个人Stream常用操作

1、list转map

我们可以使用 Collectors.toMap() 方法来实现。

• Person对象类

@Data
@Builder
@AllArgsConstructor
@NoArgsConstructor
public class Person {
    private String name;  // 姓名
    private int salary; // 薪资
    private int age; // 年龄
    private String sex; //性别
    private String area;  // 地区

}

• 重载方法一

@SpringBootTest
public class StreamDemoTest {
	@Test
	public void listToMapTest(){
		List<Person> personList = new ArrayList<Person>();
		personList.add(new Person("Tom", 8900, 17,"male", "New York"));
		personList.add(new Person("Jack", 7000,18 ,"male", "Washington"));
		personList.add(new Person("Lily", 7800,20, "female", "Washington"));
		personList.add(new Person("Anni", 8200,22, "female", "New York"));
		personList.add(new Person("Owen", 9500,20, "male", "New York"));
		personList.add(new Person("Alisa", 7900,18, "female", "New York"));
		// 转换方式一
		Map<String, Person> personMap = personList.stream()
			.collect(Collectors.toMap(Person::getName, Function.identity()));
		for (Map.Entry<String, Person> entry : personMap.entrySet()) {
			System.out.println(entry);
		}
}

我们可以将名称作为key，将对象本体作为value，结果如下

Tom=Person(name=Tom, salary=8900, age=17, sex=male, area=New York)
Owen=Person(name=Owen, salary=9500, age=20, sex=male, area=New York)
Anni=Person(name=Anni, salary=8200, age=22, sex=female, area=New York)
Alisa=Person(name=Alisa, salary=7900, age=18, sex=female, area=New York)
Jack=Person(name=Jack, salary=7000, age=18, sex=male, area=Washington)
Lily=Person(name=Lily, salary=7800, age=20, sex=female, area=Washington)

但是这样转换如果有相同的key的话会报错 java.lang.IllegalStateException: Duplicate key XXXX。

这种情况下我们可以使用Collectors.toMap() 的重载方法来进行转换

• 重载方法二
  当我们的map中可能存在重复key的时候，我们可以使用 Collectors.toMap() 的重载方法，包含了一个 mergeFunction 合并函数来处理重复key数据。

@Test
public void listToMapTest(){
	List<Person> personList = new ArrayList<Person>();
	personList.add(new Person("Tom", 8900, 17,"男", "New York"));
	personList.add(new Person("Tom", 7000,18 ,"女", "Washington"));
	personList.add(new Person("Lily", 7800,20, "女", "Washington"));
	personList.add(new Person("Anni", 8200,22, "女", "New York"));
	personList.add(new Person("Tom", 9500,20, "男", "New York"));
	personList.add(new Person("Lily", 7900,18, "男", "New York"));
	// 转换为map时使用已存在数据
	Map<String, Person> personMap = personList.stream()
		.collect(Collectors.toMap(Person::getName, Function.identity(),
			(exists,newest)-> exists ));
	/*  ==================== 打印结果 =================
            Tom=Person(name=Tom, salary=8900, age=17, sex=male, area=New York)
            Anni=Person(name=Anni, salary=8200, age=22, sex=female, area=New York)
            Lily=Person(name=Lily, salary=7800, age=20, sex=female, area=Washington)
         */
	// 转换为map时使用最新的数据替换旧数据
	Map<String, Person> personMap1 = personList.stream()
		.collect(Collectors.toMap(Person::getName, Function.identity(),
			(exists,newest)-> newest ));
	/*  ==================== 打印结果 =================
            Tom=Person(name=Tom, salary=9500, age=20, sex=male, area=New York)
            Anni=Person(name=Anni, salary=8200, age=22, sex=female, area=New York)
            Lily=Person(name=Lily, salary=7900, age=18, sex=female, area=New York)
         */

	// 也可以进行判断后转换，当key重复时，如果最新数据性别为男则使用最新数据替换，否则使用已存在的旧数据
	Map<String, Person> personMap2 = personList.stream()
		.collect(Collectors.toMap(Person::getName, Function.identity(),
			(exists,newest)-> newest.getSex().equals("男") ? newest : exists));
	/*  ==================== 打印结果 =================
            Tom=Person(name=Tom, salary=9500, age=20, sex=男, area=New York)
            Anni=Person(name=Anni, salary=8200, age=22, sex=女, area=New York)
            Lily=Person(name=Lily, salary=7900, age=18, sex=男, area=New York)
        */
}

• 重载方法三
  可以指定转换的map类型

        TreeMap<String, Person> treeMap = personList.stream()
                .collect(Collectors.toMap(Person::getName, Function.identity(), 
			(existing, replacement) -> existing, TreeMap::new));