在写入文件时需要帮助

Question

i have the following class that writes to a file but is not working as i want. I call the write method in a while loop so that it writes a string to a new line. It only writes the last string. All the previous ones are not written except the last one.

Here is the class:

import java.io.FileReader;
import java.io.FileWriter;
import java.io.IOException;
import java.io.PrintWriter;

public class Writer {

 private static PrintWriter  outputStream = null;

 public Writer(String algorithmName){
  try {
   outputStream = new PrintWriter(new FileWriter(algorithmName+".txt"));
  } catch (IOException e) {
   e.printStackTrace();
  }
 }
     public  void write(String str){
        try {
                outputStream.append(str);
        }catch(Exception exc){

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

example code:

Writer w = new Writer("filename");
for(int = i; i < 10; i++){
w.write(i);
}

In this case i get as results:

9

instead of

0
1
2
3
...
9

what am i doing wrong?

thanks

Answer 1

You're closing the output stream after every write operation. You should call it one time after having written everything.

Answer 2

everytime you write, you are closing the stream which then forces a new file to be created that is why all that is in the file is 9.

Writer output = new BufferedWriter(new FileWriter(algorithmName+".txt"));
try {
  for(int = i; i < 10; i++){
      output.write( i );
  }
}
finally {
  output.close();
}

Answer 3

I'll be a bit off topic and recommend the Google Guava library to you, since it contains a lot of useful everyday functionality.

Using it, the solution to your problem becomes almost a one-liner with:

for(int i = 0; i < 10; ++i) {
    Files.append(String.valueOf(i) + "\n", new File("filename"), Charsets.UTF_8);
}

Edit: you shouldn't use this in large loops because the library is probably effectively opening and closing the file every single time. Not efficient at all.

Answer 4

You have several bad mistakes here. You have used a standard class name as your own class name; you are closing the stream after every write, as the others have noted; the stream is a static member when it should be an instance member; and you have an empty catch block which is preventing you from seeing any errors whatsoever. Never do that.