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Exceptions

Exceptions. Three categories of errors: Syntax errors Runtime errors Logic errors Syntax errors: rules of the language have not been followed. Runtime error: program is running and environment detects an operation that is impossible to carry out.

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Exceptions

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  1. Exceptions • Three categories of errors: • Syntax errors • Runtime errors • Logic errors • Syntax errors: rules of the language have not been followed. • Runtime error: program is running and environment detects an operation that is impossible to carry out. • Logic errors: the program doesn’t perform the way it intend to

  2. Runtime error • Runtime errors occur for various reason • Invalid input • Open a file which doesn’t exist • Access out-of-bounds array • … • Runtime errors cause exceptions • Exception are handled differently from the events of GUI programming • A event may be ignored in GUI programming, but an exception cannot be ignored. • Java provides programmers with the capability to elegantly handle runtime errors.

  3. An example import javax.swing.JOptionPAne; public class test{ public static void main(String[] args){ String input = JOptionPane.showInputDialog(null, “Please enter an integer”); int number = Integer.parseInt(input); JOptionPane.shoeMessageDialog(null, “the number entered is “ + number); } } // if you entered a floating-point value, the program terminates

  4. Catch this error: try–catch block import javax.swing.JOptionPAne; public class test{ public static void main(String[] args){ try{ String input = JOptionPane.showInputDialog (null, “Please enter an integer”); int number = Integer.parseInt(input); JOptionPane.shoeMessageDialog(null, “the number entered is “ + number); } catch{ Exception ex) { JOptionPane.showMessageDialog(null, “Incorrect input: an integer is required”); } } }

  5. Exception classes • A Java exception is an instance of a class derived from Throwable. • The Throwable class is contained in the java.lang package • Subclasses of Throwable are contained in various packages • Errors related to GUI components are included in the java.awt package; • Numeric exceptions are included in the java.lang package • You can create your own exception classes by extending Throwable or a subclass of Throwable.

  6. Exception classes • The exception classes can be classified into three types: • System errors: are thrown by the JVM and represented in the Error class. The Error class describe internal system errors • Exceptions: are represented in the Exception class, which describe errors caused by your program and by external circumstance • Runtime exceptions; are represented in the RuntimeException class, which describes programming errors, such as bad casting, accessing an out-of –bounds array, and numeric errors.

  7. Exception classes and subclasses • Object • Throwable • Exception • ClassNotFoundException • IOException • AWTException • RuntimeException • ArithmeticException • NullPointerException • IndexOutOfBoundsException • Error • LinkageError • VirtualMachineError • AWTError • …

  8. Unchecked Exceptions • RuntimeException, Error and their subclasses • Reflect programming logic errors • Not recoverable • Should be corrected in the program • Can occur anywhere • Java does not mandate that you write code to catch or declare • For example, a NullPointerException is thrown if you access an object through a reference variable before an object is assigned to it;

  9. Checked exception • All other exception are known as checked exception • The compiler forced the programmer to check and deal with them

  10. Exception handling • Java’s exception-handling model is based on three operations • Declaring an exception • Throwing an exception • Catching an exception • Every method must state the types of checked exceptions it might throw • Java doesnot require that you declare Error and RuntimeException.

  11. Declaring exception • Other exception thrown by the method must be explicitly declared • public void myMethod() throws IOException • { • … • } • If the method throws multiple exceptions • public void myMehtod() throws Exception1, Exception2, …, ExceptionN • { • }

  12. Throwing Exceptions • Suppose the argument must be non-negative, but a negative argument is passed, the program can created an instance of IllegalArgumentException and throw it • IllegalArgumentException ex = new IllegalArgumentException(“Wrong Argument”); • throw ex; • Or • throw new IllegalArgumentException(“Wrong Argument”);

  13. Catching Exceptions • try{ • statements; • } • catch (Exception1 e1) • { • } • catch (Exception2 e2) • { • } • finally{ • }

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