利用有序Map把一维数据转换为树结构的方法

转换为树结构的工具类：

import java.util.ArrayList;
import java.util.LinkedHashMap;
import java.util.List;
import java.util.Map;

public class AssmTreeUtil {
	/**
	* 组装树结构数据的方法
	*/
	public static List<TreeNode> assmTree(List<TreeNode> singleTreeNodes) {
		// 判断排序数据是否为空
		if(singleTreeNodes == null || singleTreeNodes.isEmpty()) {
			return null;
		}
		// 用有序Map把传参组装起来
		Map<String,TreeNode> nodeId2treeNodes = new LinkedHashMap<String,TreeNode>();
		for(TreeNode node : singleTreeNodes){
			TreeNode treeNode = new TreeNode();
			treeNode.setNodeId(node.getNodeId());
			treeNode.setNodeName(node.getNodeName());
			treeNode.setPid(node.getPid());
			nodeId2treeNodes.put(node.getNodeId(), treeNode);
		}
		// 用来保存组装好的数据, 作为返回值
		List<TreeNode> topNodeTrees = new ArrayList<TreeNode>();
		// 遍历所有节点
		for(String nodeId : nodeId2treeNodes.keySet()){
			TreeNode treeNode = nodeId2treeNodes.get(nodeId);
			String pid = treeNode.getPid();
			// 当前节点的父id为空 或 整个列表中没有以此父id作为自己的id的情况
			// 即当前节点是根节点
			if(pid==null || pid.length()==0 || !nodeId2treeNodes.containsKey(pid)){
				// treeNode.setPid(""); // 此处可以根据需要调整父id的值
				topNodeTrees.add(treeNode);
			}else{ // 不是父节点, 作为子节点添加进去
				TreeNode parentTreeNode = nodeId2treeNodes.get(pid);
				if(parentTreeNode.getChildren()==null){
					parentTreeNode.setChildren(new ArrayList<TreeNode>());
				}
				parentTreeNode.getChildren().add(treeNode);
			}
		}
		return topNodeTrees;
	}
}

TreeNode bean类：

import java.util.List;

public class TreeNode {
	/**
	 * 父节点ID
	 */
	private String pid;
	/**
	 * 节点ID
	 */
	private String nodeId;
	/**
	 * 节点名称
	 */
	private String nodeName;
	/**
	 * 子节点
	 */
	private List<TreeNode> children;
	public String getPid() {
		return pid;
	}
	public void setPid(String pid) {
		this.pid = pid;
	}
	public String getNodeId() {
		return nodeId;
	}
	public void setNodeId(String nodeId) {
		this.nodeId = nodeId;
	}
	public String getNodeName() {
		return nodeName;
	}
	public void setNodeName(String nodeName) {
		this.nodeName = nodeName;
	}
	public List<TreeNode> getChildren() {
		return children;
	}
	public void setChildren(List<TreeNode> children) {
		this.children = children;
	}
}

测试：

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

/**
 * 测试组装树结构方法
 * @Author: Rebecca Zhong
 * @Description:
 * @Date: Created in 2019/3/7 10:31
 * @Modified By:
 */
public class Test {
    public static void main(String[] args) {
        List<TreeNode> singleNodes = new ArrayList<TreeNode>();
        singleNodes.add(new TreeNode("a","1","节点1"));
        singleNodes.add(new TreeNode("b","2","节点2"));
        singleNodes.add(new TreeNode("c","3","节点3"));
        singleNodes.add(new TreeNode("1","4","节点1-1"));
        singleNodes.add(new TreeNode("2","5","节点2-1"));
        singleNodes.add(new TreeNode("1","6","节点1-3"));
        singleNodes.add(new TreeNode("1","7","节点1-2"));
        singleNodes.add(new TreeNode("2","8","节点2-2"));
        List<TreeNode> treeNodes = AssmTreeUtil.assmTree(singleNodes);
        outputResult(treeNodes);
    }

    private static void outputResult(List<TreeNode> treeNodes) {
        for (TreeNode treeNode : treeNodes) {
            System.out.println("nodeId: " + treeNode.getNodeId() + "; pid: " + treeNode.getPid() + "; name: "+treeNode.getNodeName());
            List<TreeNode> childrenNode = treeNode.getChildren();
            if (childrenNode != null) {
                outputResult(childrenNode);
            }
        }
    }
}

期望结果：

• 节点1
  • 节点1-1
  • 节点1-3
  • 节点1-2
• 节点2
  • 节点2-1
  • 节点2-2
• 节点3

输出结果：

nodeId: 1; pid: a; name: 节点1
nodeId: 4; pid: 1; name: 节点1-1
nodeId: 6; pid: 1; name: 节点1-3
nodeId: 7; pid: 1; name: 节点1-2
nodeId: 2; pid: b; name: 节点2
nodeId: 5; pid: 2; name: 节点2-1
nodeId: 8; pid: 2; name: 节点2-2
nodeId: 3; pid: c; name: 节点3

小结：

利用ArrayList也可以实现相关功能，但代码整体较长，不利于维护。
利用LinkedHashMap代码更简洁一些。