JAVA警告-取消引用可能的空指针。 我如何正确摆脱此警告？

Question

I am learning JAVA.

I cannot seem to find a way to get rid of this 'possible null derefence' warning. I have created fOut in the proper scope as null, otherwise I get a 'may not have been initialized' error.

I was unable to find an simple example that would help me fix this. I know it is probably a simple answer.

Thank you.

public static int waveToFile (String filename, byte[] byteWave)
{
    FileOutputStream fOut = null;


    File file = new File (filename);
    try
    {
        fOut = new FileOutputStream(file);
    }
    catch (IOException e)
    {

    }
    try{
        fOut.write(byteWave); //THIS IS WARNING of POSSIBLE DE-REFERENCE 
        fOut.close();                                  //OF NULL POINTER
       }
         catch (IOException e)
             {

             }


    return mErr;
}

Answer 1

If an exception is thrown fOut can be null . Therefore the compiler is warning you.

To avoid it, check that it is not null :

finally {
    if(fOut != null) {
        fOut.close();
    }
}

As a side note:

• do not just swallow exception (catching them doing nothing)
• put the close in a finally block to make sure it is executed
• do not write in fOut if there has been an exception

You can also use a try-with-resources statement which is perfectly safe and does the work for you:

try(fOut = new FileOutputStream(file)) {
    fOut.write(byteWave);
} catch(IOException e) {
    // do something with e
}

Answer 2

The fact is that you are constructing the file object and using it in two different try-catch blocks. So if this fails:

fOut = new FileOutputStream(file);

no value will be assigned to fOut , the first catch will handle the exception and the excution will continue with:

fOut.write(byteWave); //THIS IS WARNING of POSSIBLE DE-REFERENCE 
fOut.close();                                  //OF NULL POINTER

Hah! fOut is null because the previous block failed. The warning is well justified.

First of all, wrap the two operations in the same try-catch block:

FileOutputStream fOut = null;
File file = new File (filename);

try {
    fOut = new FileOutputStream(file);
    fOut.write(byteWave);
    fOut.close();
}
catch (IOException e) {
    // Do something
}

This piece of code has still a problem: if write fails and an exception is thrown, close will never be called. You can use a finally block to ensure that the file is closed no matter what or, better, a closing context:

File file = new File (filename);

try (FileOutputStream fOut = new FileOutputStream(file)){
    fOut.write(byteWave);
}
catch (IOException e) {
    // Do something
}

Answer 3

you need two try block.

public static int waveToFile (String filename, byte[] byteWave)
{
    FileOutputStream fOut = null;


    File file = new File (filename);
    try
    {
        fOut = new FileOutputStream(file);
        fOut.write(byteWave); 
        if(fOut !=null){
          fOut.close();  
        }

    }
    catch (IOException e)
    {

    }
    return mErr;
}