空指针异常

问题

CategoryVO类，

package com.imooc.mall.model.vo;

import java.io.Serializable;
import java.util.ArrayList;
import java.util.Date;
import java.util.List;

public class CategoryVO implements Serializable {

    private Integer id;

    private String name;

    private Integer type;

    private Integer parentId;

    private Integer orderNum;

    private Date createTime;

    private Date updateTime;

    private List<CategoryVO> childCategory = new ArrayList<>();

    public List<CategoryVO> getChildCategory() {
        return childCategory;
    }

    public void setChildCategory(List<CategoryVO> childCategory) {
        this.childCategory = childCategory;
    }

    public Integer getId() {
        return id;
    }

    public void setId(Integer id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name == null ? null : name.trim();
    }

    public Integer getType() {
        return type;
    }

    public void setType(Integer type) {
        this.type = type;
    }

    public Integer getParentId() {
        return parentId;
    }

    public void setParentId(Integer parentId) {
        this.parentId = parentId;
    }

    public Integer getOrderNum() {
        return orderNum;
    }

    public void setOrderNum(Integer orderNum) {
        this.orderNum = orderNum;
    }

    public Date getCreateTime() {
        return createTime;
    }

    public void setCreateTime(Date createTime) {
        this.createTime = createTime;
    }

    public Date getUpdateTime() {
        return updateTime;
    }

    public void setUpdateTime(Date updateTime) {
        this.updateTime = updateTime;
    }
}

CategoryServiceImpl方法:

@Override
    @Cacheable(value = "listCategoryForCustomer")
    public List<CategoryVO> listCategoryForCustomer(Integer parentId) {
        ArrayList<CategoryVO> categoryVOList = new ArrayList<>();
        recursivelyFindCategories(categoryVOList, parentId);
        return categoryVOList;
    }

    private void recursivelyFindCategories(List<CategoryVO> categoryVOList, Integer parentId) {
        //递归获取所有子类别，并组合成为一个“目录树”
        List<Category> categoryList = categoryMapper.selectCategoriesByParentId(parentId);
        if (!CollectionUtils.isEmpty(categoryList)) {
            for (int i = 0; i < categoryList.size(); i++) {
                Category category = categoryList.get(i);
                CategoryVO categoryVO = new CategoryVO();
                BeanUtils.copyProperties(category, categoryVO);
                categoryVOList.add(categoryVO);
                recursivelyFindCategories(categoryVO.getChildCategory(), categoryVO.getId());
            }
        }
    }

错误，

空指针

解决

CategoryVO类中的childCategory属性，这是接口引用，没有进行初始化，改成

List<CategoryVo> childCategory=new ArrayList<>();