【Stream】Java中List＜Object＞,List＜~＞ 使用Stream函数式

EXP List 转List

场景一 Object是已知对象，单纯去某一个成员变量为List<~>

public class Main {
	public static void main(String[] args) {
		List<Person> list = new ArrayList<>();
		list.add(new Person("张三", 1));
		list.add(new Person("李四", 2));
		// 这里只需要id
		List<Long> ids = list.stream().map(Person::getId()).collect(Collectors.toList());
		
	}
	@Data
	class Person {
		private String name;
		private Long id;

		public Person(String name, String id) {
			this.name = name;
			this.id = id;
		}
		
	}
}
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场景二 Object其实String，但是强转感觉很LOW

public class Main {
	public static void main(String[] args) {
		List<Object> list = new ArrayList<>();
		list.add("A");
		list.add("B");
		List<String> collect = list.stream().map(String::valueOf()).collect(Collectors.toList());
	}
}
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List中使用stream() 实现大小写转换

场景一 Object其实是String

public class Main {
	public static void main(String[] args) {
		List<Object> list = new ArrayList<>();
		list.add("A");
		list.add("B");
		List<String> collect = list.stream().map(item -> String.valueOf(item).toLowerCase(Locale.ROOT)).collect(Collectors.toList());
	}
}
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场景二 Object是对象

public class Main {
	public static void main(String[] args) {
		List<Person> list = new ArrayList<>();
		list.add(new Person("A", 1));
		list.add(new Person("B", 2));
		// 这里只需要id
		List<String> names = list.stream().map(item -> item.getName().toLowerCase(Locale.ROOT)).collect(Collectors.toList());
		
	}
	@Data
	class Person {
		private String name;
		private Long id;

		public Person(String name, String id) {
			this.name = name;
			this.id = id;
		}
		
	}
}
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List 转Map

// 首先定义一个User类,我们用主键ID作为key，然后value就是当前对象
// 举一反三
List<User> users = new ArrayList<>();
Map<String, User> usersMap = users.stream().collect(Collectors.toMap(User::getId, Function.identity(), (key1, key2) -> key2));
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