Java程序辅导

COMP5028 Object Oriented Analysis and Design  Semester 2, 2004 
 1
 
 
Laboratory session Six  
Storie Dixson 432 A,B 
Wednesday Sep 8, 2004 
School of Information Technologies 
The University of Sydney 
SOLUTION 
 
 
Objective 
• Understand COMMAND pattern 
• Understand the message exchange in the COMMAND pattern 
• Being able to implement simple COMMAND pattern design 
Tasks 
A. Figure 6.1 illustrate a design class diagram for a simple calculator. This calculator can 
only perform integer addition and subtraction operations. It keeps a history of all 
operations it has performed so far and can redo or undo previous operations for unlimited 
times. Identify different participants in this Command pattern such as “Invoker”, 
“Receiver”, “Client” and “Concrete Command”.  
 
Figure 6.1 Simple Calculator system 
 
(The Invoker, Receiver, Client and Concrete Command are all illustrated in the code sample) 
B. Draw a sequence diagram to show the message exchange when client invokes an 
AddCommand in the calculator 
COMP5028 Object Oriented Analysis and Design  Semester 2, 2004 
 2
 
(There are many alternative ways to draw this diagram, students might create the 
Calculator in TestCalculator rather than in User, which is indeed a better solution) 
 
C. Write a simple implementation of this calculator in Java. The code for TestCalculator is 
given below.  This application is adapted from the C# sample given here 
http://www.dofactory.com/patterns/PatternCommand.aspx You will get some helpful 
hints or even pieces of code from there. You can download and try the demonstration 
from the course web. Run it: java –jar tc.jar.  
COMP5028 Object Oriented Analysis and Design  Semester 2, 2004 
 3
TestCalculator.java: 
 
import java.util.ArrayList; 
 
interface Command{ 
 public void execute(); 
 public void unexecute(); 
} 
 
//concrete method 1 
 
class AddCommand implements Command{ 
 
 int operand; 
 Calculator calculator; 
  
 //Constructor 
 public AddCommand (Calculator calculator, int operand){ 
  this.calculator = calculator; 
  this.operand = operand; 
 } 
  
 public void execute(){ 
  calculator.add(operand); 
 } 
 public void unexecute(){ 
  calculator.sub(operand); 
 } 
} 
 
//concrete method 2 
 
class SubCommand implements Command{ 
  
 int operand; 
 Calculator calculator; 
 
 //Constructor 
 public SubCommand(Calculator calculator, int operand){ 
  this.calculator = calculator; 
  this.operand = operand; 
 } 
  
 public void execute(){ 
  calculator.sub(operand); 
 } 
 
 public void unexecute(){ 
  calculator.add(operand); 
 } 
} 
 
COMP5028 Object Oriented Analysis and Design  Semester 2, 2004 
 4
// "Receiver" 
class Calculator{ 
 private int total = 0; 
 
 public void add(int operand){ 
  total += operand; 
  System.out.println("Total becomes " + total +" after + 
" + operand); 
 } 
 
 public void sub(int operand){ 
  total -= operand; 
  System.out.println("Total becomes " + total + " after 
- " + operand); 
 } 
} 
 
//"Invoker" 
class User{ 
 private Calculator calculator = new Calculator(); 
 private ArrayList commands = new ArrayList(); 
 private int current = 0; 
 
 public void redo (int levels){ 
  System.out.println("---- Redo " + levels + " levels "); 
  for (int i = 0; i < levels; i ++) 
   if (current < commands.size() -1) 
   
 ((Command)commands.get(current++)).execute(); 
 } 
  
 public void undo (int levels){ 
  System.out.println("---- Undo " + levels + " levels "); 
  for (int i = 0; i < levels; i ++) 
   if (current > 0) 
    ((Command)commands.get(--
current)).unexecute(); 
 } 
 
 public void compute (char operator, int operand){ 
  // create command option and execute it 
  Command command; 
  switch (operator){ 
     case '+': 
      command = new AddCommand(calculator,operand); 
      break; 
  case '-': 
   command = new SubCommand(calculator, operand); 
   break; 
  default: 
   command = new AddCommand(calculator, operand); 
   break; 
COMP5028 Object Oriented Analysis and Design  Semester 2, 2004 
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  } 
  command.execute(); 
   
  //Add command to history 
  commands.add(command); 
  current ++; 
 } 
} 
 
// client 
public class TestCalculator{ 
 public static void main(String argv[]){ 
  // create user and let her compute 
  User user = new User(); 
  user.compute('+', 100); 
  user.compute('-', 50); 
  user.compute('-', 2); 
  user.compute('+', 20); 
   
  // undo the previous 3 operations 
  user.undo(3); 
  // redo the previous 2 operations  
  user.redo(2); 
 } 
}