Java8 list.stream()操作使用心得

准备数据

实体类

@Data
public class UserInfo {
	
	String personNo;
	
	String name;

	int num;

	int age;

}
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类似输入

[
{"personNo": "123","name":"张三","age":20},
{"personNo": "123","name":"张三","age":21},
{"personNo": "1234","name":"李四","age":19}
]
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1，把list中某个字段用逗号相连

joining(“,”)

String result = list.stream().map(UserInfo::getPersonNo).collect(Collectors.joining(","))
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输出

123,123,1234
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2，根据某个字段分组

groupingBy()

Map<String,List<UserInfo>> result = list.stream().collect(Collectors.groupingBy(UserInfo::getName)):
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key为null时不报错的处理方法

public static <T,A> Collector<T ,?, Map<A,List<T>>> groupByWithNullKeys(Function<? super T,
? extends A> classifier){
	return Collectors.toMap(
			classifier,
			Collections::singletonList,
			(List<T> oldList,List<T> newEl) -> {
				List<T> newList = new ArrayList<>(oldList.hashCode()+1);
				newList.addAll(oldList);
				newList.addAll(newEl);
				return newList;
				}
	);
}
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输出如下

key  value
李四 [{"personNo": "1234","name":"李四","age":19}]
张三 [{"personNo": "123","name":"张三","age":20},{"personNo": "123","name":"张三","age":21}]
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3，取出list中某个字段最大值的记录

max

UserInfo userInfo = list.stream().max(Comparator.comparing(UserInfo::getAge)).get();
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输出如下

{"personNo": "123","name":"张三","age":21}
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4，根据某个字段倒序

sorted，默认升序，可通过reversed来变成降序

List<UserInfo> result = list.stream().sorted(Comparator.comparing(UserInfo::getNum).reversed()).collect(Collectors.toList());
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输出如下：

[
{"personNo": "123","name":"张三","age":21},
{"personNo": "123","name":"张三","age":20},
{"personNo": "1234","name":"李四","age":19}
]
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5，根据某个字段求和

mapToInt sum

int result = list.stream().mapToInt(UserInfo::getAge).sum();
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输出如下

60
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6，根据某个字段过滤

filter

List<UserInfo> result = list.stream().filter(u -> "李四".equals(u.getName())).collect(Collectors.toList());
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输出如下：

{"personNo": "1234","name":"李四","age":19}
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7，全字段去重

distinct count，全部字段值都一样才会去重，部分字段一样不会去重

int count = list.stream().distinct().count();
List<UserInfo> result = list.stream().distinct().collect(Collectors.toList());
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输出如下

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[
{"personNo": "123","name":"张三","age":21},
{"personNo": "123","name":"张三","age":20},
{"personNo": "1234","name":"李四","age":19}
]
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