无法写入Java txt文件

Question

I am currently using blueJ to learn java and I have an assignment where I have to write to a txt file, check if the file exists and read the file. The code I have is below which compiles but when I try to run the write() method,I receive the follow error java.lang.nullpointerexception;

I do not know where I am going wrong and it is beginning to drive me nuts at this stage.

import java.io.*;

public class ReadWrite
{
// instance variables - replace the example below with your own
private String file;
private String text;

/**
 * Constructor for objects of class ReadWrite
 */
public ReadWrite(String file, String text)
{
    // initialise instance variables
    file=this.file;
    text=this.text;
}

public void write() 
{


    try{
        FileWriter writer = new FileWriter(file);

        writer.write(text);
        writer.write('\n');
        writer.close();
    }
    catch(IOException e)
    {
        System.out.print(e);
    }


}

public boolean writeToFile()
{

    boolean ok;

    try{

        FileWriter writer = new FileWriter(file);

        {
          write();
        }

        ok=true;
    }

    catch(IOException e) {

        ok=false;

    }

    return ok;

    }

public void read(String fileToRead)
{
    try {
         BufferedReader reader = new BufferedReader(new        FileReader(fileToRead));
         String line = reader.readLine();

           while(line != null) {
               System.out.println(line);
               line = reader.readLine();
            }

            reader.close();

                }
                catch(FileNotFoundException e) {


                }  
                catch(IOException e) {

                } 
}

}

Answer 1

Your constructor is assigning the values in reverse. At the moment you have

public ReadWrite(String file, String text)
{
    // initialise instance variables
    file=this.file;
    text=this.text;
}

This is assigning the incoming variables file and text to the instance variables, which are null.

What you need to have is this:

public ReadWrite(String file, String text)
{
    // initialise instance variables
    this.file = file;
    this.text = text;
}

A useful way to avoid this in the future is to make your parameters final - this means you can't assign anything to them, and you'll catch this in the compiler.

public ReadWrite(final String file, final String text)
{
    // won't compile!
    file = this.file;
    text = this.text;
}

A further improvement would be to make the instance variables file and text final , which means they have to be assigned. This way, you're using the compiler to help you catch errors.

public class ReadWrite
{
    private final String file;
    private final String text;

    public ReadWrite(final String file, 
                     final String text)
    {
        this.file = file;
        this.text = text;
    }

    // ...
}