Lecture 28. Exceptions and Inner Classes. Goals. We are going to talk in more detail about two advanced Java features:

Transcription

1 Lecture 28 Exceptions and Inner Classes Goals We are going to talk in more detail about two advanced Java features: Exceptions supply Java s error handling mechanism. Inner classes ease the overhead of subclassing and implementing interfaces. They make using the Java event model much more pleasant. 2 1

2 Throwing an Exception public static double average( double [] darray ) throws IllegalArgumentException if ( darray.length == 0 ) throw new IllegalArgumentException(); else double sum = 0.0; for ( int i = 0; i < darray.length; i++ ) sum += darray[ i ]; return sum / darray.length; 3 Declaring an Exception If a method can throw an exception, you can always declare the type of the exception in the header after the keyword throws public static double average( double [] darray ) throws IllegalArgumentException The compiler requires you to declare the possible exception throw if the exception class is not derived from RuntimeException or Error. These are called checked exceptions (checked by the compiler). Exceptions derived from RuntimeException or Error are called unchecked exceptions and their declaration is optional. IllegalArgumentException is actually unchecked. 4 2

3 Checked vs. Unchecked Exceptions The distinction between checked and unchecked exceptions is fairly arbitrary. In principle, checked exceptions are those which you, the programmer, are supposed to be able to fix at runtime, such as a FileNotFoundException. Very often these are generated in system code by user, not programmer error. Unchecked exceptions are supposed to be able to occur anywhere (so hard to check for) and to be the result of programmer error (so the best way of handling them is to fix the program). Good examples, NullPointerException, ArrayIndexOutOfBoundsException. Bad example, NumberFormatException, thrown e.g. by Integer.parseInt(String s). 5 Creating Exception Instances Exceptions are objects. You must create a new instance of an exception before you can throw it if ( darray.length == 0 ) throw new IllegalArgumentException(); Exceptions can be arbitrarily complex, but the exceptions supplied with the JDK possess a default constructor and a constructor that takes a single error message string. 6 3

4 Catching Exceptions Normal execution of a method ceases at the point an exception is thrown. The runtime environment then looks for an enclosing try block with a matching catch clause. After executing the catch clause, the program resumes with the first statement after the catch block. 7 throw/try/catch Pattern try if ( error ) throw new MyException(); //skip futher execution catch ( MyException e ) // handle exception e //resume execution 8 4

5 Catching Exceptions, 2 If there is no enclosing try block in the current method, or one with an appropriately typed catch clause, then the Java Virtual Machine goes hunting up the call stack for a matching try/catch pair. If the JVM can't find such a pair on the call stack, the default behavior is to print out an error message and halt the thread (often the whole program). 9 Catching Exception Up the Call Stack double [] mydoubles =... ; double a = 0.0; try a = average( mydoubles ); catch ( IllegalArgumentException e ) // do something about it System.out.println( Average = + a ); 10 5

6 Not Catching Exception Up the Call Stack import javax.swing.*; public class BadArgument public static void main( String [] args ) while ( true ) String answer = JOptionPane.showInputDialog( "Enter an integer" ); int intanswer = Integer.parseInt( answer ); // What happens if the user types %!Z$ if ( intanswer == 42 ) break; System.exit( 0 ); 11 Writing Your Own Exception Classes Writing your own exception class is simple. New exception classes allow you to handle a new type of error separately. Exception classes extend java.lang.exception. public class DataFormatException extends java.lang.exception public DataFormatException() super(); public DataFormatException(String s) super( s ); 12 6

7 Exceptions and Error Handling The Zen of error handling starts with the realization that the place you discover an error is almost never the place where you can fix it. Older languages like C use the horrible kludge of error codes. C++ introduced exceptions into the C family. But many programmers simply don t test for errors. 13 The Exception Strategy The goal of using exceptions is to split the problem of detecting errors from the problem of handling them. 14 7

8 Exceptions in the Calculator From CalculatorController:doOp() try... switch( et )... case Calculator.I_DIV: setacc( model.div() ); break; catch ( EmptyStackException e ) error(); 15 Exceptions in the Calculator, 2 Where does the exception get thrown? Not in CalculatorModel. public double div() throws EmptyStackException double bot = pop(); double top = pop(); return top / bot; 16 8

9 Exceptions in the Calculator, 3 CalculatorController: public double doop() does and puts FSM in ERROR state CalculatorModel: public double div() doesn t catch it! ArrayStack: public Object pop() throws EmptyStackException 17 Exceptions in the Calculator, 4 Despite the fact that EmptyStackException is an unchecked exception, it is caused here by user error (not entering enough operands) not programmer error. ArrayStack doesn't know how to fix it so it throws it. CalculatorModel doesn't know how to fix it, so it throws it. Finally, CalculatorController can do something about it so it catches the exception and puts the whole calculator into the Error state until the user clears it. 18 9

10 Exceptions and Inheritance Since exceptions are instances of classes, exception classes may use inheritance. AFileNotFoundException is a derived class of IOException. When an error is detected, you should create and throw a new instance of an appropriate type of exception. The stack is searched for the nearest catch statement matching the exception class or one of its superclasses. 19 static Nested Classes Now on to inner ( and nested) classes. You can define a static nested class inside another class: public abstract class java.awt.geom.line2d public static class Double... public static class Float

11 static Nested Classes, 2 It behaves like any other top level class except that its true name is the outer class name concatenated with the inner class name: e.g., Line2D.Double A nested class is considered to be part of the enclosing class: Make it public if you want methods in other classes to use it Make it private if you are only going to use it in the enclosing class 21 private static Nested Class public class SLinkedList implements List private int length = 0; private SLink first = null; private SLink last = null; private static class SLink Object item; SLink next; SLink( Object o, SLink n ) item = o; next = n; SLink( Object o ) this( o, null ); 22 11

12 Inner Classes If a nested class is not static, we call it an inner class. Instances of inner classes are usually created using new in an instance method of the enclosing class. Inner class methods have access to the instance variables and methods of the enclosing class instance. Inner classes can have constructors with or without arguments. 23 Inner Class Example public class BankAccount... private Bank home; public class Deposit private int id;... public boolean hascleared() Transaction t = home.gettrans(id); return t.cleared(); 24 12

13 Anonymous Inner Classes Are cheap inner classes in the sense that they are easy to define They are unnamed, so you can only create a single instance at the place you define them. They must extend a class or implement an interface. They can t have a defined constructor and, therefore, always use a default constructor. Are commonly used in Swing code to implement listeners or adapters. 25 Listener Example public class AnonExample extends JFrame private JLabel countlabel; private int count = 0; public AnonExample() JPanel mypanel = new JPanel(); JButton myb = new JButton( "Increment" ); mypanel.add( myb ); countlabel = new JLabel( "0" ); mypanel.add( countlabel ); 26 13

14 Listener Example, 2 myb.addactionlistener( new ActionListener() public void actionperformed(actionevent e) countlabel.settext( String.valueOf( ++count )); Variables from the enclosing ); class getcontentpane().add( mypanel, BorderLayout.CENTER ); 27 Listener Example Without Anonymous Class public class MyAction implements ActionListener private int count; private JLabel myl; public MyAction( JButton b, JLabel l ) count = 0; myl = l; b.addactionlistener(this); public void actionperformed(actionevent e) l.settext( String.valueOf( count++ )); 28 14

15 Adapters Some listener interfaces have many methods and you may only be interested in using one. For most Listener interfaces with multiple methods, there is a corresponding Adapter, a class with null implementations of the listener methods. Pattern: create an anonymous inner class that extends the adapter and override the single method you are interested in. 29 Adapter Example package java.awt.event; public interface MouseMotionListener extends EventListener public void mousedragged(mouseevent e); public void mousemoved(mouseevent e); public abstract class MouseMotionAdapter implements MouseMotionListener public void mousedragged(mouseevent e) public void mousemoved(mouseevent e) 30 15

16 Adapter Example, 2 import java.awt.event.*; import javax.swing.*; public class MouseMotion extends JFrame public MouseMotion() setsize( 600, 400 ); setdefaultcloseoperation( EXIT_ON_CLOSE ); getcontentpane().addmousemotionlistener( new MouseMotionAdapter() public void mousedragged( MouseEvent e ) System.err.println( "Dragged: " + e.getx() + ", " + e.gety() ); ); 31 16