listpostfixevaluator.java

来自「一个普通的计算器程序」· Java 代码 · 共 72 行

//************************************************************
//  ListPostfixEvaluator.java       Authors:  Lewis/Chase
//  
//  Provides an evaluator for integer postfix expressions
//  presented as a list.
//************************************************************
import jss2.*;
import java.util.Iterator;

public class ListPostfixEvaluator
{
  /**********************************************************
    Constructor
  **********************************************************/
  public ListPostfixEvaluator()
  {
  
  }
    
  /**********************************************************
    Returns the result of a postfix expression
  **********************************************************/
  public int evaluate(ArrayUnorderedList<String> tokenList)
  {
    ArrayStack<Integer> inStack = new ArrayStack<Integer>();
    int result, toPush=0,operand1=0, operand2=0;
    char tempChar;
    String tempToken;
    
    while (tokenList.size() > 0)
    {
      tempToken = tokenList.removeFirst();
      
      /** operator of length greater than 1 */
      if(tempToken.length()>1)
        inStack.push(new Integer(Integer.parseInt(tempToken)));
      
      else if(tempToken.length()==1)
      {
        tempChar = tempToken.charAt(0);
        
        /** if operator */
        if(tempChar >= '0' && tempChar <= '9')    
          inStack.push(new Integer(Integer.parseInt(tempToken)));
        
        /** if operand */
        else if( tempToken.equals("*")|| tempToken.equals("/") || 
                 tempToken.equals("+")|| tempToken.equals("-") )
        {
          /** get operator/operands for calculation */
          operand2 = inStack.pop();
          operand1 = inStack.pop();
          tempChar = tempToken.charAt(0);
          
          /** calculate */
          switch (tempChar) 
          {
            case '*': toPush = operand1 * operand2; break;
            case '/': toPush = operand1 / operand2; break;
            case '+': toPush = operand1 + operand2; break; 
            case '-': toPush = operand1 - operand2; break;  
          }
          
          inStack.push(new Integer(toPush));
        }
      }
    }
    
    return (inStack.pop());
  }
}