java8不但可以提高代码的运行的性能，而且实现起来很优雅，因此学习它是不错的选择。
今天写这篇文章，是因为看到我们部门大佬写的代码，因此将其还原成匿名内部类的时候发现这个retrun是不能省掉的，省去会报错。同时还可以学习到map如果在筛选条件中只有一行的时候，是可以不需要return的，这个是与中间有处理过程是不同的。因此就有了下面的学习：
代码语言：javascript复制public List<CmPatientBasePO> listAll(Integer fkHospId, Integer fkTenantId) {
CmPatient cmPatient = new CmPatient();
cmPatient.setFkHospId(Objects.isNull(fkHospId) ? UserUtils.getHospId() : fkHospId);
cmPatient.setFkTenantId(Objects.isNull(fkTenantId) ? UserUtils.getTenantId() : fkTenantId);
List<CmPatient> cmPatientList = cmPatientPOMapper.listByCondition(cmPatient);
if (CollectionUtils.isEmpty(cmPatientList)) {
return Collections.emptyList();
}
return cmPatientList.stream().filter(Objects::nonNull).map(patient -> {
CmPatientBasePO cmPatientBasePO = new CmPatientBasePO();
BeanUtils.copyProperties(patient, cmPatientBasePO);
cmPatientBasePO.setAliasInitials(patient.getSpellInitials());
cmPatientBasePO.setPatientName(patient.getAliasName());
return cmPatientBasePO; //需要返回值，此时不可省略
}).collect(Collectors.toList());
}首先map和peek之间的区别和使用
map是有返回值的，而peek作为中间处理过程，其返回的值是void，这是两者最大的区别，其次官方推荐使用map。而使用peek可以进行中间处理，方便对数据的处理。
代码语言：javascript复制/**
* @author lyz
* @date 2020/5/12 17:03
**/
public class Demo {
public static void main(String[] args) {
Stream.of("小王:18","小杨:20").map(new Function<String, People>() {
@Override
public People apply(String s) {
String[] str = s.split(":");
People people = new People(str[0],Integer.valueOf(str[1]));
return people;
}
}).forEach(people-> System.out.println("people = " people));
}
}运行结果：
代码语言：javascript复制people = People{name='小王', age=18}
people = People{name='小杨', age=20}修改成map实现：
代码语言：javascript复制/**
*
* @description: lambda学习
* @author: lyz
* @date: 2020/05/12 14:48
**/
public class demo2 {
public static void main(String[] args) {
Stream.of("小王:18","小杨:20").map((String s)->{
String[] str = s.split(":");
People people = new People(str[0],Integer.valueOf(str[1]));
return people; //此处不可省略
}).forEach(people-> System.out.println("people = " people));
}
}运行结果：
代码语言：javascript复制people = People{name='小王', age=18}
people = People{name='小杨', age=20}修改成peek实现：
代码语言：javascript复制/**
*
* @description: lambda学习
* @author: lyz
* @date: 2020/05/12 15:16
**/
public class Demo5 {
public static void main(String[] args) {
Stream.of("小王:18","小杨:20").peek((String s)->{
String[] str = s.split(":");
People people = new People(str[0],Integer.valueOf(str[1])); //这里没有return，因此返回的void
}).forEach(people-> System.out.println("people = " people));
}
}运行结果：
代码语言：javascript复制people = 小王:18
people = 小杨:20进行数据过滤
代码语言：javascript复制/**
*
* @description: stream流学习
* @author: lyz
* @date: 2020/05/12 15:22
**/
public class Demo6 {
public static void main(String args[]){
List<Map<String,Object>> list=new ArrayList<>();
for(int i=0;i<5;i ){
Map<String,Object> map=new HashMap<>();
map.put("type",i);
list.add(map);
}
System.out.println("list过滤前的数据:" list);
System.out.println("list过滤前的数量:" list.size());
//过滤获取 type=2的数据
List<Map<String,Object>> list2 = list.stream().filter((Map a) -> ("4".equals(a.get("type").toString()))).collect(Collectors.toList());
//只获取数量也可以这样写
Long list2Count = list.stream().filter((Map a) -> ("4".equals(a.get("type").toString()))).count();
System.out.println("list过滤后的数据:" list2);
System.out.println("list过滤后的数量:" list2Count);
System.out.println("list过滤后的数量:" list2.size());
}
}运行结果：
代码语言：javascript复制list过滤前的数据:[{type=0}, {type=1}, {type=2}, {type=3}, {type=4}]
list过滤前的数量:5
list过滤后的数据:[{type=4}]
list过滤后的数量:1
list过滤后的数量:1进行数据过滤：
代码语言：javascript复制/**
*
* @description: lambda学习 使用过滤 循环
* @author: lyz
* @date: 2020/05/12 15:24
**/
public class Demo7 {
public static void main(String[] args) {
/* List<String> strArr = Arrays.asList("1", "2", "3", "4");
strArr.stream().filter(str ->{
return "2".equals(str)?true:false;
}).forEach(str ->{
System.out.println(str);
System.out.println(str 1);
});*/
List<String> strArr = Arrays.asList("1", "2", "3", "4");
strArr.stream().filter(str -> "2".equals(str)?true:false).forEach(str ->{
System.out.println(str);
System.out.println(str 1);
});
}
}运行结果：
代码语言：javascript复制2
21基于扁平流实现：
代码语言：javascript复制/**
*
* @description: lambda学习 扁平流使用
* @author: lyz
* @date: 2020/05/12 16:00
**/
public class Demo12 {
public static void main(String[] args) {
List<Student> students = new ArrayList<>(4);
students.add(new Student("百花", 22, 175));
students.add(new Student("白条", 40, 178));
students.add(new Student("略一", 40, 180));
students.add(new Student("畅享", 50, 185));
List<Student> list = students.stream()
.filter(stu -> stu.getStature() < 180)
.collect(Collectors.toList());
System.out.println(list);
List<Student> students1 = new ArrayList<>();
students1.add(new Student("拉普斯", 22, 175));
students1.add(new Student("洛夫斯基", 40, 178));
List<Student> studentList = Stream.of(students, students1)
.flatMap(stu -> stu.stream()).collect(Collectors.toList());
System.out.println(studentList);
System.out.println("==============================");
Optional<Student> max = students.stream()
.max(Comparator.comparing(stu -> stu.getAge()));
Optional<Student> min = students.stream()
.min(Comparator.comparing(stu -> stu.getAge()));
//判断是否有值
if (max.isPresent()) {
System.out.println(max.get());
}
if (min.isPresent()) {
System.out.println(min.get());
}
long count = students.stream().filter(s1 -> s1.getAge() < 45).count();
System.out.println("年龄小于42岁的人数是：" count);
}运行结果：
代码语言：javascript复制[Student{name='百花', age=22, stature=175}, Student{name='白条', age=40, stature=178}]
[Student{name='百花', age=22, stature=175}, Student{name='白条', age=40, stature=178}, Student{name='略一', age=40, stature=180}, Student{name='畅享', age=50, stature=185}, Student{name='拉普斯', age=22, stature=175}, Student{name='洛夫斯基', age=40, stature=178}]
==============================
Student{name='畅享', age=50, stature=185}
Student{name='百花', age=22, stature=175}
年龄小于42岁的人数是：3进行字符串拼接
代码语言：javascript复制/**
*
* @description: 学习java8,字符串拼接
* @author: lyz
* @date: 2020/05/12 16:23
**/
public class Demo13 {
public static void main(String[] args) {
List<Student> students = new ArrayList<>(3);
students.add(new Student("绿衣", 22, 175));
students.add(new Student("红妆", 40, 180));
students.add(new Student("水月", 50, 185));
String names = students.stream()
.map(Student::getName).collect(Collectors.joining(",","[","]"));
System.out.println(names);
}
}运行结果：
代码语言：javascript复制[绿衣,红妆,水月]进行分组操作
代码语言：javascript复制/**
*
* @description: 进行lambda学习,进行分组操作
* @author: lyz
* @date: 2020/05/12 16:43
**/
public class Demo15 {
public static void main(String[] args) {
List<User> users = Lists.newArrayList(
new User("高三1班", "stu01", "男"),
new User("高三1班", "stu02", "女"),
new User("高三2班", "stu11", "男"),
new User("高三2班", "stu12", "女"),
new User("高三3班", "stu21", "女"),
new User("高三3班", "stu22", "男"),
new User("高三3班", "stu23", "女"));
Map<String, List<User>> collect = users.stream().collect(groupingBy(User::getClassName));
System.out.println(JSON.toJSONString(collect));
System.out.println();
Map<String, Long> collect1 = users.stream().collect(groupingBy(User::getClassName, Collectors.counting()));
System.out.println(JSON.toJSONString(collect1));
System.out.println();
Map<String, Map<String, User>> collect2 = users.stream().collect(groupingBy(User::getClassName, Collectors.toMap(User::getStudentName, o -> o)));
System.out.println(JSON.toJSONString(collect2));
System.out.println();
}
private static class User {
private String className;
private String studentName;
private String sex;
//省略get/set与构造函数
}
}结果：
代码语言：javascript复制{"高三3班":[{"className":"高三3班","sex":"女","studentName":"stu21"},{"className":"高三3班","sex":"男","studentName":"stu22"},{"className":"高三3班","sex":"女","studentName":"stu23"}],"高三2班":[{"className":"高三2班","sex":"男","studentName":"stu11"},{"className":"高三2班","sex":"女","studentName":"stu12"}],"高三1班":[{"className":"高三1班","sex":"男","studentName":"stu01"},{"className":"高三1班","sex":"女","studentName":"stu02"}]}
{"高三3班":3,"高三2班":2,"高三1班":2}
{"高三3班":{"stu21":{"className":"高三3班","sex":"女","studentName":"stu21"},"stu23":{"className":"高三3班","sex":"女","studentName":"stu23"},"stu22":{"className":"高三3班","sex":"男","studentName":"stu22"}},"高三2班":{"stu12":{"className":"高三2班","sex":"女","studentName":"stu12"},"stu11":{"className":"高三2班","sex":"男","studentName":"stu11"}},"高三1班":{"stu02":{"className":"高三1班","sex":"女","studentName":"stu02"},"stu01":{"className":"高三1班","sex":"男","studentName":"stu01"}}}
