java - 为什么getDocumentFilter需要转换为AbstractDocument？

/*
 * To change this template, choose Tools | Templates
 * and open the template in the editor.
 */
package texteditor;
import javax.swing.*;
import javax.swing.text.*;
import javax.swing.event.*;

/**
 *
 * @author
 */
public class TextPad implements DocumentListener,ChangeListener{
    private JTextPane textArea;
    private Document textDoc;

    private int selectionOffset;
    private int selectionLength;

    public void init()
    {
        //System.out.println("Constructor invoked");
        // TODO code application logic here
        JFrame window = new JFrame("Text Editor");
        //JMenuBar menuBar = window.getJMenuBar();

        /**
        //Create menu bar
        JMenuBar menuBar= new JMenuBar();
        //File Menu
        JMenu fileMenu = new JMenu("File");
        fileMenu.add(new JMenuItem("Save"));
        * */

       // menuBar.add(fileMenu);
       //window.setJMenuBar(menuBar);

        this.textArea= new JTextPane();

        this.textDoc = this.textArea.getDocument();
        this.textDoc.addDocumentListener(this);
        this.textArea.getCaret().addChangeListener(this);
        //System.out.println(d.getClass());

        //override  default text generation ****THIS LINE******
       ((AbstractDocument)this.textDoc).getDocumentFilter();

        //Add scorllable interface in jtextpane
        window.add(new JScrollPane(this.textArea));

        window.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        window.setVisible(true);
    }
    public void changedUpdate(DocumentEvent e)
    {
       //System.out.println("changed");

    }
    public void removeUpdate(DocumentEvent e)
    {
          System.out.println("removed");
    }

    public void insertUpdate(DocumentEvent e)
    {

        try
        {
          System.out.println(this.textDoc.getText(0,this.textDoc.getLength()));

        }
        catch(Exception ex)
        {
            System.err.println(ex);
        }

    }
    /**
     *
     * @param e
     */
    public void stateChanged(ChangeEvent e)
    {
        System.out.println(this.textArea.getCaret().getMark());
    }

}

为什么将abstractDocument强制转换为((AbstractDocument)this.textDoc).getDocumentFilter();却不能像这样强制转换
this.textDoc.getDocumentFilter();确实抛出无法找到方法的错误。谁能解释一下？

编辑：

if(this.textDoc instanceof AbstractDocument)
        {
            System.out.println("Yes it is");
        }

打印Yes it is，这也意味着它是AbstractDocument的一种。我不明白，为什么AbstractDocument的调用方法会引发错误。

最佳答案

HoverCraft正确。使用Java，您只能根据其转换方式访问对象的方法和属性。因此，以证明

public interface Fooer {
    public int doFoo();
}

public class Foo implements Fooer{
    public int doFoo(){return 0;}
    public void doNoFooer(){}
}

public class Bar extends Foo {
    public void doYesFooer(){}
}

如果使用这些定义，将会发生以下情况：

Foo foo = new Bar();
foo.doNoFooer(); // fine because it is a method of Foo
foo.doFoo();
foo.doYesFooer(); // causes an error because the variable is improperly typed.

Fooer fooer = foo;
fooer.doFoo(); // find because it is part of the definition of Fooer
fooer.doYesFoo(); // causes an error because the variable is improperly typed.
fooer.doNoFoo(); // causes an error because the variable is improperly typed.

Bar bar = (Bar) foo;
// notice that the next three do not cause errors.
bar.doYesFooer();
bar.doNoFooer();
bar.doFoo();