Java基础系列：HashMap与null

拼搏现实的明天。 2022-09-16 11:30 174阅读 0赞

# 1 插入数据 #

## 1.1 插入null键 ##

/**
         * 添加null键.
         */
        @Test
        public void mapPutNullKeyTest() {
        
            Map<String, String> map = new HashMap<>();
            map.put(null, "test1");
            String value = map.get(null);
            logger.info(">>>>>>>>>Value:" + value);
            
        }

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## 1.2 插入null值 ##

/**
         * 添加null值.
         */
        @Test
        public void mapPutNullValueTest() {
        
            Map<String, String> map = new HashMap<>();
            map.put("2", null);
            String value1 = map.get("2");
            logger.info(">>>>>>>>>Value:" + value1);
        }

![在这里插入图片描述][656647db28484d59adf075c039c5432d.png_pic_center]

# 2 查询数据 #

## 2.1 查询null键 ##

/**
         * 使用null作为键查询.
         */
        @Test
        public void mapKeyNullTest() {
        
            Map<String, String> map = new HashMap<>();
            map.put("1", "test1");
            map.put("2", "test2");
            String value = map.get(null);
            logger.info(">>>>>>>>>Value:" + value);
        }

![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]

## 2.3 查询不存在的键 ##

/**
         * 查询没有key的值.
         */
        @Test
        public void mapReturnNullTest() {
        
            Map<String, String> map = new HashMap<>();
            map.put("1", "test1");
            map.put("2", "test2");
            String value = map.get("3");
            logger.info(">>>>>>>>>Float:" + value);
        }

![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]

## 2.4 查询第一条数据 ##

/**
         * 获取第一个元素.
         */
        @Test
        public void mapFindFirstTest() {
        
            Map<String, String> map = new HashMap<>();
            map.put("1", "test1");
            map.put("2", "test2");
            Optional<Map.Entry<String, String>> firstElement = map.entrySet().stream().findFirst();
            firstElement.ifPresent(s -> logger.info(">>>>>>>>>Element:" + s));
            firstElement.ifPresent(s -> logger.info(">>>>>>>>>key:" + s.getKey() + ", value:" + s.getValue()));
        }

![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 小结 #

<table> 
 <thead> 
  <tr> 
   <th>序号</th> 
   <th>描述</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td>1</td> 
   <td>HashMap键可以为null</td> 
  </tr> 
  <tr> 
   <td>2</td> 
   <td>HashMap值可以为null</td> 
  </tr> 
  <tr> 
   <td>3</td> 
   <td>HashMap使用null作为键查询时，不会抛异常</td> 
  </tr> 
 </tbody> 
</table>

# 4 完整样例 #

package com.monkey.java_study.functiontest;
    
    import org.apache.logging.log4j.LogManager;
    import org.apache.logging.log4j.Logger;
    import org.junit.Test;
    
    import java.util.HashMap;
    import java.util.Map;
    import java.util.Optional;
    
    /**
     * Map测试.
     *
     * @author xindaqi
     * @date 2021-10-19 15:23
     */
    public class MapTest {
        
    
        private static final Logger logger = LogManager.getLogger(MapTest.class);
    
        /**
         * 添加null键.
         */
        @Test
        public void mapPutNullKeyTest() {
        
            Map<String, String> map = new HashMap<>();
            map.put(null, "test1");
            String value = map.get(null);
            logger.info(">>>>>>>>>Value:" + value);
    
        }
    
        /**
         * 添加null值.
         */
        @Test
        public void mapPutNullValueTest() {
        
            Map<String, String> map = new HashMap<>();
            map.put("2", null);
            String value1 = map.get("2");
            logger.info(">>>>>>>>>Value:" + value1);
        }
    
        /**
         * 查询没有key的值.
         */
        @Test
        public void mapReturnNullTest() {
        
            Map<String, String> map = new HashMap<>();
            map.put("1", "test1");
            map.put("2", "test2");
            String value = map.get("3");
            logger.info(">>>>>>>>>Value:" + value);
        }
    
        /**
         * 使用null作为键查询.
         */
        @Test
        public void mapKeyNullTest() {
        
            Map<String, String> map = new HashMap<>();
            map.put("1", "test1");
            map.put("2", "test2");
            String value = map.get(null);
            logger.info(">>>>>>>>>Value:" + value);
        }
    
        /**
         * 获取第一个元素.
         */
        @Test
        public void mapFindFirstTest() {
        
            Map<String, String> map = new HashMap<>();
            map.put("1", "test1");
            map.put("2", "test2");
            Optional<Map.Entry<String, String>> firstElement = map.entrySet().stream().findFirst();
            firstElement.ifPresent(s -> logger.info(">>>>>>>>>Element:" + s));
            firstElement.ifPresent(s -> logger.info(">>>>>>>>>key:" + s.getKey() + ", value:" + s.getValue()));
        }
    }

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