Java递归原来如此简单

递归思想

把大问题转换为小问题（问题简单化），有明显的结束条件（不会无限循环）

递归例子

层级路径设置

    @Data
    @NoArgsConstructor
    @AllArgsConstructor
    static class Recursion {
        private Integer id;
        private String path;
        private Integer parentId;
    }

    public static void main(String[] args) {
        List<Recursion> allList = Lists.newArrayList(
                new Recursion(3, "3", 2),
                new Recursion(1, "1", null),
                new Recursion(5, "5", null),
                new Recursion(2, "2", 1),
                new Recursion(4, "4", 3));

        List<Recursion> rootList = allList.stream().filter(item -> Objects.isNull(item.getParentId())).collect(Collectors.toList());
        recursionMethod(rootList, allList);
        // [{"id":3,"parentId":2,"path":"1,2,3"},{"id":1,"path":"1"},{"id":5,"path":"5"},{"id":2,"parentId":1,"path":"1,2"},{"id":4,"parentId":3,"path":"1,2,3,4"}]
        System.out.println(JSON.toJSONString(allList));

    }

    private static void recursionMethod(List<Recursion> rootList, List<Recursion> allList) {
        for (Recursion root : rootList) {
            List<Recursion> childList = allList.stream().filter(item -> Objects.equals(root.getId(), item.getParentId())).collect(Collectors.toList());
            if (CollectionUtils.isNotEmpty(childList)) {
                childList.forEach(child -> child.setPath(root.getPath().concat(Constant.comma).concat(child.getPath())));
                recursionMethod(childList, allList);
            }
        }

    }

分析

当我们要设置Recursion全路径时，就会发现，常用的遍历方法复杂而且未知条件层数，无从下手，这时我们运用递归的思想，把问题简单化。
其实就是：遍历设置路径；而递归的结束条件是没有子元素，找出这两个条件就可以很容易的实现递归逻辑。