Threads that read from text files concurently and place data in arrays

Question

The goal of this code is to run four threads, which will open four text files, read the words from them and then place them into a string array,

Main problems i know of:

1- I am not putting the concurrent function in the void run function, I want to be able to pass parameters into that function

2- I am not sure if I am modifying the global arrays of strings or not either

First is the main method:

 public static void main(String[] args) throws IOException 
    {
         //declare the threads
         Thread thread1 =  new Thread(ReadFile("list1.txt", Global.array1,"thread1"));
         Thread thread2 =  new Thread(ReadFile("list2.txt", Global.array2,"thread2"));
         Thread thread3 =  new Thread(ReadFile("list3.txt", Global.array3,"thread1"));
         Thread thread4 =  new Thread(ReadFile("list4.txt", Global.array4,"thread2"));

         /*error message from netbeans: cannot find symbol
            symbol:   method ReadFile(java.lang.String,java.lang.String[])

            it says it for every delcaration of the thread*/


         thread1.start();  //putting the threads to work
         thread2.start();
         thread3.start();
         thread4.start();

         thread1.join();   //telling the threads to finish their work
         thread2.join();
         thread3.join();
         thread4.join();


         // merging the arrays into one
         List list = new ArrayList(Arrays.asList(Global.array1));
         list.addAll(Arrays.asList(Global.array2));
         list.addAll(Arrays.asList(Global.array3));
         list.addAll(Arrays.asList(Global.array4));
         Object[] theArray = list.toArray();

           -------------------------etc----------------------------

This is the "thread class" if my vocab is right

public class ReadFile implements Runnable
{
    public void run(){
            //I should get stuff here, that's my problem!!!!
    }


    private String path;
    Thread runner;

    public ReadFile(String filePath, String[] toArray, String threadName) throws IOException
    {
        String path = filePath;
        FileReader fr = new FileReader(path);
        BufferedReader textReader = new BufferedReader(fr);

        int numOfLines = readLines();  
        toArray = new String[numOfLines];

        int i;
        for (i=0; i<numOfLines; i++)
        {
            toArray[i]= textReader.readLine();    //place next line into string array
        }

        textReader.close();
    }


   int readLines() throws IOException
    {
        FileReader fr = new FileReader(filePath);
        BufferedReader bf = new BufferedReader(fr);

        String aLine;
        int noOfLines = 0;

        while((aLine = bf.readLine()) != null)
        {
            noOfLines++;
        }
        bf.close();
        return noOfLines;
    }

    }

---

Finally i did a class for the global variable, and i don't know if it's a good idea

public class Global 
{
    public static String[] array1;
    public static String[] array2;
    public static String[] array3;
    public static String[] array4;
}

please let me know what you guys think, any help or explanations or tips would be greatly appreciated

Solution 1

If we first fix your file reading thread, so that the main work occurs in the run() method:

public class FileReader extends Thread {

  private final File file;
  private String[] lines;

  public FileReader(File file) {
    this.file = file;
  }

  @Override
  public void run() {
    // Read file here (populate `lines`)..
  }

  public String[] getLines() {
    return lines;
  }
}

Then we can utilise this in the main method, as follows:

public static void main(String[] args) throws Exception {
  List<FileReader> threads = new ArrayList<FileReader>();

  threads.add(new FileReader(new File("foo1")));
  threads.add(new FileReader(new File("foo2")));
  threads.add(new FileReader(new File("foo3")));
  threads.add(new FileReader(new File("foo4")));

  for (FileReader t : threads) {
    t.start();
  }

  List<String> allLines = new ArrayList<String>();

  for (FileReader t : threads) {
    t.join();
    allLines.addAll(Arrays.asList(t.getLines()));
  }    

  // File lines now in allLines
}

Solution 2

Three mistakes:

1. ReadFile is a class implementing Runnable, so you should pass an instance to the Thread constructor:

   Thread thread1 = new Thread(new ReadFile("list1.txt", Global.array1),"thread1");

2. Why create a Thread inside ReadFile? Remove that, no additional runner needed!

3. The ReadFile constructor has an additional parameter threadName which has to be removed.

Solution 3

Your constructor signature for ReadFile:

public ReadFile(String filePath, String[] toArray, String threadName)

does not match with the calls in main (you are only providing String, String[]). I think you meant:

Thread thread1 =  new Thread(new ReadFile("list1.txt", Global.array1, "thread1"));
Thread thread2 =  new Thread(new ReadFile("list2.txt", Global.array2, "thread2"));
Thread thread3 =  new Thread(new ReadFile("list3.txt", Global.array3, "thread1"));
Thread thread4 =  new Thread(new ReadFile("list4.txt", Global.array4, "thread2"));

You were probably confused by the fact that ReadFile's constructor takes the threadName as the third parameter, but the Thread class itself also has a similar parameter.

Comments

[Tudor]

1. That's what he's doing. 2. I agree with this.

[Arne Burmeister]

1. no, he missed the keyword new before ReadFile!