如何在 Java 中以树状结构显示列表数据？

Question

我有一个 arrayList，我想以分层结构显示。

我希望结果看起来像这样。如果它没有任何子节点，我希望项目用连字符缩进：

User Design
    Lectures
        Week 1
            -Apr 5
            -Apr 8
        Week 2
            -Apr 12
    Activities
        Personas
            Male
                -George
            Female
                -Allison
                -Jessica

我正在尝试使用递归方法显示列表，但没有得到想要的结果。这是我迄今为止尝试过的：

import java.util.*;

class Materials {
    public int id;
    public String content;
    public int pid; 

    public Materials(int id, String content, int pid) {
        this.id = id;
        this.content = content;
        this.pid = pid; 
    }
}
public class nestedList {

public static ArrayList<Materials> material;

public static void main(String[] args) {
    material = new ArrayList<Materials>();
    material.add(new Materials(1,"User Design", 0));
    material.add(new Materials(2,"Lectures", 1));
    material.add(new Materials(3,"Activities", 1));
    material.add(new Materials(4,"Week 1", 2));
    material.add(new Materials(5,"Apr 5", 4));
    material.add(new Materials(6,"Apr 8", 4));
    material.add(new Materials(7,"Week 2", 2));
    material.add(new Materials(8,"Apr 12", 7));
    material.add(new Materials(9,"Personas", 3));
    material.add(new Materials(10,"Male", 9));
    material.add(new Materials(11,"Female", 9));
    material.add(new Materials(12,"George", 10));
    material.add(new Materials(13,"Allison", 11));
    material.add(new Materials(14,"Jessica", 11));

    displayContent(material);

}

static void displayContent(ArrayList<Materials> materials) {
    ArrayList<Materials> childs = new ArrayList<Materials>();
    for (Materials material : materials) {
        childs = selectChild(material.id);
        System.out.println(material.content);
        displayContent(childs);
    }

}

static ArrayList<Materials> selectChild(int id) {
    ArrayList<Materials> list = new ArrayList<Materials>();

    for (int i = 0; i < material.size(); i++) {
        if(material.get(i).pid == id) {
            list.add(material.get(i));
        }
    }

    return list;
}
}

当我运行此代码时，arrayList 中的项目重复了太多次。它最初显示正确的层次结构，然后开始以随机变化重复。

谁能指点我正确的方向？

Answer 1

问题在于，在您第一次调用displayContent 时，您传递了整个材料列表。所以它会这样做：

Display the entire hierarchy for "User Design"
Display the entire hierarchy for "Lectures"
Display the entire hierarchy for "Activities"
etc.

您真正想要做的是在您的初始调用中仅将根元素传递给displayContent。所以在你的主要：

ArrayList<Materials> topLevel = selectChild(0);
displayContent(topLevel);

Answer 2

static void displayContent(ArrayList<Materials> materials) {
    ArrayList<Materials> childs = new ArrayList<Materials>();
    for (Materials material : materials) {
        childs = selectChild(material.id);
        if(childs.isEmpty()) {
            System.out.print(" - ");
        }
        System.out.println(material.content);
        displayContent(childs);
    }

}

在打印材料内容之前检查列表是否为空将为最后一个元素添加 -。

对于系统选项卡，您必须计算遍历次数（例如，child of child 的 child 为 3）并添加那么多选项卡。 This answer 可以提供帮助。感谢杜克林指出。