爲什麼不返回對當前對象的引用總是返回相同的引用？

Question

我對Java相對比較陌生，我正在嘗試創建一個名爲VisualObject的類，其實例可以在JPanel之間拖放。

這是getTransferData功能VisualObject，它實現Transferable：

public Object getTransferData(DataFlavor d) 
     throws UnsupportedFlavorException, IOException { 

     if (d != visualObjectDataFlavor) { 
      throw new UnsupportedFlavorException(d); 
     } 
     return this; 
    } 

然而，在TransferHandler我JPanel的I跑這條線

System.out.println(t.getTransferData(VisualObject.visualObjectDataFlavor).equals(t.getTransferData(VisualObject.visualObjectDataFlavor))); 

而且它總是返回false。我怎樣才能實際返回一個指向正在傳輸的對象的一致指針？或者有更好的方法來傳輸它？

第二個編輯：

我原來的代碼仍然低於，但問題仍然出現在這個大大簡化代碼：

package trytwo; 

import java.awt.datatransfer.DataFlavor; 
import java.awt.datatransfer.Transferable; 
import java.awt.datatransfer.UnsupportedFlavorException; 
import java.io.IOException; 

public class SimplifiedVisualObject implements Transferable { 

    public static void main(String[] args) {   
     SimplifiedVisualObject a, b; 
     a = new SimplifiedVisualObject(); 
     b = new SimplifiedVisualObject(); 
     try { 
      System.out.println(a.getTransferData(DataFlavor.imageFlavor).equals(b.getTransferData(DataFlavor.imageFlavor))); 
     } catch (UnsupportedFlavorException e) { 
      e.printStackTrace(); 
     } catch (IOException e) { 
      e.printStackTrace(); 
     } 
    } 

    @Override 
    public Object getTransferData(DataFlavor flavor) 
      throws UnsupportedFlavorException, IOException { 
     return this; 
    } 

    @Override 
    public DataFlavor[] getTransferDataFlavors() { 
     return null; 
    } 

    @Override 
    public boolean isDataFlavorSupported(DataFlavor flavor) { 
     return false; 
    } 

} 

編輯：

下面是三個類我寫道：VisualObject（Transferable），ObjectFrame（它在內容窗格中保存着VisualObject）和Main，它們只是創建兩個ObjectFrame s。

VisualObject：

package tryone; 

import java.awt.Color; 
import java.awt.Graphics; 
import java.awt.Graphics2D; 
import java.awt.datatransfer.DataFlavor; 
import java.awt.datatransfer.Transferable; 
import java.awt.datatransfer.UnsupportedFlavorException; 
import java.awt.geom.Ellipse2D; 
import java.io.IOException; 
import javax.swing.JComponent; 

public class VisualObject extends JComponent implements Transferable { 

    private static final long serialVersionUID = -7105793092357377791L; 
    public static DataFlavor visualObjectDataFlavor = new DataFlavor(VisualObject.class, "Visual Object"); 

    public VisualObject() { 
     setOpaque(true); 
     setLayout(null); 
    } 


    @Override 
    protected void paintComponent(Graphics g) { 
     Graphics2D g2d = (Graphics2D) g; 
     g2d.setColor(Color.black); 
     g2d.fill(new Ellipse2D.Double(0, 0,getWidth(),getHeight())); 
    } 

    @Override 
    public Transferable getTransferData(DataFlavor d) 
      throws UnsupportedFlavorException, IOException { 

     if (d != visualObjectDataFlavor) { 
      throw new UnsupportedFlavorException(d); 
     } 
     return this; 
    } 


    @Override 
    public DataFlavor[] getTransferDataFlavors() { 
     DataFlavor d[] = {visualObjectDataFlavor}; 
     return d; 
    } 


    @Override 
    public boolean isDataFlavorSupported(DataFlavor d) { 
     if (d == visualObjectDataFlavor) { 
      return false; 
     } 
     return false; 
    } 

    public VisualObject getThis() { 
     return this; 
    } 
} 

ObjectFrame：

package tryone; 

import java.awt.BorderLayout; 
import java.awt.Color; 
import java.awt.Cursor; 
import java.awt.datatransfer.Transferable; 
import java.awt.datatransfer.UnsupportedFlavorException; 
import java.awt.dnd.DnDConstants; 
import java.awt.dnd.DragGestureEvent; 
import java.awt.dnd.DragGestureListener; 
import java.awt.dnd.DragSource; 
import java.awt.event.WindowAdapter; 
import java.awt.event.WindowEvent; 
import java.io.IOException; 
import javax.swing.JFrame; 
import javax.swing.JPanel; 
import javax.swing.TransferHandler; 


public class ObjectFrame extends JFrame implements DragGestureListener { 

    private static final long serialVersionUID = 4330669242071962048L; 
    protected Cursor draggingCursor = Cursor.getPredefinedCursor(Cursor.HAND_CURSOR); 
    private JPanel panel; 



    public ObjectFrame() { 
     setVisible(true); 
     setSize(300, 400); 
     setLayout(new BorderLayout()); 
     setTitle("Object Tray"); 

     addWindowListener(new WindowAdapter() { 
      @Override 
      public void windowClosing(WindowEvent e) { 
       System.exit(0); 
      } 
     }); 

     panel = new JPanel(); 
     panel.setBackground(Color.white); 
     panel.setLayout(null); 
     add(panel, BorderLayout.CENTER); 


     final DragSource ds = new DragSource(); 
     final DragGestureListener handle = this; 

     panel.setTransferHandler(new ObjectTransferHandler()); 

     java.awt.EventQueue.invokeLater(new Runnable() { 

      public void run() { 
       VisualObject v = null; 


       v = new VisualObject(); 
       v.setSize(50, 50); 
       v.setLocation(100, 100); 
       ds.createDefaultDragGestureRecognizer(v, 
         DnDConstants.ACTION_COPY, handle); 
       panel.add(v); 

       panel.repaint(); 
      } 
     }); 
    } 

    @Override 
    public void dragGestureRecognized(DragGestureEvent dge) { 
     if (dge.getComponent() instanceof Transferable) { 
      dge.startDrag(draggingCursor, (Transferable) dge.getComponent()); 
     } 
    } 

    private class ObjectTransferHandler extends TransferHandler { 
     private static final long serialVersionUID = 5072686271469629699L; 

     public ObjectTransferHandler() { 
     } 

     public boolean canImport(TransferSupport t) { 
      if (t.isDataFlavorSupported(VisualObject.visualObjectDataFlavor)) { 
       return true; 
      } 
      return false;   
     } 

     public boolean importData(TransferSupport ts) { 
      Transferable t = ts.getTransferable(); 
      try { 
       System.out.println(t.getTransferData(VisualObject.visualObjectDataFlavor) == (t.getTransferData(VisualObject.visualObjectDataFlavor))); 

       return true; 
      } catch (UnsupportedFlavorException e) { 
       // TODO Auto-generated catch block 
       e.printStackTrace(); 
      } catch (IOException e) { 
       // TODO Auto-generated catch block 
       e.printStackTrace(); 
      }   
      return false; 
     } 
    } 
} 

Main：

package tryone; 

import javax.swing.JFrame; 

public class Main { 

    public static void main(String[] args) { 
     JFrame f1 = new ObjectFrame(); 
     JFrame f2 = new ObjectFrame(); 
    } 

} 

Answer 1

關於SimplifiedVisualObject版本：該getTransferData()方法換貨政... this。由於您正在創建兩個不同的實例，因此您正在比較a.equals(b)，除非equals()被重寫，否則通常總是爲false。

關於你的原始問題：如果你調試你的代碼，你會發現這也是VisualObject的情況。每次您撥打getTransferData()時都會返回一個不同的實例，因此equals()比較返回false。

如果您使用DataFlavor.imageFlavor它將按預期工作，但我無法準確解釋原因。

也許當使用未知風味時，默認拖拽&拖放實現不知道如何處理它。結果是可傳輸對象在傳輸過程中被序列化和反序列化，導致始終創建不同的實例。

編輯：其實答案由Java tutorial that suggests因爲當你知道你只會傳遞本地對象（在同一應用程序內），你應該創建新DataFlavor像visualObjectDataFlavor = new DataFlavor(DataFlavor.javaJVMLocalObjectMimeType + ";class=tryone.VisualObject");（把它作爲一個靜態字段在你的例子中使用一個靜態初始化塊）。這將讓系統知道您正在傳遞應用程序中的本地對象，以便它將返回通過對象比較測試的相同實例。