java - 在 Java 中实现命令模式的问题

Question

我试图在java中简单地实现命令模式。但是，我收到以下错误:

java.lang.ClassNotFoundException: AddCommand
Exception in thread "main" java.lang.NullPointerException
at com.programming.sample.TransactionCommand.execute(TestTransactionCommand.java:64)
at com.programming.sample.CommandManager.runCommands(TestTransactionCommand.java:33)
at com.programming.sample.TestTransactionCommand.main(TestTransactionCommand.java:124)

代码:

            package com.programming.sample;

            //TestTransactionCommand.java
            import java.util.*;
            final class CommandReceiver {
              private int[] c;
              private CommandArgument a;
                 private CommandReceiver(){
                   c = new int[2];
                 }
                 private static CommandReceiver cr = new CommandReceiver();
                 public static CommandReceiver getHandle() {
                 return cr;
                 }
                 public void setCommandArgument(CommandArgument a) {
                 this.a = a;
                 }
                 public void methAdd() {
                 c = a.getArguments();
                     System.out.println("The result is " + (c[0]+c[1]));
                 }
                 public void methSubtract() {
                 c = a.getArguments();
                     System.out.println("The result is " + (c[0]-c[1]));
                 }
            }
             class CommandManager {
               private Command myCommand;
               public CommandManager(Command  myCommand) {
                 this.myCommand  =  myCommand ;    
               }
               public void runCommands( ) {
                        myCommand.execute();     
               }
             }
            class TransactionCommand implements Command {
              private CommandReceiver commandreceiver;
              private Vector commandnamelist,commandargumentlist; 
              private String commandname;
              private CommandArgument commandargument;
              private Command command;
              public TransactionCommand () {
                this(null,null);
              }
              public TransactionCommand ( Vector  commandnamelist, Vector
            commandargumentlist){
                this.commandnamelist = commandnamelist;
                this.commandargumentlist = commandargumentlist;
                commandreceiver =  CommandReceiver.getHandle();  
              }
              public void execute( ) {
                for (int i = 0; i < commandnamelist.size(); i++) {
                  commandname = (String)(commandnamelist.get(i));
                  commandargument = (CommandArgument)((commandargumentlist.get(i)));
                  commandreceiver.setCommandArgument(commandargument);
                  String classname = commandname + "Command";
                     try {
                       Class cls = Class.forName(classname);
                       command = (Command) cls.newInstance();
                     }
                     catch (Throwable e) {   
                              System.err.println(e);
                     }
                  command.execute();
                } 
              }
            }
             class AddCommand extends TransactionCommand {
               private CommandReceiver cr;
               public AddCommand () {
                  cr = CommandReceiver.getHandle();  
               }  
               public void execute( ) {  
                 cr.methAdd();  
               }   
             }
             class SubtractCommand extends TransactionCommand {
               private CommandReceiver cr;
               public SubtractCommand () {
                  cr = CommandReceiver.getHandle();  
               }
               public void execute( ) {
                 cr.methSubtract();
               }   
             }
             class CommandArgument {
               private int[] args;
               CommandArgument() {
                 args = new int[2];
               }
               public int[] getArguments() {
                return args;
               }
               public void setArgument(int i1, int i2) {
                     args[0] = i1; args[1] = i2;
               }
             }
             public class TestTransactionCommand {
               private  Vector clist,alist; 
               public TestTransactionCommand() {
                 clist = new Vector(); 
                   alist = new Vector();
               }
               public void clearBuffer(Vector c, Vector a) {
                 clist.removeAll(c);
                   alist.removeAll(a); 
               }
               public Vector getClist() {
                 return clist;
               }
               public Vector getAlist() {
                 return alist;
               }
                public static void main(String[] args) {

                 CommandArgument ca,ca2;
                 TestTransactionCommand t = new TestTransactionCommand();
                 ca = new CommandArgument();
                 ca.setArgument(2,8);
                 Vector myclist = t.getClist();
                 Vector myalist = t.getAlist();
                 myclist.addElement("Add"); 
                 myalist.addElement(ca);
                 TransactionCommand tc = new TransactionCommand(myclist,myalist);
                 CommandManager cm = new CommandManager(tc);       
                 cm.runCommands();

                 t.clearBuffer(myclist,myalist);
                 ca2 = new CommandArgument();
                 ca2.setArgument(5,7);
                 myclist = t.getClist();
                 myalist = t.getAlist();
                 myclist.addElement("Subtract"); 
                 myalist.addElement(ca2);
                 myclist.addElement("Add"); 
                 myalist.addElement(ca2);
                 TransactionCommand tc2 = new TransactionCommand(myclist,myalist);        
                 CommandManager cm2 = new CommandManager(tc2);       
                 cm2.runCommands();
               }
             } 

Answer 1

我想，这就是它失败的地方。

try {
    Class cls = Class.forName(classname);
    command = (Command) cls.newInstance();
}
catch (Throwable e) {   
    System.err.println(e);
}
command.execute();

显然，在类路径中找不到类 AddCommand，因此它导致了 ClassNotFoundException，您只需使用 System.err.println 忽略它(e);，然后尝试调用 command.execute();。对象command未初始化为实例。

也许，提供完整的类名，例如com.programming.sample.AddCommand，可以解决这个问题。