为什么在另一个类中使用PrintWriter时会出现PrintWriter空指针异常

Question

I have been using Java for a while for tasks and support tools, but I have never worked out how to use PrintWriter in a class that I can then use from another class, for example, I have several XML files that I want to parse, and then output to tables from a single input source (an .xls report), and I have several output files for the different XML's and child nodes. So I created this class to create a PrintWriter and report if the output file exists already:

public class PrintAdmin 
{
    public PrintWriter pout;
    public int PrintFromElement = 0;

    public PrintAdmin(String Dirname) throws IOException
    {
        PrintWriter pout = new PrintWriter(new BufferedWriter(new FileWriter((Dirname))));
        int PrintFromElement = 0;
        File f1 = new File("FirstNode.CSV");
        if(f1.exists()) 
        { PrintFromElement = 1; }
        else
        { PrintFromElement = 0; }
    }
    public void poutclose()
    {
        this.pout.close();
    }

}

This is called from:

public class AuditParser 
{
    public static File Auditor;

    public static void main(String[] args) throws IOException 
    { PrintAdmin xmltype1out = new PrintAdmin(xmltype1);
      ...
      for (int xml1 = PrintAdmin.PrintFromElement: xml1 < xml1parsed.header.size();xml1++)
      {
          ...dostuff...
      }

And this pattern is repeated 4 or 5 times.

My problem is that while I get a good compile using PrintAdmin in AuditParser , at runtime I get a NullPointerException .

I want to do it this way to have the simple class that creates a PrintWriter which instructs to print the header record or not depending on whether the file exits already. It is useful here, and I am sure it would be useful for me elsewhere....

Answer 1

You're defining a local variable in the constructor that's shadowing the field, which is never assigned. Drop the PrintWriter token in the constructor.

Answer 2

The problem is with this

public PrintWriter pout;

declared an instance variable but it is never initialized but question is why because in your constructor you did something like this

public PrintAdmin(String Dirname) throws IOException{
   PrintWriter pout = new PrintWriter(...);
}

which is creating a new local variable pout and it is initialized but the field defined above is never initialized. you can fix this by doing it this way

public PrintAdmin(String Dirname) throws IOException{
       pout = new PrintWriter(...);
    }

you also dont need that PrintFromElement again in your constructor you have already defined it as the field