Core Java - ExceptionAnexception is a problem that arises during the execution of a program. An exception can occur for many different reasons, including the following: To understand how exception handling works in Java, you need to understand the three categories of exceptions: Exception Hierarchy: All exception classes are subtypes of the java.lang.Exception class. The exception class is a subclass of the Throwable class. Other than the exception class there is another subclass called Error which is derived from the Throwable class. Errors are not normally trapped form the Java programs. These conditions normally happen in case of severe failures, which are not handled by the java programs. Errors are generated to indicate errors generated by the runtime environment. Eg : JVM is out of Memory. Normally programs cannot recover from errors. The Exception class has two main subclasses: IOException class and RuntimeException Class. ![]() Here is a list of most common checked and unchecked Java's Built-in Exceptions. Java’s Built-in Exceptions : Java defines several exception classes inside the standard java.lang package The most general of these exceptions are subclasses of the standard type UncheckedException. Since java.lang is implicitly imported into all Java programs, most exceptions derived from UncheckedException are automatically available. Java defines several other types of exceptions that relate to its various class libraries. Following is the list of Java Checked Exceptions. S.No Exception Description ---- ---------------------------- ----------------------------------------------------------------- 1 ClassNotFoundException Class not found. 2 IllegalAccessException Access to a class is denied. 3 InstantiationException Attempt to create an object of an abstract class or interface. 4 NoSuchFieldException A requested field does not exist. 5 NoSuchMethodException A requested method does not exist. 6 InterruptedException One thread has been interrupted by another thread. Following is the list of Java UnChecked Exceptions. S.No Exception Description ---- ----------------------------------- ------------------------------------------------------- 1 ArithmeticException Arithmetic error, such as divide-by-zero. 2 ArrayIndexOutOfBoundsException Array index is out-of-bounds. 3 ClassCastException Invalid cast. 4 IndexOutOfBoundsException Some type of index is out-of-bounds 5 NullPointerException Invalid use of a null reference. 6 NumberFormatException Invalid conversion of a string to a numeric format. 7 SecurityException Attempt to violate security. 8 StringIndexOutOfBoundsException Attempt to index outside the bounds of a string. Exceptions Methods : Following is the list of important methods available in the Throwable class S.No Methods 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. Syntax: try{ //Protected code }catch(ExceptionName e1){ //Catch 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. Eg: import java.io.*; public class Test { public static void main(String args[]){ try{ int a[]=new int[2]; System.out.println("Access element three :"+ a[3]); }catch(ArrayIndexOutOfBoundsException e){ System.out.println("Exception thrown :"+ e); } System.out.println("Out of the block"); } }Output : Exception thrown :java.lang.ArrayIndexOutOfBoundsException:3 Out of the block Multiple catch Blocks: A try block can be followed by multiple catch blocks. Syntax : try{ //Protected code }catch(ExceptionType1 e1){ //Catch block }catch(ExceptionType2 e2) { //Catch block }catch(ExceptionType3 e3) { //Catch block }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. Eg: try{ file =newFileInputStream(fileName); x =(byte) file.read(); }catch(IOException i){ i.printStackTrace(); return-1; }catch(FileNotFoundException f){ f.printStackTrace(); return-1; } throws/throw Keywords : If a method does not handle a checked exception, the method must declare it using the throwskeyword. 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 different in throws and throw keywords. The following method declares that it throws a RemoteException: import java.io.*; public class className{ public void deposit(double amount)throws RemoteException{ // Method implementation throw new RemoteException(); } //Remainder of class definition }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 eg, the following method declares that it throws a RemoteException and an InsufficientFundsException: import java.io.*; public class className{ public void withdraw(double amount)throws RemoteException, InsufficientFundsException { // Method implementation } //Remainder of class definition } finally Keyword : finally is a keyword which is used to create a block of code that follows a try block. A finally block of code always executes, whether or not an exception has occurred. 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: try{ //Protected code }catch(ExceptionType1 e1){ //Catch block }catch(ExceptionType2 e2){ //Catch block }catch(ExceptionType3 e3){ //Catch block }finally{ //The finally block always executes. }Eg: import java.io.*; public class Test { public static void main(String args[]){ int a[]=new int[2]; try{ System.out.println("Access element three :"+ a[3]); }catch(ArrayIndexOutOfBoundsException e){ System.out.println("Exception thrown :"+ e); } finally{ a[0]=6; System.out.println("First element value: "+a[0]); System.out.println("The finally statement is executed"); } } }Output : Exception thrown :java.lang.ArrayIndexOutOfBoundsException:3 First element value : 6 The finally statement is executed Note : Custom Exception : 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. We can define our own Exception class as below: class MyExceptio nextends Exception{ }You just need to extend the Exception class to create your own Exception class. 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. Eg: // File Name InsufficientFundsException.java import java.io.*; public class InsufficientFundsException extends Exception{ private double amount; public InsufficientFundsException(double amount){ this.amount = amount; } public double getAmount(){ return amount; } }To demonstrate using our user-defined exception, the following CheckingAccount class contains a withdraw() method that throws an InsufficientFundsException. // File Name CheckingAccount.java import java.io.*; public class CheckingAccount{ private double balance; private int number; public CheckingAccount(int number){ this.number = number; } public void deposit(double amount){ balance += amount; } public void withdraw(double amount)throws InsufficientFundsException{ if(amount <= balance){ balance -= amount; }else{ double needs = amount - balance; throw new InsufficientFundsException(needs); } } public double getBalance(){ return balance; } public int getNumber(){ return number; } }The following DemoAccount program demonstrates invoking the deposit() and withdraw() methods of CheckingAccount. // File Name DemoAccount.java public class DemoAccount{ public static void main(String[] args){ CheckingAccount c =new CheckingAccount(101); System.out.println("Depositing $500..."); c.deposit(500.00); try{ System.out.println("\nWithdrawing $100..."); c.withdraw(100.00); System.out.println("\nWithdrawing $600..."); c.withdraw(600.00); }catch(InsufficientFundsException e){ System.out.println("Sorry, but you are short $" + e.getAmount()); e.printStackTrace(); } } }Output : Depositing $500... Withdrawing $100... Withdrawing $600... Sorry, but you are short $200.0 InsufficientFundsException at CheckingAccount.withdraw(CheckingAccount.java:25) at BankDemo.main(BankDemo.java:13) Common Exceptions: In Java, it is possible to define two categories of Exceptions and Errors. |
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