Lambda表达式是Java SE 8中一个重要的新特性。你可以把 Lambda表达式 理解为是一段可以传递的代码 (将代码像数据一样进行传递)。可以写出更简洁、更灵活的代码。作为一种更紧凑的代码风格，使Java的语言表达能力得到了提升。
Lambda运算符：所有的Lambda表达式都是用新的Lambda运算符 " => ",可以叫他，“转到”或者 “成为”。运算符将表达式分为两部分，左边指定输入参数，右边是Lambda的主体。
在平常 我们可以要通过sql 对数据进行过滤、分组、转化等，如果非常频繁，会对数据库增加负担，我们这里就可以简单使用Lambda表达式
1、准备一个Object类
import java.math.BigDecimal;
public class AObject {
private String a;
private String b;
private BigDecimal money;
private Integer num;
public String getA() {
return a;
}
public void setA(String a) {
this.a = a;
}
public String getB() {
return b;
}
public void setB(String b) {
this.b = b;
}
public BigDecimal getMoney() {
return money;
}
public void setMoney(BigDecimal money) {
this.money = money;
}
public Integer getNum() {
return num;
}
public void setNum(Integer num) {
this.num = num;
}
@Override
public String toString(){
return a+" "+b;
}
}
2、Lambda使用
import java.math.BigDecimal;
import java.util.ArrayList;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;
public class StreamUtils {
public static void main(String args[]){
listToMap();
filter();
// groupingBy();
// sum();
}
/**
* List<T> -> Map
* 需要注意的是：
* toMap 如果集合对象有重复的key，会报错Duplicate key ....
* 可以用 (k1,k2)->k1 来设置，如果有重复的key,则保留key1,舍弃key2
*/
public static void listToMap() {
List<AObject> aList = new ArrayList<>();
AObject aObject = new AObject();
aObject.setA("a");
aObject.setB("B");
AObject aObject1 = new AObject();
aObject1.setA("a1");
aObject1.setB("B1");
aList.add(aObject);
aList.add(aObject1);
Map<String, AObject> map = aList.stream().
collect(Collectors.toMap(AObject::getA, AObject->AObject,(k1,k2)->k1));
for(Map.Entry<String, AObject> entry : map.entrySet()){
System.out.println("key:"+entry.getKey()+" value:"+entry.getValue().toString());
}
// key:a1 value:a1 B1
// key:a value:a B
Map<String, String> map1 = aList.stream().
collect(Collectors.toMap(AObject::getA, AObject::getB,(k1,k2)->k1));
for(Map.Entry<String, String> entry : map1.entrySet()){
System.out.println("key:"+entry.getKey()+" value:"+entry.getValue());
}
// key:a1 value:B1
// key:a value:B
}
/**
* 过滤
*/
public static void filter() {
List<String> aList = new ArrayList<>();
aList.add("1");
aList.add("a");
List<String> slist =aList.stream().filter(e -> "a".equals(e)).collect(Collectors.toList());
System.out.println(slist.toString());// [a]
List<AObject> list = new ArrayList<>();
AObject aObject = new AObject();
aObject.setA("110000");
aObject.setB("北京");
AObject aObject2 = new AObject();
aObject2.setA("330000");
aObject2.setB("浙江");
list.add(aObject);
list.add(aObject2);
List<AObject> newList =list.stream().filter(e -> e.getA().equals("110000")).collect(Collectors.toList());
System.out.println(newList.toString());// [110000 北京]
}
/**
* 分组
*/
public static void groupingBy(){
List<AObject> list = new ArrayList<>();
AObject aObject = new AObject();
aObject.setA("110000");
aObject.setB("北京");
AObject aObject1 = new AObject();
aObject1.setA("110000");
aObject1.setB("北京1");
AObject aObject2 = new AObject();
aObject2.setA("330000");
aObject2.setB("浙江");
AObject aObject3 = new AObject();
aObject3.setA("330000");
aObject3.setB("浙江1");
list.add(aObject);
list.add(aObject1);
list.add(aObject2);
list.add(aObject3);
//groupingBy 只传key 默认把整个集合关联， 也可以把集合某个数据属性 当做集合取出来，如下groupBy1
Map<String, List<AObject>> groupBy = list.stream().collect(Collectors.groupingBy(AObject::getA));
for(Map.Entry<String, List<AObject>> item : groupBy.entrySet()){
System.out.print(item.getKey()+" : ");
for(AObject a : item.getValue()){
System.out.print(a.toString()+";");
}
System.out.println();
}
//110000 : 110000 北京;110000 北京1;
//330000 : 330000 浙江;330000 浙江1;
Map<String, List<String>> groupBy1 = list.stream().collect(
Collectors.groupingBy(AObject::getA, Collectors.mapping(AObject::getB, Collectors.toList())));
for(Map.Entry<String, List<String>> item : groupBy1.entrySet()){
System.out.print(item.getKey()+" : ");
for(String a : item.getValue()){
System.out.print(a+";");
}
System.out.println();
}
// 110000 : 北京;北京1;
// 330000 : 浙江;浙江1;
}
/**
* 求和
*/
public static void sum(){
List<AObject> list = new ArrayList<>();
AObject aObject = new AObject();
aObject.setMoney(new BigDecimal("100"));
aObject.setNum(100);
AObject aObject1 = new AObject();
aObject1.setMoney(new BigDecimal("200"));
aObject1.setNum(200);
AObject aObject2 = new AObject();
aObject2.setMoney(new BigDecimal("300"));
aObject2.setNum(300);
AObject aObject3 = new AObject();
aObject3.setMoney(new BigDecimal("400"));
aObject3.setNum(400);
list.add(aObject);
list.add(aObject1);
list.add(aObject2);
list.add(aObject3);
//注意的是 求和的属性 值不能为null
BigDecimal totalMoney = list.stream().map(AObject::getMoney).
reduce(BigDecimal.ZERO, BigDecimal::add);
System.out.println("totalMoney:"+totalMoney); // totalMoney:1000
Integer totalMoney1 = list.stream().map(AObject::getNum).
reduce(0, Integer::sum);
System.out.println("totalSum:"+totalMoney1); // totalSum:1000
int sum = list.stream().mapToInt(AObject::getNum).sum();
System.out.println("sum:"+sum);// sum:1000
}
}