Java - 从文本文件中读取对象

Question

I have a program in Java with several classes, these being:

i) Shape ii) Square iii) Rectangle iv) Circle v) Triangle vi) Drawing Canvas vii) Application

Square, Rectangle, Circle and Triangle all inherit from class Shape

Now, in the Drawing Canvas class, I have two methods, one for writing objects of type Shape to a file and one for reading objects from the file.

This is the code:

The method which saves to the file works perfectly. However, I am having a problem reading the objects from the file and appending them to the Shapes ArrayList.

As it is, the application is given me the error that java.lang.String cannot be cast to shapes_assignment.Shape .

How can I read the text stored in the text file and use it to recreate the objects once more? Thanks :)

Answer 1

You need to write a reader that will take the input string, and make a shape out of it. Here is a simple example to show you how this can work, using the Java Date class . Not having your Shape constructors, I can't write your Shape for you.

Date today=new Date();
long var_to_write=today.getTime();

//Save this in to a file, then read it out as long var_in_file

Date previousTime=new Date(var_in_file);

Answer 2

you are adding a String into your ArrayList.

            while(input.hasNext())
            {
                temp.add(input.next());
            }

            Shapes.add((Shape)temp.get(i));

here you are trying to cast a String (temp.get(i) will return a String) to Shape type , which is leading to ClassCaseException .

create a Shape instance after you read from the file and populate your ArrayList (temp) with Shape instances.

      List<Shape> temp = new ArrayList<Shape>();
      //read from the scanner and make an Shape object. 
       Shape shape = new Shape(your arguments to the Shape cons);
       now, while adding into shapes you wont need a case, as you made yourself sure that your ArrayList would only accept Shape

       shapes(temp.get(i));

Answer 3

You should use an ObjectOutputStream to write the list of shapes, and retreave them from an ObjectInputStream :

out.writeObject(Shapes);

Shapes = (ArrayList)in.readObject();

assuming Shapes is an ArrayList .