组合模式（Composite），将对象组合成树形结构以表示‘部分-整体’的层次结构。组合模式使得用户对单个对象和组合对象的使用具有一致性。

主方法

public class main {
    public static void main(String[] args) {
        Composite root = new Composite("root");

        root.Add(new Leaf("Leaf A"));

        root.Add(new Leaf("Leaf B"));

        Composite comp = new Composite("Composite X");
        comp.Add(new Leaf("Leaf XA"));
        comp.Add(new Leaf("Leaf XB"));
        root.Add(comp);
        Composite comp2 = new Composite("Composite XY");
        comp2.Add(new Leaf("Leaf XYA"));
        comp2.Add(new Leaf("Leaf XYB"));
        comp.Add(comp2);

        root.Add(new Leaf("Leaf C"));

        Leaf leaf = new Leaf("Leaf D");
        root.Add(leaf);
        root.Remove(leaf);

        root.Display(1);
    }
}

声明接口

public abstract class Component {
    protected String name;
    public Component(String name) {
        this.name = name;
    }

    public abstract void Add(Component c);
    public abstract void Remove(Component c);
    public abstract void Display(int depth);
}

叶子节点

/**
 * 由于叶子没有在增加分支和树叶，所以Add和Remove方法实现它没有意义，
 * 但这样做可以消除叶子节点和枝节点在抽象层次的区别，它们具备完全一致的接口
 */
public class Leaf extends Component {
    public Leaf(String name) {
        super(name);
    }

    public void Add(Component c) {
        System.out.println("Canno add to a leaf");
    }

    public void Remove(Component c) {
        System.out.println("Cannot remove from a leaf");
    }

    public void Display(int depth) {
        String result = name;
        for (int i = 0; i < depth; i++) {
            result = '-' + result;
        }
        System.out.println(result);
    }
}

树枝节点

import java.util.ArrayList;
import java.util.List;
public class Composite extends Component {
    private List<Component> children = new ArrayList<Component>();

    public Composite(String name) {
        super(name);
    }

    public void Add(Component c) {
        children.add(c);
    }

    public void Remove(Component c) {
        children.remove(c);
    }

    public void Display(int depth) {
        String result = name;
        for (int i = 0; i < depth; i++) {
            result = '-' + result;
        }
        System.out.println(result);
        for (Component component : children) {
            component.Display(depth + 2);
        }
    }
}