Java基础系列：HashMap与null

1 插入数据 1.1 插入null键

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

1.2 插入null值

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

2 查询数据 2.1 查询null键

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

2.3 查询不存在的键

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

2.4 查询第一条数据

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

3 小结

序号	描述
1	HashMap键可以为null
2	HashMap值可以为null
3	HashMap使用null作为键查询时，不会抛异常

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;


public class MapTest {

    private static final Logger logger = LogManager.getLogger(MapTest.class);

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

    }

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

    
    @Test
    public void mapReturnNullTest() {
        Map map = new HashMap<>();
        map.put("1", "test1");
        map.put("2", "test2");
        String value = map.get("3");
        logger.info(">>>>>>>>>Value:" + value);
    }

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

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

Java基础系列：HashMap与null

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