【设计模式】结构型设计模式-组合模式的实现

定义

将对象组合成树型结构，用于表示部分-整体（树型）关系。

UML图

Java代码实现

抽象组件

public class Component {
    String name;

    Component(String name) {
        this.name = name;
    }

    void display(int depth) {
        printIndent(depth);
        System.out.println(name);
    }

    void add(Component component) {}
    void remove(Component component) {}

    Component getChild(int depth) {
        return null;
    }

    void printIndent(int depth) {
        for (int i = 0; i < depth; i++) {
            System.out.print("\t");
        }
    }
}

叶子类

public class Leaf extends Component {
    Leaf(String name) {
        super(name);
    }
}

树枝类

import java.util.List;
import java.util.ArrayList;

public class Composite extends Component {
    private final List<Component> children = new ArrayList<>();

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

    @Override
    void add(Component component) {
        children.add(component);
    }

    @Override
    void remove(Component component) {
        children.remove(component);
    }

    @Override
    Component getChild(int index) {
        return children.get(index);
    }

    @Override
    void display(int depth) {
        printIndent(depth);
        System.out.println(name + " (" + children.size() + ")");
        for (Component component : children) {
            component.display(depth + 1);
        }
    }
}

组装

public class CompositePatternDemo {
    public static void main(String[] args) {
        Component root = new Composite("Root");
        Component leaf1 = new Leaf("Leaf 1");
        Component leaf2 = new Leaf("Leaf 2");

        Component composite1 = new Composite("Composite 1");
        Component leaf3 = new Leaf("Leaf 3");
        Component leaf4 = new Leaf("Leaf 4");
        composite1.add(leaf3);
        composite1.add(leaf4);

        Component composite2 = new Composite("Composite 2");
        Component leaf5 = new Leaf("Leaf 5");
        composite2.add(leaf5);

        root.add(leaf1);
        root.add(leaf2);
        root.add(composite1);
        root.add(composite2);

        // 展示组合结构
        root.display(1);
    }
}

总结

使用组合模式可以很方便的表示有层级关系的数据结构，比如文件系统和组织结构。