设计模式之组合模式

概念：将对象组合成树形结构以表示“部分——整体”的层次结构，使得用户对单个对象和组合对象的使用具有一致性。

组合模式有三个角色：

• 抽象构件：定义公有属性和方法。
• 叶子结点：树形结构的底层结点，没有子结点，实现抽象构件的所有操作。
• 中间结点：叶子结点之前的结点，有子结点。

组合模式最经典的例子就是文件和文件夹结构。下面用一个这样的例子来帮助大家理解组合模式。

public abstract class FileComponent {
    protected String name;
    public FileComponent(String name) {
        this.name = name;
    }
    public abstract void add(FileComponent component);
    public abstract void remove(FileComponent component);
    public abstract void display(int depth);
    public String getName() {
        return name;
    }
}

public class FileNode extends FileComponent {
    public FileNode(String name) {
        super(name);
    }
    @Override
    public void add(FileComponent component) {
        throw new UnsupportedOperationException("Cannot add components to a file.");
    }
    @Override
    public void remove(FileComponent component) {
        throw new UnsupportedOperationException("Cannot remove components from a file.");
    }
    @Override
    public void display(int depth) {
        StringBuilder sb = new StringBuilder();
        for (int i = 0; i < depth; i++) {
            sb.append("--");
        }
        System.out.println(sb + name);
    }
}

public class Directory extends FileComponent {
    private List<FileComponent> children;
    public Directory(String name) {
        super(name);
        children = new ArrayList<>();
    }
    @Override
    public void add(FileComponent component) {
        children.add(component);
    }
    @Override
    public void remove(FileComponent component) {
        children.remove(component);
    }
    @Override
    public void display(int depth) {
        StringBuilder sb = new StringBuilder();
        for (int i = 0; i < depth; i++) {
            sb.append("--");
        }
        System.out.println(sb + name);
        for (FileComponent child : children) {
            child.display(depth + 1);
        }
    }
}

public class Demo {
    public static void main(String[] args) {
        Directory root = new Directory("Root");
        Directory documents = new Directory("Documents");
        Directory pictures = new Directory("Pictures");
        FileNode readme = new FileNode("Readme.txt");
        FileNode image = new FileNode("image.jpg");
        root.add(documents);
        root.add(pictures);
        documents.add(readme);
        pictures.add(image);
        System.out.println("File structure:");
        root.display(0);
    }
}