Java中是否允许多个&&语句？

Question

Is the following code allowed in Java? My program doesn't work unless I comment this line out and I am not sure if the multiple && statements in the if is allowed.

if(number.isBusy() && number2.isBusy() && number3.isBusy() && number4.isBusy() && number5.isBusy() ) {}

Edited to add: If you comment out the line that is given above, the program will actually display something in the output window (although the only thing that gets something added to it is "number5") if it is left in, all of the outputs are zero.

This is the entire program right now:

// FILE: Line.java
// This program illustrates the use of the lineSimulate method which uses
// a simple queue to simulate cars waiting at a car wash.

import java.util.LinkedList;
import java.util.Queue;
import java.util.Scanner;
// import edu.colorado.simulations.BooleanSource;
// import edu.colorado.simulations.Line ;
// import edu.colorado.simulations.Averager;

/******************************************************************************
* The <CODE>CarWash</CODE> Java application illustrates the use of
* the <CODE>lineSimulate</CODE> method.
* The illustration uses the following values:
*   <CODE>
*   <br>lineTime = 240
*   <br>arrivalTime = 0.0025
*   <br>totalTime = 6000
*   </CODE>
*
* <p><dt><b>Java Source Code for this class:</b><dd>
*   <A HREF="../applications/CarWash.java">
*   http://www.cs.colorado.edu/~main/applications/CarWash.java
*   </A>
*
* @author Michael Main 
*   <A HREF="mailto:main@colorado.edu"> (main@colorado.edu) </A>
*
* @version
*   Jun 12, 1998
******************************************************************************/
public class LineWait
{
   /**
   * The main method activates <CODE>lineSimulate</CODE> with the values:
   *   <CODE>
   *   <br>lineTime = 240
   *   <br>arrivalTime = 0.0025
   *   <br>totalTime = 6000
   *   </CODE>
   * <BR>The <CODE>String</CODE> argument (<CODE>args</CODE>) is not used in
   * this implementation.
   **/
   public static void main(String[ ] args)
   {

      final int LINETIME = 100 + (int)(Math.random() * ((400) + 1));
      final double ARRIVALPROB = (Math.random() * ((.1) + 1));
      final int TOTALTIME = 6000;

      lineSimulate(LINETIME, ARRIVALPROB, TOTALTIME);
   }

   /**
   * Simulate the running of a car washer for a specified amount of time.
   * @param <CODE>lineTime</CODE>
   *   the number of seconds required to wash one car
   * @param <CODE>arrivalProb</CODE>
   *   the probability of a customer arriving in any second, for example
   *   0.1 is 10%
   * @param <CODE>totalTime</CODE>
   *   the total number of seconds for the simulation
   * <dt><b>Precondition:</b><dd>
   *   <CODE>lineTime</CODE> and <CODE>totalTime</CODE> are positive;
   *   <CODE>arrivalProb</CODE> lies in the range 0 to 1.
   * <dt><b>Postcondition:</b><dd>
   *   The method has simulated a car wash where <CODE>lineTime</CODE> is the
   *   number of seconds needed to wash one car, <CODE>arrivalProb</CODE> is
   *   the probability of a customer arriving in any second, and
   *   <CODE>totalTime</CODE> is the total number of seconds for the
   *   simulation. Before the simulation, the method has written its three
   *   parameters to <CODE>System.out</CODE>. After the simulation, the method
   *   has written two pieces of information to <CODE>System.out</CODE>:
   *   (1) The number of cars washed, and (2) The average waiting time for
   *   customers that had their cars washed. (Customers that are still in the 
   *   queue are not included in this average).
   * @exception java.lang.IllegalArgumentException
   *   Indicates that one of the arguments violates the precondition.
   **/
   public static void lineSimulate
   (int lineTime, double arrivalProb, int totalTime)
   {  
      Queue<Integer> arrivalTimes = new LinkedList<Integer>( ); 
      Queue<Integer> arrivalTimes2 = new LinkedList<Integer>( );  
      Queue<Integer> arrivalTimes3 = new LinkedList<Integer>( );
      Queue<Integer> arrivalTimes4 = new LinkedList<Integer>( );
      Queue<Integer> arrivalTimes5 = new LinkedList<Integer>( );
      int next;
      BooleanSource arrival = new BooleanSource(arrivalProb);
      Line number = new Line(lineTime);
      Line number2 = new Line(lineTime);
      Line number3 = new Line(lineTime);
      Line number4 = new Line(lineTime);
      Line number5 = new Line(lineTime);
      Averager waitTimes = new Averager( );
      Averager waitTimes2 = new Averager();
      Averager waitTimes3 = new Averager();
      Averager waitTimes4 = new Averager();
      Averager waitTimes5 = new Averager();
      int currentSecond;
      // Write the parameters to System.out.
      System.out.println("Seconds to wait in line " + lineTime);
      System.out.print("Probability of customer arrival during a second: ");
      System.out.println(arrivalProb);
      System.out.println("Total simulation seconds: " + totalTime); 

      // Check the precondition:
      if (lineTime <= 0 || arrivalProb < 0 || arrivalProb > 1 || totalTime < 0)
         throw new IllegalArgumentException("Values out of range");        




      for (currentSecond = 0; currentSecond < totalTime; currentSecond++)
      {  // Simulate the passage of one second of time.

         // Check whether a new customer has arrived.
         if (arrival.query( ))
            {
                System.out.println("Test1");
                if(number.isBusy() && number2.isBusy() && number3.isBusy() && number4.isBusy() && number5.isBusy() )
                {
                   System.out.println("Test2");
                    if(arrivalTimes.size() < arrivalTimes2.size() && arrivalTimes.size() < arrivalTimes3.size() && arrivalTimes.size() < arrivalTimes4.size() && arrivalTimes.size() < arrivalTimes5.size()) 
                    {
                        arrivalTimes.add(currentSecond);
                        System.out.println("Test");
                    }
                    else if(arrivalTimes2.size() < arrivalTimes.size() && arrivalTimes2.size() < arrivalTimes3.size() && arrivalTimes2.size() < arrivalTimes4.size() && arrivalTimes2.size() < arrivalTimes5.size())
                    {
                        arrivalTimes2.add(currentSecond);
                        System.out.println("Test");
                    }
                    else if(arrivalTimes3.size() < arrivalTimes.size() && arrivalTimes3.size() < arrivalTimes2.size() && arrivalTimes3.size() < arrivalTimes4.size() && arrivalTimes3.size() < arrivalTimes5.size())
                    {
                        arrivalTimes3.add(currentSecond);
                        System.out.println("Test");
                    }
                    else if(arrivalTimes4.size() < arrivalTimes.size() && arrivalTimes4.size() < arrivalTimes3.size() && arrivalTimes4.size() < arrivalTimes2.size() && arrivalTimes4.size() < arrivalTimes5.size())
                    {
                        arrivalTimes4.add(currentSecond);
                        System.out.println("Test");
                    }
                    else{arrivalTimes5.add(currentSecond);}
                }

            }

             // Check whether we can put the person into a line.
             if ((!number.isBusy( ))  &&  (!arrivalTimes.isEmpty( )))
             {
                next = arrivalTimes.remove( );
                waitTimes.addNumber(currentSecond - next);
                number.startMoving( );
             }
             if ((!number2.isBusy( ))  &&  (!arrivalTimes2.isEmpty( )))
             {
                next = arrivalTimes2.remove( );
                waitTimes2.addNumber(currentSecond - next);
                number2.startMoving( );
             }
             if ((!number3.isBusy( ))  &&  (!arrivalTimes3.isEmpty( )))
             {
                next = arrivalTimes3.remove( );
                waitTimes3.addNumber(currentSecond - next);
                number3.startMoving( );
             }
             if ((!number4.isBusy( ))  &&  (!arrivalTimes4.isEmpty( )))
             {
                next = arrivalTimes4.remove( );
                waitTimes4.addNumber(currentSecond - next);
                number4.startMoving( );
             }
             if ((!number5.isBusy( ))  &&  (!arrivalTimes5.isEmpty( )))
             {
                next = arrivalTimes5.remove( );
                waitTimes5.addNumber(currentSecond - next);
                number5.startMoving( );
             }


         // Subtract one second from the remaining time in the current li
         number.reduceRemainingTime( );
         number2.reduceRemainingTime( );
         number3.reduceRemainingTime( );
         number4.reduceRemainingTime( );
         number5.reduceRemainingTime( );
        }   




      // Write the summary information about the simulation.
      System.out.println("\nCustomers served Line1: " + waitTimes.howManyNumbers( )); 
      if (waitTimes.howManyNumbers( ) > 0)
         System.out.println("Average wait Line1: " + waitTimes.average( ) + " sec");
         System.out.println("\nCustomers served Line2: " + waitTimes2.howManyNumbers( )); 
      if (waitTimes.howManyNumbers( ) > 0)
         System.out.println("Average wait Line2: " + waitTimes2.average( ) + " sec");
       System.out.println("\nCustomers served Line3: " + waitTimes3.howManyNumbers( )); 
      if (waitTimes.howManyNumbers( ) > 0)
         System.out.println("Average wait Line3: " + waitTimes3.average( ) + " sec");
       System.out.println("\nCustomers served Line4: " + waitTimes4.howManyNumbers( )); 
      if (waitTimes.howManyNumbers( ) > 0)
         System.out.println("Average wait Line4: " + waitTimes4.average( ) + " sec");
          System.out.println("\nCustomers served Line5: " + waitTimes5.howManyNumbers( )); 
      if (waitTimes.howManyNumbers( ) > 0)
         System.out.println("Average wait Line5: " + waitTimes5.average( ) + " sec");

} 
}

Answer 1

The expression is perfectly legal syntax. The && operator is a short-circuit logical AND: the evaluation proceeds left to right and will stop at the first term (if any) that evaluates to false .

EDIT

Based on your edit, I'm guessing that the problem is that the body of the if is entered only if all of number , . . ., number5 are busy. (This is just a guess because you don't post what the Line class looks like or exactly what what behavior you want to see.)

As a side note, I think your program would be a lot more readable (and maintainable) if you defined a class that encapsulated a Queue , a Line and an Averager . (Presumably these three objects taken together make up something meaningful to your problem domain.) Then you could declare an array (or, perhaps better, a List ) of these objects and work with them instead of working with them as independent variables.

Answer 2

是的，一个人可以将有效数量的逻辑运算符链接在一起。

Answer 3

Yes, this is allowed. The && is the shortcircuit operator, so once one of the predicates fails no further evaluation will occur. If you want all the conditions to be evaluated you can use & (ie only one ampersand) and all predicates will evaluated. The primary reason for wanting this is if the predicates have side-effects that you want to occur. That's usually bad practice but in your case it looks like that's what you want...

if (number.isBusy() &
    number2.isBusy() &
    number3.isBusy() &
    number4.isBusy() & 
    number5.isBusy()) {
}

each of the isBusy() will execute and the result will be the AND of the return values.

if (number.isBusy() &&
    number2.isBusy() &&
    number3.isBusy() &&
    number4.isBusy() &&
    number5.isBusy()) {
}

each isBusy() will execute until one fails, the following isBusy() 's won't be called.

Answer 4

user1247751 should provide more information for a better answer.

However, with the information provided, I can speculate that one of the isBusy methods changes the state of the system so that it does not work.