2024-03-28 Java8之Collectors类

Collectors类常用方法

1.toList、toSet、toMap

CollectorT, ?, ListT>> toList(); //收集为List集合
CollectorT, ?, SetT>> toSet(); //Set
CollectorT, ?, MapK,U>> toMap(Function? super T, ? extends K> keyMapper,Function? super T, ? extends U> valueMapper)//Map

//收集code为新List集合
ListString> strList = list.stream().map(DemoObj::getCode).collect(Collectors.toList());
//收集code为新Set集合
SetString> collect = list.stream().map(DemoObj::getCode).collect(Collectors.toSet());
//将code为key，amount为值作为map集合
MapString, Integer> collect = list.stream().collect(Collectors.toMap(DemoObj::getCode, DemoObj::getAmount));

2.joining、counting、summingInt、minBy

//指定字符拼接
CollectorCharSequence, ?, String> joining(CharSequence delimiter); 
//统计 等同于Stream的count()
T> CollectorT, ?, Long> counting();
//求和，等同于Stream的mapToInt(ToIntFunction super 服务器托管T> mapper).服务器托管sum()
T> CollectorT, ?, Integer> summingInt(ToIntFunction? super T> mapper) 
//获取最小值,等同于Stream的min(Comparator super T> comparator)
T> CollectorT, ?, OptionalT>>  minBy(Comparator? super T> comparator) 

//遍历list集合取将DemoObj对象的code属性用逗号拼接
String str = list.stream().map(DemoObj::getCode).collect(Collectors.joining(","));

Long collect = list.stream().map(DemoObj::getCode).collect(Collectors.counting());

//求和，等同于mapToInt(DemoObj::getAmount).sum()
Integer collect = list.stream().collect(Collectors.summingInt(DemoObj::getAmount));

//获取最小金额，等同于 min(Comparator.comparingInt(DemoObj::getAmount))
OptionalDemoObj> collect = list.stream().collect(Collectors.minBy(Comparator.comparingInt(DemoObj::getAmount)));

3.groupingBy

T, K> CollectorT, ?, MapK, ListT>>> groupingBy(Function? super T, ? extends K> classifier);//分组

MapInteger, ListDemoObj>> collect = list.stream().collect(Collectors.groupingBy(DemoObj::getAmount));//按金额分组

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