command.java

These are the examples from the book Java Examples in a Nutshell, 2nd Edition, by David Flanagan.

/*
 * Copyright (c) 2000 David Flanagan.  All rights reserved.
 * This code is from the book Java Examples in a Nutshell, 2nd Edition.
 * It is provided AS-IS, WITHOUT ANY WARRANTY either expressed or implied.
 * You may study, use, and modify it for any non-commercial purpose.
 * You may distribute it non-commercially as long as you retain this notice.
 * For a commercial use license, or to purchase the book (recommended),
 * visit http://www.davidflanagan.com/javaexamples2.
 */
package com.davidflanagan.examples.reflect;
import java.awt.event.*;
import java.beans.*;
import java.lang.reflect.*;
import java.io.*;
import java.util.*;

/**
 * This class represents a Method, the list of arguments to be passed
 * to that method, and the object on which the method is to be invoked.
 * The invoke() method invokes the method.  The actionPerformed() method
 * does the same thing, allowing this class to implement ActionListener
 * and be used to respond to ActionEvents generated in a GUI or elsewhere. 
 * The static parse() method parses a string representation of a method
 * and its arguments.
 **/
public class Command implements ActionListener {
    Method m;       // The method to be invoked
    Object target;  // The object to invoke it on
    Object[] args;  // The arguments to pass to the method

    // An empty array; used for methods with no arguments at all.
    static final Object[] nullargs = new Object[] {};
    
    /** This constructor creates a Command object for a no-arg method */
    public Command(Object target, Method m) { this(target, m, nullargs); }

    /** 
     * This constructor creates a Command object for a method that takes the
     * specified array of arguments.  Note that the parse() method provides
     * another way to create a Command object
     **/
    public Command(Object target, Method m, Object[] args) { 
	this.target = target;
	this.m = m;
	this.args = args;
    }

    /**
     * Invoke the Command by calling the method on its target, and passing
     * the arguments.  See also actionPerformed() which does not throw the
     * checked exceptions that this method does.
     **/
    public void invoke()
	throws IllegalAccessException, InvocationTargetException
    {
	m.invoke(target, args);  // Use reflection to invoke the method
    }

    /**
     * This method implements the ActionListener interface.  It is like
     * invoke() except that it catches the exceptions thrown by that method
     * and rethrows them as an unchecked RuntimeException
     **/
    public void actionPerformed(ActionEvent e) {
	try {
	    invoke();                           // Call the invoke method
	}
	catch (InvocationTargetException ex) {  // but handle the exceptions
	    throw new RuntimeException("Command: " + 
				       ex.getTargetException().toString());
	}
	catch (IllegalAccessException ex) { 
	    throw new RuntimeException("Command: " + ex.toString());
	}
    }

    /**
     * This static method creates a Command using the specified target object,
     * and the specified string.  The string should contain method name
     * followed by an optional parenthesized comma-separated argument list and
     * a semicolon.  The arguments may be boolean, integer or double literals,
     * or double-quoted strings.  The parser is lenient about missing commas,
     * semicolons and quotes, but throws an IOException if it cannot parse the
     * string.
     **/
    public static Command parse(Object target, String text) throws IOException
    {
	String methodname;                 // The name of the method
	ArrayList args = new ArrayList();  // Hold arguments as we parse them.
	ArrayList types = new ArrayList(); // Hold argument types.

	// Convert the string into a character stream, and use the
	// StreamTokenizer class to convert it into a stream of tokens
	StreamTokenizer t = new StreamTokenizer(new StringReader(text));

	// The first token must be the method name
	int c = t.nextToken();  // read a token
	if (c != t.TT_WORD)     // check the token type
	    throw new IOException("Missing method name for command");
	methodname = t.sval;    // Remember the method name

	// Now we either need a semicolon or a open paren
	c = t.nextToken();
	if (c == '(') { // If we see an open paren, then parse an arg list
	    for(;;) {                   // Loop 'till end of arglist
		c = t.nextToken();      // Read next token

		if (c == ')') {         // See if we're done parsing arguments.
		    c = t.nextToken();  // If so, parse an optional semicolon
		    if (c != ';') t.pushBack();
		    break;              // Now stop the loop.
		}

		// Otherwise, the token is an argument; figure out its type
		if (c == t.TT_WORD) {
		    // If the token is an identifier, parse boolean literals, 
		    // and treat any other tokens as unquoted string literals.
		    if (t.sval.equals("true")) {       // Boolean literal
			args.add(Boolean.TRUE);
			types.add(boolean.class);
		    }
		    else if (t.sval.equals("false")) { // Boolean literal
			args.add(Boolean.FALSE);
			types.add(boolean.class);
		    }
		    else {                             // Assume its a string
			args.add(t.sval);
			types.add(String.class);
		    }
		}
		else if (c == '"') {         // If the token is a quoted string
		    args.add(t.sval);
		    types.add(String.class);
		}
		else if (c == t.TT_NUMBER) { // If the token is a number
		    int i = (int) t.nval;
		    if (i == t.nval) {           // Check if its an integer
			// Note: this code treats a token like "2.0" as an int!
			args.add(new Integer(i));
			types.add(int.class);
		    }
		    else {                       // Otherwise, its a double
			args.add(new Double(t.nval));
			types.add(double.class);
		    }
		}
		else {                        // Any other token is an error
		    throw new IOException("Unexpected token " + t.sval +
					  " in argument list of " +
					  methodname + "().");
		}

		// Next should be a comma, but we don't complain if its not 
		c = t.nextToken();
		if (c != ',') t.pushBack();
	    }
	}
	else if (c != ';') { // if a method name is not followed by a paren  
	    t.pushBack();    // then allow a semi-colon but don't require it.
	}

	// We've parsed the argument list.
	// Next, convert the lists of argument values and types to arrays
	Object[] argValues = args.toArray();
	Class[] argtypes = (Class[])types.toArray(new Class[argValues.length]);

	// At this point, we've got a method name, and arrays of argument
	// values and types.  Use reflection on the class of the target object
	// to find a method with the given name and argument types.  Throw
	// an exception if we can't find the named method.
	Method method;
	try { method = target.getClass().getMethod(methodname, argtypes); }
	catch (Exception e) {
	    throw new IOException("No such method found, or wrong argument " +
				  "types: " + methodname);
	}

	// Finally, create and return a Command object, using the target object
	// passed to this method, the Method object we obtained above, and
	// the array of argument values we parsed from the string.
	return new Command(target, method, argValues);
    }

    /**
     * This simple program demonstrates how a Command object can be parsed from
     * a string and used as an ActionListener object in a Swing application.
     **/
    static class Test {
	public static void main(String[] args) throws IOException {
	    javax.swing.JFrame f = new javax.swing.JFrame("Command Test");
	    javax.swing.JButton b1 = new javax.swing.JButton("Tick");
	    javax.swing.JButton b2 = new javax.swing.JButton("Tock");
	    javax.swing.JLabel label = new javax.swing.JLabel("Hello world");
	    java.awt.Container pane = f.getContentPane();

	    pane.add(b1, java.awt.BorderLayout.WEST);
	    pane.add(b2, java.awt.BorderLayout.EAST);
	    pane.add(label, java.awt.BorderLayout.NORTH);

	    b1.addActionListener(Command.parse(label, "setText(\"tick\");"));
	    b2.addActionListener(Command.parse(label, "setText(\"tock\");"));
	    
	    f.pack();
	    f.show();
	}
    }
}