JVM is out of memory. Usually, applications cannot get over errors.

The Exception class has main subclasses: IOException class and RuntimeException class.

exceptions1

Following is a list of most common checked and unchecked Java's Built-in Exceptions

Exceptions Methods

Following is the list of important methods available in the Throwable class.

Sr.No.

Method & Description

1

public String getMessage()

Returns a detailed message about the exception that has occurred. This message is initialized in the Throwable constructor.

2

public Throwable getCause()

Returns the cause of the exception as represented by a Throwable object.

3

public String toString()

Returns the name of the class concatenated with the result of getMessage().

4

public void printStackTrace()

Prints the result of toString() along with the stack trace to System.err, the error output stream.

5

public StackTraceElement [] getStackTrace()

Returns an array containing each element on the stack trace. The element at index 0 represents the top of the call stack, and the last element in the array represents the method at the bottom of the call stack.

6

public Throwable fillInStackTrace()

Fills the stack trace of this Throwable object with the current stack trace, adding to any previous information in the stack trace.

Catching Exceptions

A method catches an exception using a combination of the try and catch keywords. A try/catch block is placed around the code that might generate an exception. Code within a try/catch block is referred to as protected code, and the syntax for using try/catch looks like the following −

Syntax

The code which is prone to exceptions is placed in the try block. When an exception occurs, that exception occurred is handled by catch block associated with it. Every try block should be immediately followed either by a catch block or finally block.

A catch statement involves declaring the type of exception you are trying to catch. If an exception occurs in protected code, the catch block (or blocks) that follows the try is checked. If the type of exception that occurred is listed in a catch block, the exception is passed to the catch block much as an argument is passed into a method parameter.

Example

The following is an array declared with 2 elements. Then the code tries to access the 3rd element of the array which throws an exception.

This will produce the following result −

Output

Multiple Catch Blocks

A try block can be followed by multiple catch blocks. The syntax for multiple catch blocks looks like the following −

Syntax

The previous statements demonstrate three catch blocks, but you can have any number of them after a single try. If an exception occurs in the protected code, the exception is thrown to the first catch block in the list. If the data type of the exception thrown matches ExceptionType1, it gets caught there. If not, the exception passes down to the second catch statement. This continues until the exception either is caught or falls through all catches, in which case the current method stops execution and the exception is thrown down to the previous method on the call stack.

Example

Here is code segment showing how to use multiple try/catch statements.

Catching Multiple Type of Exceptions

Since Java 7, you can handle more than one exception using a single catch block, this feature simplifies the code. Here is how you would do it –

The Throws/Throw Keywords

If a method does not handle a checked exception, the method must declare it using the throws keyword. The throws keyword appears at the end of a method's signature.

You can throw an exception, either a newly instantiated one or an exception that you just caught, by using the throw keyword.

Try to understand the difference between throws and throw keywords, throws is used to postpone the handling of a checked exception and throw is used to invoke an exception explicitly.

The following method declares that it throws a RemoteException −

Example

A method can declare that it throws more than one exception, in which case the exceptions are declared in a list separated by commas. For example, the following method declares that it throws a RemoteException and an InsufficientFundsException −

Example

The Finally Block

The finally block follows a try block or a catch block. A finally block of code always executes, irrespective of occurrence of an Exception.
Using a finally block allows you to run any cleanup-type statements that you want to execute, no matter what happens in the protected code.
A finally block appears at the end of the catch blocks and has the following syntax −

Syntax

Example

This will produce the following result −

Output

Note the following −

  • A catch clause cannot exist without a try statement.
  • It is not compulsory to have finally clauses whenever a try/catch block is present.
  • The try block cannot be present without either catch clause or finally clause.
  • Any code cannot be present in between the try, catch, finally blocks.

The try-with-resources

Generally, when we use any resources like streams, connections, etc. we have to close them explicitly using finally block. In the following program, we are reading data from a file using FileReader and we are closing it using finally block.

Example

try-with-resources, also referred as automatic resource management, is a new exception handling mechanism that was introduced in Java 7, which automatically closes the resources used within the try catch block.

To use this statement, you simply need to declare the required resources within the parenthesis, and the created resource will be closed automatically at the end of the block. Following is the syntax of try-with-resources statement.

Syntax

Following is the program that reads the data in a file using try-with-resources statement.

Example

Following points are to be kept in mind while working with try-with-resources statement.

  • To use a class with try-with-resources statement it should implement AutoCloseable interface and the close() method of it gets invoked automatically at runtime.
  • You can declare more than one class in try-with-resources statement.
  • While you declare multiple classes in the try block of try-with-resources statement these classes are closed in reverse order.
  • Except the declaration of resources within the parenthesis everything is the same as normal try/catch block of a try block.
  • The resource declared in try gets instantiated just before the start of the try-block.
  • The resource declared at the try block is implicitly declared as final.
  • User-defined Exceptions
  • You can create your own exceptions in Java. Keep the following points in mind when writing your own exception classes −
  • All exceptions must be a child of Throwable.
  • If you want to write a checked exception that is automatically enforced by the Handle or Declare Rule, you need to extend the Exception class.
  • If you want to write a runtime exception, you need to extend the RuntimeException class.

We can define our own Exception class as below –

You just need to extend the predefined Exception class to create your own Exception. These are considered to be checked exceptions. The following InsufficientFundsException class is a user-defined exception that extends the Exception class, making it a checked exception. An exception class is like any other class, containing useful fields and methods.

Example

To demonstrate using our user-defined exception, the following CheckingAccount class contains a withdraw() method that throws an InsufficientFundsException.

The following BankDemo program demonstrates invoking the deposit() and withdraw() methods of CheckingAccount.

Compile all the above three files and run BankDemo. This will produce the following result −

Output

Common Exceptions

In Java, it is possible to define two catergories of Exceptions and Errors.

  • JVM Exceptions − These are exceptions/errors that are exclusively or logically thrown by the JVM. Examples: NullPointerException, ArrayIndexOutOfBoundsException, ClassCastException.
  • Programmatic Exceptions − These exceptions are thrown explicitly by the application or the API programmers. Examples: IllegalArgumentException, IllegalStateException.

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