Write common code for both SWT tree and table

Question

The SWT control classes Tree and Table have several methods with the same signature and that work in the same way.

For example:

• getItems
• getSelection

But these methods are defined directly on Tree and Table, there is no common super class or interface where the methods are declared. Because of this it is hard to write code that works for both Tree and Table.

Is there a solution to this that makes it possible to write common code for both of the classes?

Answer 1

One possible solution is to create a class with the common methods, with subclasses that wraps a Tree or a Table and that delegate these methods to their wrapped control.

Using such classes generic code can look like this:

CollectionControl c = CollectionControl.create(treeOrTable);
int nrItems = c.getSelectionCount();

The following is an example of such classes:

/**
 * Wrapps a {@link Tree} or {@link Table} to make it possible to work with them
 * in a generic way.
 * 
 * This class could be an interface in Java 8, which allows static methods on interfaces.
 */
public abstract class CollectionControl {
    public abstract CollectionItem[] getItems();
    public abstract CollectionItem[] getSelection();
    public abstract int getSelectionCount();
    public abstract Control getControl();
    public abstract int getColumnCount();

    interface CollectionItem {
        String getText(int columnIx);
    }

    /**
     * @param control Either a {@link Tree} or {@link Table}..
     * @return A collection which wraps the argument an delegate method calls to it.
     * @throws IllegalArgumentException if the argument is not a Tree or a Table.
     */
    public static CollectionControl create(Control control) {
        if (control instanceof Tree) {
            return new TreeControl((Tree) control);
        } else if (control instanceof Table) {
            return new TableControl((Table) control);
        }

        throw new IllegalArgumentException();
    }


    private static class TreeControl extends CollectionControl {
        private Tree tree;

        public TreeControl(Tree tree) {
            this.tree = tree;
        }

        @Override
        public CollectionItem[] getSelection() {
            CollectionItem[] items = new CollectionItem[tree.getSelectionCount()];
            int ix = 0;
            for (TreeItem item : tree.getSelection()) {
                items[ix++] = new TreeCollectionItem(item);
            }
            return items;
        }

        @Override
        public int getSelectionCount() {
            return tree.getSelectionCount();
        }

        @Override
        public Tree getControl() {
            return tree;
        }

        @Override
        public CollectionItem[] getItems() {
            CollectionItem[] items = new CollectionItem[tree.getItemCount()];
            int ix = 0;
            for (TreeItem item : tree.getItems()) {
                items[ix++] = new TreeCollectionItem(item);
            }
            return items;
        }

        @Override
        public int getColumnCount() {
            return tree.getColumnCount();
        }

        private static class TreeCollectionItem implements CollectionItem {
            private TreeItem item;
            public TreeCollectionItem(TreeItem item) {
                this.item = item;
            }
            @Override
            public String getText(int columnIx) {
                return item.getText(columnIx);
            }
        }
    }

    private static class TableControl extends CollectionControl {
        private Table table;

        public TableControl(Table table) {
            this.table = table;
        }

        @Override
        public CollectionItem[] getSelection() {
            CollectionItem[] items = new CollectionItem[table.getSelectionCount()];
            int ix = 0;
            for (TableItem item : table.getSelection()) {
                items[ix++] = new TableCollectionItem(item);
            }
            return items;
        }

        @Override
        public int getSelectionCount() {
            return table.getSelectionCount();
        }

        @Override
        public Table getControl() {
            return table;
        }

        @Override
        public CollectionItem[] getItems() {
            CollectionItem[] items = new CollectionItem[table.getItemCount()];
            int ix = 0;
            for (TableItem item : table.getItems()) {
                items[ix++] = new TableCollectionItem(item);
            }
            return items;
        }

        @Override
        public int getColumnCount() {
            return table.getColumnCount();
        }

        private static class TableCollectionItem implements CollectionItem {
            private TableItem item;
            public TableCollectionItem(TableItem item) {
                this.item = item;
            }
            @Override
            public String getText(int columnIx) {
                return item.getText(columnIx);
            }
        }
    }
}