线上问题：stream获取值抛出空指针及源码分析

实体列表，通过stream获取数据，findFirst后，直接使用get获取数据，抛出空指针异常，复现代码如下：

@Test
    public void streamGetUnsafeTest() {

        List userEntityList = new ArrayList<>();
        userEntityList.add(new UserEntity("1", "111", "male"));
        userEntityList.add(new UserEntity("2", "222", "female"));
        userEntityList.add(new UserEntity("3", "333", "male"));
        userEntityList.add(new UserEntity("4", "444", "female"));
        UserEntity userEntity1 = userEntityList.stream().filter(s-> Objects.equals(s.getSex(), "test")).findFirst().get();
    } 

未查到数据，抛出异常，如图2.1所示。

get源码如下，由源码可知，当查不到数据（数据为null）时，get方法直接抛出异常，因此，使用stream获取数据时，需要判断
取到的值是否为null，避免出现空指针。
所在包：package java.util;
类：Optional

    
    public T get() {
        if (value == null) {
            throw new NoSuchElementException("No value present");
        }
        return value;
    }

避免通过stream出现空指针异常，主动抛出的，使用orElse给一个预知的默认值，通过默认值处理当前数据。

@Test
    public void streamGetSafeTest() {

        List userEntityList = new ArrayList<>();
        UserEntity userEntityDefault = new UserEntity();
        userEntityList.add(new UserEntity("1", "111", "male"));
        userEntityList.add(new UserEntity("2", "222", "female"));
        userEntityList.add(new UserEntity("3", "333", "male"));
        userEntityList.add(new UserEntity("4", "444", "female"));
        UserEntity userEntity1 = userEntityList.stream().filter(s-> Objects.equals(s.getSex(), "test")).findFirst().orElse(userEntityDefault);
        logger.info(">>>>>>>>>UserInfo:{}", userEntity1);
    }

测试结果如图3.1所示，查到的数据为null时，设置为预知的默认值。

package com.monkey.java_study.common.entity;



public class UserEntity {

    
    private String uid;

    
    private String nickname;

    
    private String sex = "haha";

    public UserEntity() {
    }

    public UserEntity(String uid) {
        this.uid = uid;
    }

    public UserEntity(String uid, String nickname, String sex) {
        this.uid = uid;
        this.nickname = nickname;
        this.sex = sex;
    }

    public UserEntity copy() {
        return new UserEntity(uid, nickname, sex);
    }

    public String getUid() {
        return uid;
    }

    public void setUid(String uid) {
        this.uid = uid;
    }

    public String getNickname() {
        return nickname;
    }

    public void setNickname(String nickname) {
        this.nickname = nickname;
    }

    public String getSex() {
        return sex;
    }

    public void setSex(String sex) {
        this.sex = sex;
    }

    @Override
    public String toString() {
        return "UserEntity{" +
                "uid=" + uid +
                ", nickname='" + nickname + ''' +
                ", sex='" + sex + ''' +
                '}';
    }
}

• 通过stream查询数据时，判断是否会出现空值或者抛出空指针异常；
• 安全的操作是：orElse设定一个预知的默认值。