Question

我有一个arrayList，我希望以层级结构显示。

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未获得所需结果。这是我到目前为止所尝试的：

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中的项重复次数过多。它最初显示正确的层次结构，然后开始重复随机变化。

任何人都可以指出我正确的方向吗？

Answer 1

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添加 - 为最后一个元素。

对于系统标签，您将必须计算遍历（例如，孩子的孩子的孩子是3）并添加许多标签。 This answer可以帮助解决这个问题。感谢Dukeling指出它。

Answer 2

问题是，在您第一次打电话给displayContent时，您将传递整个材料列表。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);

如何在Java中以树形结构显示列表数据？

2 个答案: