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zeusro/Main.java

Created March 9, 2018 09:41
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java8-stream
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Main.java
import com.google.gson.Gson;
import com.zeusro.Person;
import com.zeusro.Sex;
import java.time.LocalDate;
import java.time.ZoneId;
import java.util.*;
import java.util.stream.Collectors;
import java.util.stream.Stream;
import static java.lang.System.out;
/**
* The type Main <p/>
* author: <a href="http://zeusro.github.io/">Zeusro</a> <p/>
*
* @date Created in 2018.03.09 <p/>
*/
public class Main {
private static Date convertLocalDateToTimeZone(LocalDate localDate) {
ZoneId zoneID = ZoneId.systemDefault();
return Date.from(localDate.atStartOfDay(zoneID).toInstant());
}
public static void main(String[] args) {
try {
// LocalDate.of(2018, 02, 29);
//java.time.DateTimeException: Invalid date 'February 29' as '2018' is not a leap year
final Person female = new Person();
female.setBirthday(convertLocalDateToTimeZone(LocalDate.of(1981, 1, 1)));
female.setHeight(165);
female.setWeight(50);
female.setSex(Sex.Female);
female.setAddress("北京");
female.setIdentifier("1");
female.setHobbies(new ArrayList<String>() {{
add("吃飯");
add("逛街");
}});
final Person male = new Person();
male.setBirthday(convertLocalDateToTimeZone(LocalDate.of(1982, 2, 1)));
male.setHeight(170);
male.setWeight(50);
male.setSex(Sex.Male);
male.setAddress("北京");
male.setIdentifier("2");
male.setHobbies(new ArrayList<String>() {{
add("吃飯");
add("看電影");
}});
final Person x = new Person();
x.setBirthday(convertLocalDateToTimeZone(LocalDate.of(1983, 3, 1)));
x.setHeight(170);
x.setWeight(50);
x.setSex(Sex.X);
x.setAddress("北京");
x.setIdentifier("3");
x.setHobbies(new ArrayList<String>() {{
add("吃飯");
add("上網");
}});
final Person male2 = new Person();
male2.setBirthday(convertLocalDateToTimeZone(LocalDate.of(1984, 1, 1)));
male2.setHeight(150);
male2.setWeight(35);
male2.setSex(Sex.Male);
male2.setAddress("北京");
male2.setIdentifier("4");
male2.setHobbies(new ArrayList<String>() {{
add("吃飯");
add("看電影");
}});
List<Person> list1 = new ArrayList<Person>() {
{
add(female);
add(male);
add(x);
}
};
List<Person> list2 = new ArrayList<Person>() {
{
add(female);
add(male2);
}
};
//流只能用一次,重复使用会导致以下异常
//java.lang.IllegalStateException: stream has already been operated upon or closed
Stream<Person> list1Stream = list1.stream();
Date time1 = convertLocalDateToTimeZone(LocalDate.of(1990, 1, 1));
//0
Long count1 = list1.stream().filter(o -> o.getBirthday().equals(time1)).count();
Map<Sex, List<Person>> group1 = list1.stream().collect(Collectors.groupingBy(Person::getSex));
Iterator it = group1.entrySet().iterator();
while (it.hasNext()) {
Map.Entry<Sex, List<Person>> groupByItem = (Map.Entry) it.next();
Sex sex = groupByItem.getKey();
out.println(sex);
groupByItem.getValue().forEach(person -> {
out.println(new Gson().toJson(person));
});
}
/*
输出结果:
Male
{"height":170,"weight":50,"identifier":"2","address":"北京","birthday":"Feb 1, 1982 12:00:00 AM","hobbies":["吃飯","看電影"],"sex":"Male"}
Female
{"height":165,"weight":50,"identifier":"1","address":"北京","birthday":"Jan 1, 1981 12:00:00 AM","hobbies":["吃飯","逛街"],"sex":"Female"}
X
{"height":170,"weight":50,"identifier":"3","address":"北京","birthday":"Mar 1, 1983 12:00:00 AM","hobbies":["吃飯","上網"],"sex":"X"}
*/
//stream没有RemoveAll的操作
Person after90 = list1.stream()
.filter(o -> o.getBirthday().after(convertLocalDateToTimeZone(LocalDate.of(1990, 1, 1))))
.findFirst()
.orElse(null);
// null
list1.stream().forEach(o -> {
//在ForEach當中可對集合進行操作
o.setSex(Sex.X);
});
list1.forEach(o -> {
out.println(new Gson().toJson(o));
});
/*
{"height":165,"weight":50,"identifier":"1","address":"北京","birthday":"Jan 1, 1981 12:00:00 AM","hobbies":["吃飯","逛街"],"sex":"X"}
{"height":170,"weight":50,"identifier":"2","address":"北京","birthday":"Feb 1, 1982 12:00:00 AM","hobbies":["吃飯","看電影"],"sex":"X"}
{"height":170,"weight":50,"identifier":"3","address":"北京","birthday":"Mar 1, 1983 12:00:00 AM","hobbies":["吃飯","上網"],"sex":"X"}
*/
//IntStream的max方法返回的是OptionalInt,要先判断有没有值再读取值.isPresent=false 时直接getAsInt会报错.mapToLong,mapToDouble同理
OptionalInt maxHeightOption = list1.stream().mapToInt(Person::getHeight).max();
//字符串拼接、数值的 sum、min、max、average 都是特殊的 reduce。
//当集合为长度0的集合时会返回起始值Integer.MIN_VALUE,起始值也不能乱传,个中缘由我暂不清楚
int maxHeight = list1.stream().mapToInt(Person::getHeight).reduce(Integer.MIN_VALUE, Integer::max);
out.println(maxHeight);
//170
if (maxHeightOption.isPresent()) {
maxHeight = maxHeightOption.getAsInt();
out.println(maxHeight);
//170
}
//mapToInt参数的2种写法都一样,我比较喜欢以下写法,但是 idea 会报 warning
OptionalInt minWeightOption = list1.stream().mapToInt(o -> o.getHeight()).min();
int minWeight = list1.stream().mapToInt(o -> o.getHeight()).reduce(Integer.MAX_VALUE, Integer::min);
list1.stream().map(Person::getIdentifier).distinct();
//skip和 limit参数都是long, 这个要注意
list1.stream().skip(1L).limit(2L);
out.println("------------------------------------|升序|------------------------------------");
list1 = list1.stream().sorted(Comparator.comparing(Person::getBirthday)).collect(Collectors.toList());
out.println(new Gson().toJson(list1));
list1 = list1.stream().sorted((left, right) -> left.getBirthday().compareTo(right.getBirthday())).collect(Collectors.toList());
out.println(new Gson().toJson(list1));
/*
[{"height":165,"weight":50,"identifier":"1","address":"北京","birthday":"Jan 1, 1981 12:00:00 AM","hobbies":["吃飯","逛街"],"sex":"X"},{"height":170,"weight":50,"identifier":"2","address":"北京","birthday":"Feb 1, 1982 12:00:00 AM","hobbies":["吃飯","看電影"],"sex":"X"},{"height":170,"weight":50,"identifier":"3","address":"北京","birthday":"Mar 1, 1983 12:00:00 AM","hobbies":["吃飯","上網"],"sex":"X"}]
[{"height":165,"weight":50,"identifier":"1","address":"北京","birthday":"Jan 1, 1981 12:00:00 AM","hobbies":["吃飯","逛街"],"sex":"X"},{"height":170,"weight":50,"identifier":"2","address":"北京","birthday":"Feb 1, 1982 12:00:00 AM","hobbies":["吃飯","看電影"],"sex":"X"},{"height":170,"weight":50,"identifier":"3","address":"北京","birthday":"Mar 1, 1983 12:00:00 AM","hobbies":["吃飯","上網"],"sex":"X"}]
*/
out.println("------------------------------------|降序|------------------------------------");
list1 = list1.stream().sorted(Comparator.comparing(Person::getBirthday).reversed()).collect(Collectors.toList());
out.println(new Gson().toJson(list1));
list1 = list1.stream().sorted((left, right) -> right.getBirthday().compareTo(left.getBirthday())).collect(Collectors.toList());
out.println(new Gson().toJson(list1));
/*
[{"height":170,"weight":50,"identifier":"3","address":"北京","birthday":"Mar 1, 1983 12:00:00 AM","hobbies":["吃飯","上網"],"sex":"X"},{"height":170,"weight":50,"identifier":"2","address":"北京","birthday":"Feb 1, 1982 12:00:00 AM","hobbies":["吃飯","看電影"],"sex":"X"},{"height":165,"weight":50,"identifier":"1","address":"北京","birthday":"Jan 1, 1981 12:00:00 AM","hobbies":["吃飯","逛街"],"sex":"X"}]
[{"height":170,"weight":50,"identifier":"3","address":"北京","birthday":"Mar 1, 1983 12:00:00 AM","hobbies":["吃飯","上網"],"sex":"X"},{"height":170,"weight":50,"identifier":"2","address":"北京","birthday":"Feb 1, 1982 12:00:00 AM","hobbies":["吃飯","看電影"],"sex":"X"},{"height":165,"weight":50,"identifier":"1","address":"北京","birthday":"Jan 1, 1981 12:00:00 AM","hobbies":["吃飯","逛街"],"sex":"X"}]
*/
out.println("------------------------------------|交集 list1 ∩ list2|------------------------------------");
list1.stream().filter(o -> list2.contains(o)).collect(Collectors.toList());
out.println("------------------------------------|并集list1 ∪ list2 |------------------------------------");
list1.addAll(list2);
list1.stream().distinct().collect(Collectors.toList());
out.println("------------------------------------|差集list1 - list2|------------------------------------");
list1.stream().filter(item1 -> !list2.contains(item1)).collect(Collectors.toList());
out.println("------------------------------------|数据结构转换|------------------------------------");
List<Person> list3 = list1.stream().filter(o -> true).collect(Collectors.toList());
ArrayList<Person> list4 = list1.stream().filter(o -> true).collect(Collectors.toCollection(ArrayList::new));
Set<Person> list5 = list1.stream().filter(o -> true).collect(Collectors.toSet());
Object[] list6 = list1.stream().toArray();
Person[] list7 = list1.stream().toArray(Person[]::new);
out.println("------------------------------------|其他需要注意的地方|------------------------------------");
//2个数组合并用这种方法的话, list2会为空
Stream.of(list1, list2).collect(Collectors.toList());
//以下才是正确用法
Stream.of(list1, list2).flatMap(List::stream).collect(Collectors.toList());
out.println("------------------------------------|reduce|------------------------------------");
//字符串拼接、数值的 sum、min、max、average 都是特殊的 reduce。
maxHeight = list1.stream().mapToInt(Person::getHeight).reduce(Integer.MIN_VALUE, Integer::max);
minWeight = list1.stream().mapToInt(o -> o.getHeight()).reduce(Integer.MAX_VALUE, Integer::min);
int sumWeight = -1;
OptionalInt sumWeightOption = list1.stream().mapToInt(Person::getHeight).reduce(Integer::sum);
if (sumWeightOption.isPresent()) {
sumWeight = sumWeightOption.getAsInt();
}
sumWeight = list1.stream().mapToInt(Person::getHeight).reduce(0, (a, b) -> a + b);
sumWeight = list1.stream().mapToInt(Person::getHeight).reduce(0, Integer::sum);
out.println("------------------------------------|peek ,forEach ,forEachOrdered |------------------------------------");
out.println("forEach后面无法继续执行方法");
list1.stream().forEach(o -> {
out.println(new Gson().toJson(o));
});
/*
forEach后面无法继续执行方法
{"height":170,"weight":50,"identifier":"3","address":"北京","birthday":"Mar 1, 1983 12:00:00 AM","hobbies":["吃飯","上網"],"sex":"X"}
{"height":170,"weight":50,"identifier":"2","address":"北京","birthday":"Feb 1, 1982 12:00:00 AM","hobbies":["吃飯","看電影"],"sex":"X"}
{"height":165,"weight":50,"identifier":"1","address":"北京","birthday":"Jan 1, 1981 12:00:00 AM","hobbies":["吃飯","逛街"],"sex":"X"}
{"height":165,"weight":50,"identifier":"1","address":"北京","birthday":"Jan 1, 1981 12:00:00 AM","hobbies":["吃飯","逛街"],"sex":"X"}
{"height":150,"weight":35,"identifier":"4","address":"北京","birthday":"Jan 1, 1984 12:00:00 AM","hobbies":["吃飯","看電影"],"sex":"Male"}
*/
out.println("先排序,然后遍历");
list1.stream().forEachOrdered(o -> {
out.println(new Gson().toJson(o));
});
/*
先排序,然后遍历
{"height":170,"weight":50,"identifier":"3","address":"北京","birthday":"Mar 1, 1983 12:00:00 AM","hobbies":["吃飯","上網"],"sex":"X"}
{"height":170,"weight":50,"identifier":"2","address":"北京","birthday":"Feb 1, 1982 12:00:00 AM","hobbies":["吃飯","看電影"],"sex":"X"}
{"height":165,"weight":50,"identifier":"1","address":"北京","birthday":"Jan 1, 1981 12:00:00 AM","hobbies":["吃飯","逛街"],"sex":"X"}
{"height":165,"weight":50,"identifier":"1","address":"北京","birthday":"Jan 1, 1981 12:00:00 AM","hobbies":["吃飯","逛街"],"sex":"X"}
{"height":150,"weight":35,"identifier":"4","address":"北京","birthday":"Jan 1, 1984 12:00:00 AM","hobbies":["吃飯","看電影"],"sex":"Male"}
*/
out.println("peek*2");
List<Integer> l = Stream.iterate(0, (Integer n) -> n + 1) //生成一个每次递增1的等差数列
.peek(n -> out.println("number generated:" + n))
.filter(n -> (n % 2 == 0))
.peek(n -> out.println("Even number filter passed for" + n)) //遇到偶数时继续输出
.limit(5).collect(Collectors.toList());
out.println(new Gson().toJson(l));
//可以看到在生成到9时,这个操作结束了,是因为设置了limit
/*
peek*2
number generated:0
Even number filter passed for0
number generated:1
number generated:2
Even number filter passed for2
number generated:3
number generated:4
Even number filter passed for4
number generated:5
number generated:6
Even number filter passed for6
number generated:7
number generated:8
Even number filter passed for8
[0,2,4,6,8]
*/
} catch (Exception e) {
out.println(e);
} finally {
out.println("done");
}
}
}
Raw
Person.java
package com.zeusro;
import java.util.Date;
import java.util.List;
/**
* The type Person <p/>
* author: <a href="http://zeusro.github.io/">Zeusro</a> <p/>
*
* @date Created in 2018.03.08 <p/>
*/
public class Person implements Cloneable {
// 身高
private int height;
//体重-
private int weight;
//身份证号
private String identifier;
//地址
private String address;
//生日
private Date birthday;
//爱好
private List<String> hobbies;
//性别
private Sex sex;
@Override
protected Object clone() throws CloneNotSupportedException {
return (Person) super.clone();
}
public Sex getSex() {
return sex;
}
public void setSex(Sex sex) {
this.sex = sex;
}
public String getAddress() {
return address;
}
public void setAddress(String address) {
this.address = address;
}
public Date getBirthday() {
return birthday;
}
public void setBirthday(Date birthday) {
this.birthday = birthday;
}
public int getHeight() {
return height;
}
public void setHeight(int height) {
this.height = height;
}
public List<String> getHobbies() {
return hobbies;
}
public void setHobbies(List<String> hobbies) {
this.hobbies = hobbies;
}
public String getIdentifier() {
return identifier;
}
public void setIdentifier(String identifier) {
this.identifier = identifier;
}
public int getWeight() {
return weight;
}
public void setWeight(int weight) {
this.weight = weight;
}
}
Raw
Sex.java
package com.zeusro;
public enum Sex {
//男
Male,
//女
Female,
//第三性 https://zh.wikipedia.org/wiki/%E7%AC%AC%E4%B8%89%E6%80%A7
X,
}
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