设置丢弃模式

When enabling drop on a component, such as a list, you need to decide how you want the drop location to be interpreted. For example, do you want to restrict the user to replacing existing entries? Do you want to only allow adding or inserting new entries? Do you want to allow both? To configure this behavior, the JList class provides the setDropMode method which supports the following drop modes.

• The default drop mode for JList is DropMode.USE_SELECTION. When dragging in this mode, the selected item in the list moves to echo the potential drop point. On a drop the selected item shifts to the drop location. This mode is provided for backwards compatibility but is otherwise not recommended.
• In DropMode.ON, the selected item in the list moves to echo the potential drop point, but the selected item is not affected on the drop. This mode can be used to drop on top of existing list items.
• In DropMode.INSERT, the user is restricted to selecting the space between existing list items, or before the first item or after the last item in the list. Selecting existing list items is not allowed.
• DropMode.ON_OR_INSERT is a combination of the ON and INSERT modes.

The JTree class provides the same set of drop modes and the JTable class has several more specific to adding rows or columns.

To obtain the location of the drop, the TransferSupport class provides the getDropLocation method that returns the precise point where the drop has occurred. But for a list component, the index of the drop is more useful than a pixel location, so JList provides a special subclass, called JList.DropLocation. This class provides the getIndex and isInsert methods, which handle the math for you.

The table, tree, and text components each provide an implementation of DropLocation with methods that make the most sense for each component. The JTable.setDropMode method has the most choices. The following table shows the methods for all four classes:

Next is a demo that implements a custom transfer handler for a list component so that it fully participates in drag and drop.