List

List is a user interface component that occupies the whole main pane of the screen and allows the user to select one or multiple items from a predefined set. The List layout varies based on four attributes:

  • list type

  • optional icon attached to the elements

  • font

  • fit policy for long elements

When a List becomes the foreground Displayable, the currently selected List element becomes automatically focused. Row elements are composed of a text part, an optional icon and an optional font attribute. A List is almost similar to the Form component ChoiceGroup and they both have a common Choice interface.

Lists come in three different configurations: IMPLICIT, MULTIPLE and EXCLUSIVE. A common usage for a List is a MIDlet main menu.

Figure: A simple MULTIPLE List

Source code for the example:

import javax.microedition.midlet.*;
import javax.microedition.lcdui.*;

public class ExampleList extends MIDlet implements CommandListener {
    private List ls;
    private Command exit;
 
    public ExampleList() {
        //List(label,type,items,Images)
        ls = new List("Multiple list", List.MULTIPLE, new String[] {"Choice 1", "Choice 2", "Choice 3", "Choice 4"}, null);
        exit = new Command("Exit", Command.EXIT, 1);
        ls.addCommand(exit);
        ls.setCommandListener(this);
    }      
    public void startApp() {
        Display display=Display.getDisplay(this);
        display.setCurrent(ls);
    }      
    public void pauseApp() {
    }
    public void destroyApp(boolean unconditional) {
    }
    public void commandAction(Command command, Displayable displayable) {
        if (command == exit) {
            destroyApp(false);
            notifyDestroyed();
        }
    }
}

When List becomes the foreground Displayable, the first List element becomes automatically focused

Default font face (FACE_SYSTEM) is always used.

The Font property of element can be set independently from the other elements in the same List. The default font used in elements is the same as returned by Font.getDefaultFont().

Lists with no data are presented so that the user clearly sees that they are empty, a no data text is displayed in the user interface. It is not possible for the user to trigger a SELECT_COMMAND, or select Command (MIDP 2.1) notification in an empty IMPLICIT List.

Fit policy

Fit policy defines how the text is laid out in a List. The initial value of the element fit policy is TEXT_WRAP_DEFAULT. The size of Images determines whether the default fit policy is TEXT_WRAP_ON or TEXT_WRAP_OFF as follows:

  • If the element image fits into one row, the fit policy is TEXT_WRAP_OFF.

  • If the element image height is greater than one text line, the fit policy is TEXT_WRAP_ON.

The same fit policy is applied to all elements of a List.

Application programmer can override the default functionality. If the fit policy is set to TEXT_WRAP_OFF, only one row is reserved for the text part regardless of the size of the image or the length of the text. If the fit policy is set to TEXT_WRAP_ON, two rows are reserved for the text part regardless of the size of the image or the length of the text.

Wrapping text to more than two lines is not possible. This implies that if the text does not fit into two lines, it is truncated regardless of the fit policy.

List layout

The following parameters are used to determine the List layout:

  • Whether there is a selection graphic (EXCLUSIVE, MULTIPLE) or not (IMPLICIT);

  • Whether there are images, and what is the image size; and

  • Whether the text needs one or two lines.

Also the following principles are applied:

  • It is assumed that all the Images in List have the same size, so the biggest Image determines the reserved space. There are two possible values: big Images and small Images. If Images are larger than the space reserved for them, they are clipped.

  • The text can wrap to two lines. If any of the Choice elements require wrapping, all elements have two lines; otherwise the size of the icons determines the height of an element.

  • If the text does not fit into one or two lines as defined by fit policy, it is truncated, and in which case an ellipsis is used to indicate this. If the MIDlet adds or removes elements, re-layout can happen.

For information on List implementation in Touch UI -enabled devices, see section Displayables and commands.

For an example on using List, see article Using List in Java ME in Forum Nokia.

© Nokia 2010.