轉載:https://blog.csdn.net/u011212394/article/details/96432029
Stream是什麼
Stream是Java8中新加入的api,更準確的說:
Java 8 中的 Stream 是對集合（Collection）對象功能的增強，它專注於對集合對象進行各種非常便利、高效的聚合操作，或者大批量數據操作 。Stream API 藉助於同樣新出現的 Lambda 表達式，極大的提高編程效率和程序可讀性。下面例子中包括利用Java8 stream方式對於對象集合的分組、list轉map、條件過濾、去重、排序、求和、求最大最小值。
import lombok.Data;
import java.math.BigDecimal;
import java.util.*;
import java.util.stream.Collectors;
/**
* Java8 集合操作
*
* @Author hujy
* @Date Created in 2019-07-16 11:38
* @Version 1.0
*/
@Data
public class Fruit {
private Integer id;
private String name;
private BigDecimal money;
private Integer num;
public Fruit(Integer id, String name, BigDecimal money, Integer num) {
this.id = id;
this.name = name;
this.money = money;
this.num = num;
}
/**
* 分組
* @author hujy
* @date 2019-07-18 15:03
* @param fruitList
* @return void
*/
private static void groupBy(List<Fruit> fruitList) {
Map<Integer, List<Fruit>> groupBy =
fruitList.stream().collect(Collectors.groupingBy(Fruit::getId));
System.out.println("groupBy:" + groupBy);
}
/**
* List轉Map
* @author hujy
* @date 2019-07-18 15:04
* @param fruitList
* @return void
*/
private static void listToMap(List<Fruit> fruitList) {
// 注:如果集合對象有重複的key，會報Duplicate key錯誤，可以用 (k1, k2) -> k1 來設置，如果有重複的key,則保留key1,捨棄key2
Map<Integer, Fruit> fruitMap =
fruitList.stream().collect(Collectors.toMap(Fruit::getId, a -> a, (k1, k2) -> k1));
System.out.println("toMap:" + fruitMap);
}
/**
* 按條件過濾
* @author hujy
* @date 2019-07-18 15:07
* @param fruitList
* @return void
*/
private static void filter(List<Fruit> fruitList) {
List<Fruit> filterList =
fruitList.stream().filter(a -> a.getName().equals("香蕉")).collect(Collectors.toList());
System.out.println("filter:" + filterList);
}
/**
* 去重
* @author hujy
* @date 2019-07-18 15:08
* @param fruitList
* @return void
*/
private static void toUnique(List<Fruit> fruitList) {
// 根據id去重
List<Fruit> uniqueList =
fruitList.stream().collect(
Collectors.collectingAndThen(Collectors.toCollection(() -> new TreeSet<>(Comparator.comparing(o -> o.getId()))),
ArrayList::new));
System.out.println("unique:" + uniqueList);
}
/**
* 升序
* @author hujy
* @date 2019-07-18 15:09
* @param fruitList
* @return void
*/
private static void sortAsc(List<Fruit> fruitList) {
// 按id升序排序
List<Fruit> sortAsc=
fruitList.stream().sorted(Comparator.comparing(Fruit::getId)).collect(Collectors.toList());
System.out.println("sortAsc:" + sortAsc);
}
/**
* 降序
* @author hujy
* @date 2019-07-18 15:09
* @param fruitList
* @return void
*/
private static void sortDesc(List<Fruit> fruitList) {
// 按id降序排序
List<Fruit> sortDesc =
fruitList.stream().sorted(Comparator.comparing(Fruit::getId).reversed()).collect(Collectors.toList());
System.out.println("sortDesc:" + sortDesc);
}
/**
* 先升序再降序
* @author hujy
* @date 2019-07-18 15:09
* @param fruitList
* @return void
*/
private static void sortAscDesc(List<Fruit> fruitList) {
// 按id升序排序，id一樣的按num降序
List<Fruit> sortAscDesc = fruitList.stream().sorted(Comparator.comparing(Fruit::getId).reversed()
.thenComparing(Fruit::getNum).reversed()).collect(Collectors.toList());
System.out.println("sortAscDesc:" + sortAscDesc);
}
/**
* 求和
* @author hujy
* @date 2019-07-18 15:10
* @param fruitList
* @return void
*/
private static void sum(List<Fruit> fruitList) {
// 按money求和
BigDecimal totalMoney = fruitList.stream().map(Fruit::getMoney).reduce(BigDecimal.ZERO, BigDecimal::add);
System.out.println("totalMoney:" + totalMoney);
}
/**
* 求最大值
* @author hujy
* @date 2019-07-18 15:10
* @param fruitList
* @return void
*/
private static void getMax(List<Fruit> fruitList) {
// 求money最大值
Optional<BigDecimal> max = fruitList.stream().map(Fruit::getMoney).max(BigDecimal::compareTo);
System.out.println("max:" + max.get());
}
/**
* 求最小值
* @author hujy
* @date 2019-07-18 15:10
* @param fruitList
* @return void
*/
private static void getMin(List<Fruit> fruitList) {
// 求money最小值
Optional<BigDecimal> min = fruitList.stream().map(Fruit::getMoney).min(BigDecimal::compareTo);
System.out.println("min:" + min.get());
}
public static void main(String[] args) {
Fruit fruit1 = new Fruit(1, "蘋果1", new BigDecimal("2.25"), 10);
Fruit fruit2 = new Fruit(1, "蘋果2", new BigDecimal("1.85"), 20);
Fruit fruit3 = new Fruit(2, "香蕉", new BigDecimal("1.89"), 30);
Fruit fruit4 = new Fruit(3, "櫻桃", new BigDecimal("8.99"), 40);
List<Fruit> fruitList = Arrays.asList(fruit1, fruit2, fruit3, fruit4);
// 1.分組
groupBy(fruitList);
// groupBy:{1=[Fruit(id=1, name=蘋果1, money=2.25, num=10), Fruit(id=1, name=蘋果2, money=1.85, num=20)], 2=[Fruit(id=2, name=香蕉, money=1.89, num=30)], 3=[Fruit(id=3, name=櫻桃, money=8.99, num=40)]}
// 2.List轉Map
listToMap(fruitList);
// toMap:{1=Fruit(id=1, name=蘋果1, money=2.25, num=10), 2=Fruit(id=2, name=香蕉, money=1.89, num=30), 3=Fruit(id=3, name=櫻桃, money=8.99, num=40)}
// 3.按條件過濾
filter(fruitList);
// filter:[Fruit(id=2, name=香蕉, money=1.89, num=30)]
// 4.根據id去重
toUnique(fruitList);
// unique:[Fruit(id=1, name=蘋果1, money=2.25, num=10), Fruit(id=2, name=香蕉, money=1.89, num=30), Fruit(id=3, name=櫻桃, money=8.99, num=40)]
// 5.按id升序
sortAsc(fruitList);
// sortAsc:[Fruit(id=1, name=蘋果1, money=2.25, num=10), Fruit(id=1, name=蘋果2, money=1.85, num=20), Fruit(id=2, name=香蕉, money=1.89, num=30), Fruit(id=3, name=櫻桃, money=8.99, num=40)]
// 6.按id降序
sortDesc(fruitList);
// sortDesc:[Fruit(id=3, name=櫻桃, money=8.99, num=40), Fruit(id=2, name=香蕉, money=1.89, num=30), Fruit(id=1, name=蘋果1, money=2.25, num=10), Fruit(id=1, name=蘋果2, money=1.85, num=20)]
// 7.按id升序排序，id一樣的按num降序
sortAscDesc(fruitList);
// sortAscDesc:[Fruit(id=1, name=蘋果2, money=1.85, num=20), Fruit(id=1, name=蘋果1, money=2.25, num=10), Fruit(id=2, name=香蕉, money=1.89, num=30), Fruit(id=3, name=櫻桃, money=8.99, num=40)]
// 8.money求和
sum(fruitList);
// totalMoney:14.98
// 9.求money最大值
getMax(fruitList);
// max:8.99
// 10.求money最小值
getMin(fruitList);
// min:1.85
// 11集合轉數組
Fruit[] array = list.stream.toArray(Stirng[] new);
}
}
將一個List的屬性賦值給另一個List
List<OrderOpHis> records = orderIds.stream().map(orderId -> {
OrderOpHis record = new OrderOpHis();
record.setOrderId(orderId);
record.setOperatorName("system");
return record;
}).collect(Collectors.toList());