Java流Stream-1：数据过滤filter

àì夳堔傛蜴生んèń 2022-09-16 03:58 285阅读 0赞

`重要：默认过滤数据实体对象时为引用传递。`  
（1）若修改过滤后的值，原始值会改变。  
（2）若过滤后的数据需要变更，且原始值不受影响，需要过滤时新建对象。

# 1 filter方法 #

按照指定条件过滤数据，保留原始数据类型。

# 2 Usage #

## 2.1 基础类型数据过滤 ##

过滤字符串为例：构造`List<String>`

/**
         * 1.1过滤数据：构造字符串列表.List<A>->List<A>
         */
        @Test
        public void filterStringWithSpecificCondition() {
        
            List<String> list = Stream.of("female", "male", "female", "male").collect(Collectors.toList());
            logger.info(">>>>>>>>>>原始数据：\n{}", list);
            List<String> listAfterFilter = list.stream().filter(s -> null != s && !"male".equals(s)).collect(Collectors.toList());
            logger.info(">>>>>>>>>>过滤后的数据：\n{}", listAfterFilter);
        }

![在这里插入图片描述][watermark_type_ZHJvaWRzYW5zZmFsbGJhY2s_shadow_50_text_Q1NETiBA5aSp54S2546p5a62_size_20_color_FFFFFF_t_70_g_se_x_16_pic_center]

## 2.2 对象过滤 ##

过滤对象中满足条件的数据，返回对象

*  测试样例

/**
         * 1.2过滤数据：构造实体列表.List<A>->List<A>
         */
        @Test
        public void  filterEntityWithSpecificCondition() {
        
            List<UserEntity> 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"));
            logger.info(">>>>>>>>>>原始数据：\n{}", userEntityList);
            List<UserEntity> listAfterFilter = userEntityList.stream().filter(s -> null != s && !"male".equals(s.getSex())).collect(Collectors.toList());
            logger.info(">>>>>>>>>>过滤后的数据：\n" + listAfterFilter);
        }

*  测试结果![在这里插入图片描述][watermark_type_ZHJvaWRzYW5zZmFsbGJhY2s_shadow_50_text_Q1NETiBA5aSp54S2546p5a62_size_20_color_FFFFFF_t_70_g_se_x_16_pic_center 1]
 *  引用传递  
    过滤后的数据为仍为原始的对象，因此，修改过滤后的数据，原始数据同时会变化，如果原始数据有其他用途，则需要在过滤时新建对象，保证值传递。  
    ![在这里插入图片描述][watermark_type_ZHJvaWRzYW5zZmFsbGJhY2s_shadow_50_text_Q1NETiBA5aSp54S2546p5a62_size_20_color_FFFFFF_t_70_g_se_x_16_pic_center 2]
 *  值传递

/**
         * 1.3过滤数据：构造实体列表，创建新对象.List<A>->List<A>
         */
        @Test
        public void  filterEntityWithSpecificConditionAndCreateNewObject() {
        
            List<UserEntity> 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"));
            logger.info(">>>>>>>>>>原始数据：\n{}", userEntityList);
            List<UserEntity> listAfterFilter = userEntityList.stream().filter(s -> null != s && !"male".equals(s.getSex())).map(UserEntity::copy).collect(Collectors.toList());
            logger.info(">>>>>>>>>>过滤后的数据：\n" + listAfterFilter);
        }

*  UserEntity

package com.monkey.java_study.common.entity;
    
    /**
     * User实体
     *
     * @author xindaqi
     * @since 2021-01-23
     */
    
    public class UserEntity {
        
    
        /**
         * 用户id
         */
        private String uid;
    
        /**
         * 用户名称
         */
        private String nickname;
    
        /**
         * 用户性别
         */
        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 + '\'' +
                    '}';
        }
    }

![在这里插入图片描述][watermark_type_ZHJvaWRzYW5zZmFsbGJhY2s_shadow_50_text_Q1NETiBA5aSp54S2546p5a62_size_20_color_FFFFFF_t_70_g_se_x_16_pic_center 3]

# 3 完整样例 #

package com.monkey.java_study.functiontest;
    
    import com.monkey.java_study.common.entity.UserEntity;
    import org.junit.Test;
    import org.slf4j.Logger;
    import org.slf4j.LoggerFactory;
    
    import java.util.ArrayList;
    import java.util.List;
    import java.util.stream.Collectors;
    import java.util.stream.Stream;
    
    /**
     * Stream测试.
     *
     * @author xindaqi
     * @date 2021-10-15 10:06
     */
    public class StreamTest {
        
    
        private static final Logger logger = LoggerFactory.getLogger(StreamTest.class);
    
        /**
         * 1.1过滤数据：构造字符串列表.List<A>->List<A>
         */
        @Test
        public void filterStringWithSpecificCondition() {
        
            List<String> list = Stream.of("female", "male", "female", "male").collect(Collectors.toList());
            logger.info(">>>>>>>>>>原始数据：\n{}", list);
            List<String> listAfterFilter = list.stream().filter(s -> null != s && !"male".equals(s)).collect(Collectors.toList());
            logger.info(">>>>>>>>>>过滤后的数据：\n{}", listAfterFilter);
        }
    
        /**
         * 1.2过滤数据：构造实体列表.List<A>->List<A>
         */
        @Test
        public void  filterEntityWithSpecificCondition() {
        
            List<UserEntity> 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"));
            logger.info(">>>>>>>>>>原始数据：\n{}", userEntityList);
            List<UserEntity> listAfterFilter = userEntityList.stream().filter(s -> null != s && !"male".equals(s.getSex())).collect(Collectors.toList());
            logger.info(">>>>>>>>>>过滤后的数据：\n" + listAfterFilter);
        }
    
        /**
         * 1.3过滤数据：构造实体列表，创建新对象.List<A>->List<A>
         */
        @Test
        public void  filterEntityWithSpecificConditionAndCreateNewObject() {
        
            List<UserEntity> 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"));
            logger.info(">>>>>>>>>>原始数据：\n{}", userEntityList);
            List<UserEntity> listAfterFilter = userEntityList.stream().filter(s -> null != s && !"male".equals(s.getSex())).map(UserEntity::copy).collect(Collectors.toList());
            logger.info(">>>>>>>>>>过滤后的数据：\n" + listAfterFilter);
        }
    }

[watermark_type_ZHJvaWRzYW5zZmFsbGJhY2s_shadow_50_text_Q1NETiBA5aSp54S2546p5a62_size_20_color_FFFFFF_t_70_g_se_x_16_pic_center]: /images/20220828/c3fb6ccd37a148b4937c8313e3c339d3.png
[watermark_type_ZHJvaWRzYW5zZmFsbGJhY2s_shadow_50_text_Q1NETiBA5aSp54S2546p5a62_size_20_color_FFFFFF_t_70_g_se_x_16_pic_center 1]: /images/20220828/4654e4b44a584bd78299c2643801b685.png
[watermark_type_ZHJvaWRzYW5zZmFsbGJhY2s_shadow_50_text_Q1NETiBA5aSp54S2546p5a62_size_20_color_FFFFFF_t_70_g_se_x_16_pic_center 2]: /images/20220828/eafe0f3266be4625abdbd2a382f224bb.png
[watermark_type_ZHJvaWRzYW5zZmFsbGJhY2s_shadow_50_text_Q1NETiBA5aSp54S2546p5a62_size_20_color_FFFFFF_t_70_g_se_x_16_pic_center 3]: /images/20220828/a1db4414d4474fc8a164fb68547eecc0.png