Iterate / recurse through Containers and Components to find objects of a given class?

问题:

I've written a MnemonicsBuilder class for JLabels and AbstractButtons. I would like to write a convenience method setMnemonics( JFrame f ) that will iterate through every child of the JFrame and select out the JLabels and AbstractButtons. How can I obtain access to everything contained in the JFrame? I've tried:

LinkedList<JLabel> harvestJLabels( Container c, LinkedList<JLabel> l ) {     Component[] components = c.getComponents();     for( Component com : components )     {         if( com instanceof JLabel )         {             l.add( (JLabel) com );         } else if( com instanceof Container )         {             l.addAll( harvestJLabels( (Container) com, l ) );         }     }     return l; } 

In some situations, this works just fine. In others, it runs out of memory. What am I not thinking of? Is there a better way to search for child components? Is my recursion flawed? Is this not a picture of how things "Contain" other things in Swing - e.g., is Swing not a Rooted Tree?

JFrame | |\__JMenuBar |   | |    \__JMenu |       | |        \__JMenuItem | |\__JPanel |   | |   |\__JButton  |   | |   |\__JLabel |   | |   |\__ ... JCheckBoxes, other AbstractButtons, etc. 

回答1:

Agree with Tom here... Your problem is that you're already passing the List to add the JLabels down to your recursive method AND you're also returning it - thus adding the same items to your list more than once. In more politically correct terms - the List is your accumulator.

Your method should instead look like this:

public void harvestJLabels(Container c, List<JLabel> l) {     Component[] components = c.getComponents();     for(Component com : components) {         if(com instanceof JLabel) {             l.add((JLabel) com);         } else if(com instanceof Container) {             harvestJLabels((Container) com, l));         }     } } 

Then you can have a helper method to initiate this harvesting:

public List<JLabel> harvestJLabels(Container c) {     List<JLabel> jLabels = new ArrayList<JLabel>();     harvestJLabels(c, jLabels);     return jLabels; } 

回答2:

Here's your problem:

LinkedList<JLabel> harvestJLabels( Container c, LinkedList<JLabel> l ) {     ...                 l.addAll( harvestJLabels( (Container) com, l ) );     ...     return l; } 

You only have a single list. You are appending a list to another list. Therefore you are adding a list to itself. That may work in some sense, but you are going to have a doubling of length (exponential growth).

Either have a single List (usually there is no need to specify an algorithm in declarations) or create a new list instance each time the method is called. Avoid returning a reference you don't need to - it's just misleading.

Also ArrayList would be more appropriate than LinkedList. LinkedList is almost always the wrong choice.

回答3:

What if you had two components that each had one another in their components collection? It would infinetly recurse through them, adding to your collection.

You could have a circular reference somewhere, that may not be as obvious or simple as what I described. I'm not familiar with JFrame's, so I'm not sure if this is possible.

When doing this sort of thing you might need a "visited" property of some sort, so that you can mark objects as visited, and if so, don't do the recursive call on them.

回答4:

You can try to remove recursion to release the arrays of components:

LinkedList<JLabel> harvestJLabels( Container c, LinkedList<JLabel> l ) {     List<Container> containers = new ArrayList<Container();     containers.add(c);     while (!containers.isEmpty()) {         Container cont = containers.remove(0);         Component[] components = cont.getComponents();         for( Component com : components )         {             if( com instanceof JLabel )             {                     l.add( (JLabel) com );             } else if( com instanceof Container )             {                 containers.add((Container)com);             }         }     }     return l; } 

回答5:

Another utility method to stand on the shoulders of others such as @Maurice and @Cem:

public <T extends Component> List<T> harvestMatches(Container root, Class<T> clazz) {     List<Container> containers = new LinkedList<>();     List<T> harvested = new ArrayList<>();      containers.add(root);     while (!containers.isEmpty()) {         Container container = containers.remove(0);         for (Component component : container.getComponents()) {             if (clazz.isAssignableFrom(component.getClass())) {                 harvested.add((T) component);             } else if (component instanceof Container) {                 containers.add((Container) component);             }         }     }      return Collections.unmodifiableList(harvested); } 

and call it with something like:

List<AbstractButton> buttons = harvestMatches(root, AbstractButton.class); 

or

List<JLabel> labels = harvestMatches(root, JLabel.class); 

Although I also like the Swing-Fest approach by using org.fest.swing.core.GenericTypeMatcher

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