一、JDK 1.8 Lambda简述
在jdk1.8中lambda表达式提供了集合的一些很实用的操作方法，函数式编程既减少了很多繁杂的foreach代码，又简洁明了，通俗易懂.
二、Lambda表达式
Lambda表达式本质上是一段匿名内部类，也可以是一段可以传递的代码
三、上干货
1. 新建一个Bean（ User），新建一个List list = new Array<>(); 并把多个student添加到list中
public class Student {
private String no;
private String name;
private String classes;
public Student(String no, String name, String classes) {
this.no = no;
this.name = name;
this.classes = classes
}
public String getNo() {
return no;
}
public void setNo(String no) {
this.no = no;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public String getClasses() {
return classes;
}
public void setClasses(String classes) {
this.classes = classes;
}
}
2. 新建一个List list = new Array<>(); 并把多个student添加到list中
List<Student> list = new ArrayList<>();
list.add(new Student("4","小米","二班"));
list.add(new Student("2","小丽","一班"));
list.add(new Student("1","小红","一班"));
list.add(new Student("3","小名","二班"));
list.add(new Student("3","小名","二班"));
3. 按照班级分组groupingBy()
Map<String, List<Student>> listMap = list.stream().collect(Collectors.groupingBy(Student::getClasses));
4. 筛选二班的学生filter()
List<Student> filterList = list.stream().filter(student -> student.getClass().equals("二班")).collect(Collectors.toList());
5. 获取list某个字段组装新list map()
List<String> newList = list.stream().map(student -> student.getName()).collect(Collectors.toList());
6. 去重distinct().
List<Student> filterList = list.stream().distinct().collect(Collectors.toList());
//去重
List<Map<String, String>> list = teacherList.stream().collect(Collectors.toMap(m -> m.get("course") + m.get("teacherTel"), Function.identity(), (oldValue, newValue) -> oldValue)).values().stream().collect(Collectors.toList());
7. 单字段多字段排序sort()、sorted()
//第一种
list.sort(Comparator.comparing(Student::getNo));
//注：若选择排序字段为null值，正序可
list.sort(Comparator.comparing(Student::getNo,Comparator.nullsFirst(Comparator.naturalOrder())));
//第二种
List<Student> sorterList = list.stream().sorted(Comparator.comparing(Student::getNo)).collect(Collectors.toList());
//第三种
List<Student> sorterList = list.stream().sorted((s1,s2)->{return s1.getNo().compareTo(s2.getNo());}).collect(Collectors.toList());
8. 多字段排序sort()、sorted()
//第一种
list.sort(Comparator.comparing(Student::getNo).thenComparing(Comparator.comparing(Student::getName)));
//第二种
List<Student> sorterList = list.stream().sorted(Comparator.comparing(Student::getNo).thenComparing(Student::getName)).collect(Collectors.toList());
//第三种
List<Student> sorterList = list.stream().sorted((s1, s2) -> {
int v1 = s1.getNo().compareTo(s2.getNo());
if (v1 != 0)
return v1;
int v2 = s1.getName().compareTo(s2.getName());
return v2;
}).collect(Collectors.toList());
9. list转map
//其中，若集合对象key有重，可根据(k1,k2)->k1设置<保留k1，舍弃k2>
Map<String, Student> studentMap = list.stream().collect(Collectors.toMap(Student::getNo, t -> t, (k1, k2) -> k1));
10. 求和
int sum = list.stream().mapToInt(s->Integer.valueOf(s.getNo())).sum();
11. 最大值
int max = list.stream().mapToInt(s -> Integer.valueOf(s.getNo())).max().getAsInt();