Hello,

So I've got trouble with a program I'm working on. The program is supposed to register studends and teachers to a register, and when I close it, I want to save all data with DataInputStream/DataOutputStream. So when I open the program, I want to see what I've registered. It's currently looking like this:

public abstract class School {
     private String name, address;
     private int zipcode;
     School next;

     public School(String n, String a, int z) {
         name = n;
         address = a;
         zipcode = z;
     }

     public void writeObjectToFile(DataOutputStream dos) {
         try {
             dos.writeUTF(name);
             dos.writeUTF(address);
             dos.writeInt(zipcode);
             dos.flush();
         }
         catch(IOException e) {
             // methods
         }
     }

     public boolean readObjectFromFile(DataInputStream dis) {
         try {
              while(true){
              name = dis.readUTF();
              address = dis.readUTF();
              zipcode = dis.readInt();
              }
         }
         catch(EOFException eof) {
              try {
                 dis.close();
              }
              catch(IOException e) {
                 // methods
              }
         }
         catch(IOException e2) {
            // methods
         }
         return true;
     }
     
     public String toString() {
          // methods for output
     }   
}

class Students extends School {
     private String name, iClass;
     private int age;

     public School(String n, String a, int z, String na, String c, int age) {
         super(n,a,z);
         name = na;
         iClass = c;
         age = a;
     }

     public void writeObjectToFile(DataOutputStream dos) {
         try {
             dos.writeUTF(this.getClass().getName()); // for identifying the class. 
             dos.writeUTF(name);
             dos.writeUTF(iClass);
             dos.writeInt(age);
             dos.flush();
         }
         catch(IOException e) {
             // methods
         }
     }

     public boolean readObjectFromFile(DataInputStream dis) {
         try {
              while(true){
                  dis.readUTF(); // identify Student object/class
                  name = dis.readUTF();
                  iClass = dis.readUTF();
                  age = dis.readInt();
              }
         }
         catch(EOFException eof) {
              try {
                  dis.close();
              }
              catch(IOException e) {
                 // methods
              }
         }
         catch(IOException e2) {
            // methods
         }
         return true;
     }
     
     public String toString() {
          // methods for output
     }
     
}

// I also have similar codes for class Teacher.

This is my Register class. I'll copy/paste the readFromFile- and writeToFile methods.

public class Register {
     // methods for insert a object and write a list. 
     School first;
     
     public void writeToFile(String filename) {
         try {
             DataOutputStream dos = new DataOutputStream(new FileOutputStream(filename));
             School node = first;
             while(node != null){
                  next.writeObjectToFile(dos);
                  node = node.next;
             }
             dos.close();
         }
         catch(IOException e) {
             // methods
         }
     }

     public void readFromFile(String filename) {
         DataInputStream dis = null;
         try {
              dis = new DataInputStream(new FileInputStream(filname));
              while(true){
                  String objectType = dis.readUTF(); // supposed to read the object type. for example Student. 
                  School object = newObject(objectType); // create new object with information on file. 
                  object.readObjectFromFile(dis); // fill the object with data
                  register(object); //I have a register method. Nothing wrong with this..
              }
         }
         catch(EOFException eof) {
              try {
                 dis.close();
              }
              catch(IOException e) {
                 // methods
              }
         }
         catch(IOException e2) {
            // methods
         }
     }

     public School newObject(String objectType) {
         switch(objectType) {
             case "Students": return new Students("", "", 0, "", "", 0); // not sure what values to give here. values will be given when I insert from GUI class. 
             case "Teachers": return new Teachers("", "", 0, "", "", 0);
         }
         return null; 
     }
}

I also have a GUI class and a main method class. GUI class got these methods:

public void writeToFile( String filename )
{
     register.writeToFile(filname);
}

public void readFromFile( String filename )
{
     register.readFromFile(filname);
}

My error is:

java.io.IOException: Stream Closed
	at java.io.FileInputStream.read(Native Method)
	at java.io.DataInputStream.readUnsignedShort(DataInputStream.java:337)
	at java.io.DataInputStream.readUTF(DataInputStream.java:589)
	at java.io.DataInputStream.readUTF(DataInputStream.java:564)
	at Register.readFromFile(Register.java:59)
	at GUIclass.readFromFile(GUIclass.java:215)
	at MainClass.main(MainClass.java:10)

on MainClass I have

gui.readFromFile("data.dat");

What I've done wrong? My readFromFile in register class is supposed to create a new object from the information in the file. Then I will fill this object with data with help of readObjectFromFile method. Then I'll add the object to the list.

Edited 4 Years Ago by Chario0z: n/a

how can you have a School constructor at all, not to mention, how can you have one in your Students class?

you say it doesn't run? I doubt it even compiles. check the error messages you get, those you can solve, solve them, those you can't, paste them here, and we'll help you out.

I could just make a new file Student.java and paste the code there. This used to be a finish program without data I/O-strams. We had to extend it with data I/O-streams.

It compiles fine, and it runs. But when I open it again to check if my data is saved, I get the error message I pasted in my last post. That's why it doesn't run properly.

yes, but we can't see what is at line 10 of MainClass and the part of GUIClass you show is 9 lines, so how are we to know what you do on line 215?

point for my previous post in: you have a constructor in an abstract class... you can't instantiate an abstract class.
next to that, you have a constructor for that abstract class in a class that inherits mentioned abstract class. seriously, did that even compile?

on my mainClass i have that line i posted in the first post. also gui.readFromFile("data.dat");

on line 215 i have readFromFile method which you can find in the box with 9 lines.

this program compiles and runs fine, call my teacher stupid for the constructors and the abstrsct methods. it just dont want to run the way i want it to, something wrong with the readFromFile and readObjectFromFile methods i guess.

well .. having a Students class with a School constructor.
looks to me like you would have a problem there. AFAIK, it's just not possible to get that compiled.

What's the worry with abstract class constructors? They are perfectly valid and normal.
When you instantiate a concrete subclass the superclass constructors will always be called, even if the superclass is abstract. Just try a new B() with these classes:

abstract class A {
   A() {System.out.println("A constructor");}
   abstract void method();
}

class B extends A {
   B(){System.out.println("B constructor");}
   void method(){}
}

well yes, but in your class B, you did not have a:

public A(){

}

constructor, which is what he is doing. agreed, was a bit off on the use of abstract constructors, they do can have their purpose (which I overlooked), but in the Abstract class itself, not in the inheriting class.

That's line 55 in the original code? Yes, that's an error. Considering that it begins with a valid call to super(...) I guess that was just a small slip. It continues with other errors (eg line 59 - that's why variable names are important!) so that's obviously not a compilable version.

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