UI 组件-Tree 组件
Tree 组件可以用来显示具有层次关系的数据源，比如文件系统。Tree 组件的一个典型应用是作为菜单显示。
final Object[][] planets = new Object[][]{
new Object[]{"Mercury"},
new Object[]{"Venus"},
new Object[]{"Earth", "The Moon"},
new Object[]{"Mars", "Phobos", "Deimos"},
new Object[]{"Jupiter", "Io", "Europa", "Ganymedes",
"Callisto"},
new Object[]{"Saturn", "Titan", "Tethys", "Dione",
"Rhea", "Iapetus"},
new Object[]{"Uranus", "Miranda", "Ariel", "Umbriel",
"Titania", "Oberon"},
new Object[]{"Neptune", "Triton", "Proteus", "Nereid",
"Larissa"}};
Tree tree = new Tree("The Planets and Major Moons");
// Add planets as root items in the tree.
for (int i=0; i String planet = (String) (planets[i][0]);
tree.addItem(planet);
if (planets[i].length == 1) {
// The planet has no moons so make it a leaf.
tree.setChildrenAllowed(planet, false);
} else {
// Add children (moons) under the planets.
for (int j=1; j String moon = (String) planets[i][j];
// Add the item as a regular item.
tree.addItem(moon);
// Set it to be a child.
tree.setParent(moon, planet);
// Make the moons look like leaves.
tree.setChildrenAllowed(moon, false);
}
// Expand the subtree.
tree.expandItemsRecursively(planet);
}
}
main.addComponent(tree);
当然你可以选择合适的 Menu 风格使得 Tree 看起来更像菜单，比如：
注：到这里你可能注意到到目前为止在介绍 UI 组件时文章都避免涉及 CSS，Theme 应用，这是为的使Vaadin 应用开发更像开发桌面应用，后面会集中介绍主题的使用，对于一般应用你可以使用缺省主题。
同 Field 组件一样，你可以使用 getValue,setValue 设置或取得 Tree 组件当前选中的 Item 项。Tree 组件对应的 Container 类型为 HierarchicalContainer.
