How to get a value from lambda in Java?

28 июля, 2017 rius

If you reached this article, you most likely face one of these errors, and trying to get rid of one, get another: «variable used in lambda expression should be final or effectively final», «variable may be not initialized», «cannot assign a value to final variable».

For my explanation I will use vaadin’s Window. 

public class SelectionWindow extends Window {
	private Button okButton = null;
	private BeanItemContainer container;
	private LegalEntity selected;
 
	public SelectionWindow(List items){
	...
		setClosable(false);
		container = new BeanItemContainer<>(Entity.class, items);
		grid = new Grid(container);
		grid.addSelectionListener(event -> {
			Object selectedObj = grid.getSelectedRow();
			selected = (LegalEntity) selectedObj;
			if (okButton != null) {
				okButton.setEnabled(selected != null);
			}
 		});
	}
 
	// the only way to close the window
	public void setOkButtonListener(Button.ClickListener listener) {
        okButton.addClickListener(listener);
    }
 
	// this way we get selected item
	public Entity getSelected() {
        return selected;
    }
 
}

So, to pass params into the Window you just add arguments to the constructor. In my case I don’t need a private field List items. If I did, I would add:

private List items;

and a line to constructor:

this.items = items;

You can use setters as well.

Case 1. You just need to read selected value and to use right in the code of the listener.

SelectionWindow selectionWindow = new SelectionWindow(items);
selectionWindow.setOkButtonListener(clickEvent -> {
    Entity selected = selectionWindow.getSelected();
    populateFromSelection(selected);
    selectionWindow.close();
});
 
private void populateFromSingleSuggestion(LegalEntity legalEntity) {
	// react upon the new value
}

The point is that you know in the code when your Window gets closed. On this event you can read any field/state from it (you can add getters) and update your UI.

Case 2. You want to read a value and assign to a variable with a wider scope. Use atomic reference.

final AtomicReference reference = new AtomicReference<>();
 
selectionWindow.setOkButtonListener(clickEvent -> {
                reference.set(selectionWindow.getSelected());
                selectionWindow.close();
            });

Then get your item with reference.get().

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