java8lambda-list转map、过滤、求和、分组等
java8中开始引⼊了lambda表达式,该表达式⼤⼤简化了代码量,使⽤lambda处理list列表很⽅便,下⾯介绍⼀些常⽤的使⽤⽅式数据对象:
import java.io.Serializable;
import java.math.BigDecimal;
import java.util.Objects;
import lombok.AllArgsConstructor;
import lombok.Data;
import lombok.NoArgsConstructor;
/**
* ⼈类
* @create 2018-10-17 12:40
**/
@Data
@NoArgsConstructor
@AllArgsConstructor
public class Person implements Serializable {
private static final long serialVersionUID = -3236552889204227101L;
private Integer id;
private Integer age;
private String name;
private BigDecimal money;
public Person(Integer id, String name) {
this.id = id;
this.name = name;
}
@Override
public boolean equals(Object o) {
if (this == o) return true;
if (o == null || getClass() != o.getClass()) return false;
Person person = (Person) o;
return Objects.equals(id, person.id) &&
Objects.equals(name, person.name);
}
@Override
public int hashCode() {
return Objects.hash(id, name);
}
}
list中添加对象列表:
Person p1 = new Person(1,10,"zs",new BigDecimal(10.11).setScale(2, BigDecimal.ROUND_HALF_UP));
Person p2 = new Person(2,11,"zs2",new BigDecimal(5.52).setScale(2, BigDecimal.ROUND_HALF_UP));
Person p3 = new Person(3,15,"zs",new BigDecimal(6.29).setScale(2, BigDecimal.ROUND_HALF_UP));
List<Person> personList = new ArrayList<>(3);
personList.add(p1);
personList.add(p2);
personList.add(p3);
1、过滤
List<Person> personFilterList = personList.stream().filter(person -> "zs".Name())).List());
System.out.println(personFilterList);
运⾏结果:[Person(id=1, age=10, name=zs, money=10.11), Person(id=3, age=15, name=zs, money=6.29)]
2、计算总数(sum)
Long totalAges = personList.stream().mapToLong(Person::getAge).sum();
System.out.println("totalAges:" + totalAges);
运⾏结果:totalAges:36
3、对象属性转换成list
List<Integer> ids = personList.stream().map(Person::getId).List());
System.out.println("ids:" + ids);
运⾏结果:ids:[1, 2, 3]
4、分组
Map<String, List<Person>> personGroupMap = personList.stream().upingBy(Person::getName));
System.out.println("personGroupMap:" + personGroupMap);
运⾏结果:personGroupMap:{zs2=[Person(id=2, age=11, name=zs2, money=5.52)], zs=[Person(id=1, age=10, name=zs, money=10.11), Person(id=3, age=15, name=zs, money=6.29)]}
5、计算BigDecial类型数据和
BigDecimal totalMoney = personList.stream().map(Person::getMoney).reduce(BigDecimal.ZERO, BigDecimal::add);
System.out.println("totalMoney:" + totalMoney);
运⾏结果:totalMoney:21.92
6、list转成map,不允许有重复key,存在重复key时会报错
Map<Integer,Person> personMap = personList.stream().Map(Person::getId, Function.identity()));
personMap.forEach((k,v) -> System.out.println("key:" + k + ", value=" + v));
运⾏结果:
key:1, value=Person(id=1, age=10, name=zs, money=10.11)
key:2, value=Person(id=2, age=11, name=zs2, money=5.52)
key:3, value=Person(id=3, age=15, name=zs, money=6.29)
7、list转成map,存在重复key时⾃⼰决定怎么样处理
Person p1 = new Person(1,10,"zs",new BigDecimal(10.11).setScale(2, BigDecimal.ROUND_HALF_UP));
Person p2 = new Person(1,11,"zs2",new BigDecimal(5.52).setScale(2, BigDecimal.ROUND_HALF_UP));
Person p3 = new Person(3,15,"zs",new BigDecimal(6.29).setScale(2, BigDecimal.ROUND_HALF_UP));
List<Person> personList = new ArrayList<>(3);
personList.add(p1);
personList.add(p2);
personList.add(p3);
/
/ list 转成map,重复key处理,k1,k2 重复时取k2,即采⽤后者覆盖前者的⽅式
Map<Integer,Person> duplicateKeyMap = personList.stream().Map(Person::getId, Function.identity(), (k1, k2) -> k2));        duplicateKeyMap.forEach((k,v) -> System.out.println("key:" + k + ", value=" + v));
运⾏结果:
key:1, value=Person(id=1, age=11, name=zs2, money=5.52)
key:3, value=Person(id=3, age=15, name=zs, money=6.29)
完成测试类代码:
import com.py.Person;
import java.math.BigDecimal;
import java.util.ArrayList;
import java.util.List;
import java.util.Map;
import java.util.function.Function;
import java.util.stream.Collectors;
bigdecimal转换为integerimport org.junit.Test;
/**
* 测试类
*
* @date 2019-04-03 14:44
**/
public class LambdaTest {
@Test
public void test(){
Person p1 = new Person(1,10,"zs",new BigDecimal(10.11).setScale(2, BigDecimal.ROUND_HALF_UP));
Person p2 = new Person(2,11,"zs2",new BigDecimal(5.52).setScale(2, BigDecimal.ROUND_HALF_UP));
Person p3 = new Person(3,15,"zs",new BigDecimal(6.29).setScale(2, BigDecimal.ROUND_HALF_UP));
List<Person> personList = new ArrayList<>(3);
personList.add(p1);
personList.add(p2);
personList.add(p3);
// 过滤
List<Person> personFilterList = personList.stream().filter(person -> "zs".Name())).List());
System.out.println(personFilterList);
// 计算总数
Long totalAges = personList.stream().mapToLong(Person::getAge).sum();
System.out.println("totalAges:" + totalAges);
// list 转成map, 重复key会报错
Map<Integer,Person> personMap = personList.stream().Map(Person::getId, Function.identity()));
personMap.forEach((k,v) -> System.out.println("key:" + k + ", value=" + v));
// list 转成map,重复key处理,k1,k2 重复时取k2,即采⽤后者覆盖前者的⽅式
/*Map<Integer,Person> duplicateKeyMap = personList.stream().Map(Person::getI
d, Function.identity(), (k1, k2) -> k2));        duplicateKeyMap.forEach((k,v) -> System.out.println("key:" + k + ", value=" + v));*/
// 对象属性转成list
List<Integer> ids = personList.stream().map(Person::getId).List());
System.out.println("ids:" + ids);
// 分组
Map<String, List<Person>> personGroupMap = personList.stream().upingBy(Person::getName));
System.out.println("personGroupMap:" + personGroupMap);
// 计算BigDecial类型数据和
BigDecimal totalMoney = personList.stream().map(Person::getMoney).reduce(BigDecimal.ZERO, BigDecimal::add);
System.out.println("totalMoney:" + totalMoney);
}
}

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