Java输入错误的if语句

Question

I'm running into the strangest error in this program, which is confirmed when debugging it. I have the following code (boiled down to highlight the problem, of course):

BHFrame.java

public class BHFrame
{
  private boolean uSS;
  private StateSaver stateSaver;

  public BHFrame(boolean useInternalStateSaver)
  {
    //Init code

    uSS = useInternalStateSaver;

    //More init code
    System.out.println(uSS);
    if (uSS)
    {System.out.println("Entered 1");
      stateSaver = new StateSaver(title, false);
      stateSaver.addSaveable(getThis());
    }

    //More init code
    System.out.println(uSS);
    if (uSS)
    {System.out.println("Entered 2");
      try
      {
        stateSaver.loadState();
        stateSaver.putState(getThis());
      }
      catch (IOException ex)
      {
        alertUserOfException(ex);
      }
    }
  }
}

GUI.java

public class GUI extends BHFrame
{
  public GUI(boolean useInternalStateSaver)
  {
    super(useInternalStateSaver);
  }
}

Main.java

public class Main
{
  public static void main(String[] args)
  {
    GUI gui = new GUI(false);
  }
}

Output

false
false
Entered 2
Exception in thread "main" java.lang.NullPointerException
    at bht.tools.comps.BHFrame.<init>(BHFrame.java:26)
    at bhms.GUI.<init>(GUI.java:5)
    at bhms.Main.main(Main.java:5)

The class BHFrame is extended and run from a child class that calls this constructor, but that really shouldn't affect this behavior. The problem is that, when false is passed to the constructor as useInternalStateSaver , the first if (uSS) is skipped, but the second is entered. Upon debugging, I found that uSS is false throughout runtime, including on the line of the second if statement, here. Why would Java enter an if statement when the condition returns false ? Before you ask, I did delete the .class files and recompile it just in case there was some residual code messing with it, but I got the same result. And rest assured, all the references to the uSS variable are displayed here.

Solution

---

As it turns out, this appears to be a bug in NetBeans 7.1 Build 201109252201, wherein the IDE doesn't properly insert new code into the compiled .class files. The problem was fixed by compiling the files externally. A bug report has been submitted.

Answer 1

Whatever's throwing that exception is probably not in your posted code.

It's not being caught by your catch statement, which only catches IOException.

It's a NullPointerException and can occur anywhere.

You have shown no indication that the code inside your if block is actually executing. In your screenshot, there is absolutely know way of knowing if your if block is entered or not. There are no logging statements.

Add debugging messages at various points to see exactly what is happening. Or, you know, look at line 26 (wayyyyy before your posted code) to see why you're getting a NullPointerException.

Answer 2

I've seen crazy stuff like this when there is bad RAM on the machine. You might want to run memtest86.

You might also consider deleting all of your project class files, and then doing a build. Maybe you changed Main.java, but it was never recompiled. I hate that when that happens.

Answer 3

This is just a guess, because I can't see the code you are mentioning, but I reckon you have defined a local variable uSS in the second //More init code segment.

Once you define a local variable named the same as an instance variable, it 'hides' the instance variable. Better to qualify all instance variables with this .

So, try qualifying all above accesses of uSS with this. ... ( this.uSS )

Even if this isn't the issue, it might be better to post the full code anyway.

HTH