java list转树形结构
由于项目需要，需要把list转为树形结构，一般list转为树形结构时，需要关心prentCode,及子节点对应的父节点，并且父节点对应的子节点应该放在哪一个属性。及节点的结构层次。
在实现前，先了解下lambda 方法引用获取字段，该方法优势是解决硬编码问题，同时符合java编码规范，不会直接写死，并且当字段名有改变，不能编译通过，及时发现错误。
博客涉及断言使用，主要是用于基本参数的校验，不做详细介绍。
一、lambda 方法引用获取字段
jdk8提供关键字::，用于类、方法及属性的应用，通过下面代码及可拿到相应的属性名。
//直接通过类应用方法，解决硬编码问题
//list 列表；
//menu对象节点，转化树形结构在其对应子节点 ；
//Menu::getCode表示跟节点值，可为空；
//Menu::getParentCode表示其父节点
//Menu::getChildren表示子节点
new ListTransferTree(list, menu, Menu::getCode, Menu::getParentCode, Menu::getChildren)
//构造方面
public ListTransferTree(List list, T t, TypeFunction parent, TypeFunction children, TypeFunction node) {
//断言用于参数校验
Assert.notEmpty(list, "error: prams list must not belist empty");
Assert.notNull(t, "error: prams t must not be null");
this.list = list;
this.t = t;
//获取字段名
this.columnParentName = TypeFunction.getLambdaColumnName(parent);
this.columnChildrenName = TypeFunction.getLambdaColumnName(children);
this.columnNodeName = TypeFunction.getLambdaColumnName(node);
}
//定义接口，参考mybaits-plus实现
@FunctionalInterface
public interface TypeFunction extends Serializable {
Object get(T source);
/**
* 获取列名称
* @param lambda
* @return String
*/
static String getLambdaColumnName(Serializable lambda) {
try {
//核心代码，函数式接口继承Serializable时，编译器在编译Lambda表达式时，生成了一个writeReplace方法，这个方法会返回SerializedLambda，可以反射调用这个方法；
Method method = lambda.getClass().getDeclaredMethod("writeReplace");
method.setAccessible(Boolean.TRUE);
SerializedLambda serializedLambda = (SerializedLambda) method.invoke(lambda);
String getter = serializedLambda.getImplMethodName();
String fieldName = Introspector.decapitalize(getter.replace("get", ""));
return fieldName;
} catch (ReflectiveOperationException e) {
throw new RuntimeException(e);
}
}
}
二、 list转树形结构
list转树形结构在项目中经常使用，比如项目菜单列表，通常涉及多级菜单。下面是具体实现代码：
/**
* 将普通的entity的集合转成存在树状结构的集合
* @return
*/
public void tree() throws Exception {
if (list == null || list.isEmpty()) {
return;
}
//查找子节点
findChildren(list, t);
}
/**
* 获取所有子级
* @param t
* @param list
* @return
*/
private void findChildren(List list, T t) throws Exception {
if (list == null || list.isEmpty()) {
return;
}
String columnParentValue = BeanUtils.getProperty(t, columnParentName);
List items = new ArrayList();
list.stream().filter(e -> {
String columnValue = null;
try {
columnValue = BeanUtils.getProperty(e, columnChildrenName);
} catch (Exception ex) {
ex.printStackTrace();
}
if (columnParentValue == null && columnValue == null) {
return true;
}
return columnParentValue != null && columnParentValue.equals(columnValue);
}).forEach((s) -> {
items.add(s);
});
if (items == null || items.isEmpty()) {
return;
}
BeanUtils.setProperty(t, columnNodeName, items);
items.stream().forEach(e -> {
try {
//递归查找
this.findChildren(list, e);
} catch (Exception ex) {
ex.printStackTrace();
}
});
}
三、测试
测试代码如下:
//测试代码
public static void main(String[] args) throws Exception {
Menu menu = new Menu();
List测试结果:
//json
{
"children": [
{
"children": [
{
"children": [
{
"code": "M4",
"id": 4,
"level": 3,
"name": "三级菜单-a-a",
"parentCode": "M2",
"parentId": 2
},
{
"code": "M5",
"id": 5,
"level": 3,
"name": "三级菜单-a-b",
"parentCode": "M2",
"parentId": 2
},
{
"code": "M6",
"id": 6,
"level": 3,
"name": "三级菜单-a-c",
"parentCode": "M2",
"parentId": 2
}
],
"code": "M2",
"id": 2,
"level": 2,
"name": "二级菜单-a",
"parentCode": "M1",
"parentId": 1
},
{
"children": [
{
"code": "M7",
"id": 7,
"level": 3,
"name": "三级菜单-b-a",
"parentCode": "M3",
"parentId": 3
}
],
"code": "M3",
"id": 3,
"level": 2,
"name": "二级菜单-b",
"parentCode": "M1",
"parentId": 1
}
],
"code": "M1",
"id": 1,
"level": 1,
"name": "顶级菜单-1",
"parentCode": ""
},
{
"children": [
{
"children": [
{
"code": "M11",
"id": 11,
"level": 3,
"name": "三级菜单-a-a-2",
"parentCode": "M9",
"parentId": 9
},
{
"code": "M12",
"id": 12,
"level": 3,
"name": "三级菜单-a-b-2",
"parentCode": "M9",
"parentId": 9
},
{
"code": "M13",
"id": 13,
"level": 3,
"name": "三级菜单-a-c-2",
"parentCode": "M9",
"parentId": 9
}
],
"code": "M9",
"id": 9,
"level": 2,
"name": "二级菜单-a-2",
"parentCode": "M8",
"parentId": 8
},
{
"children": [
{
"code": "M14",
"id": 14,
"level": 3,
"name": "三级菜单-b-a-2",
"parentCode": "M10",
"parentId": 10
}
],
"code": "M10",
"id": 10,
"level": 2,
"name": "二级菜单-b-2",
"parentCode": "M8",
"parentId": 8
}
],
"code": "M8",
"id": 8,
"level": 1,
"name": "顶级菜单-2",
"parentCode": ""
}
],
"code": ""
}
四、demo地址
https://github.com/13162576590/List-Transfer-Tree