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封装集合的stream操作

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zhangtao 2020-11-17 17:19:36 +08:00
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202
hutool-core/src/main/java/cn/hutool/core/collection/CollectionStream.java Normal file
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package cn.hutool.core.collection;
import cn.hutool.core.map.MapUtil;
import java.util.*;
import java.util.function.BiFunction;
import java.util.function.Function;
import java.util.stream.Collectors;
/**
* 常用stream操作集合<br></br>
* 做这个集合的封装的原因在于list.stream后续的写法虽然简便，但是别人看不易不懂，甚至自己写了两周后都看不懂,维护成本高<br></br>
* 于是笔者采用见名知意的方法封装，降低阅读难度，减轻了维护成本，使用者只需要提供一些简单的lambda表达式<br></br>
* 当然本人才疏学浅，名字未必能够做到信雅达，欢迎各位大佬指教<br></br>
* 另外，CollectionStream封装的都是一些常用的stream方法，当然考虑到一些方法不常用(比如map->list)，并未提交，如果有一些好的意见，欢迎联系本人528910437@QQ.COM
*
* @version 1.0
*/
public class CollectionStream {
/**
* 将collection转化为类型不变的map
* <br></br>
* <B>{@code Collection<E> ----> Map<T,E>}</B>
*
* @param collection 需要转化的集合
* @param key E类型转化为T类型的lambda方法
* @param <E> collection中的泛型
* @param <T> map中的key类型
* @return 转化后的map
*/
public static <E, T> Map<T, E> toIdentityMap(Collection<E> collection, Function<E, T> key) {
if (CollUtil.isEmpty(collection)) {
return Collections.EMPTY_MAP;
}
return collection.stream().collect(Collectors.toMap(key, Function.identity()));
}
/**
* E
* 将collection转化为map(value类型与collection的泛型不同)
* <br></br>
* <B>{@code Collection<E> -----> Map<T,U> }</B>
*
* @param collection 需要转化的集合
* @param key E类型转化为T类型的lambda方法
* @param value E类型转化为U类型的lambda方法
* @param <E> collection中的泛型
* @param <T> map中的key类型
* @param <U> map中的value类型
* @return 转化后的map
*/
public static <E, T, U> Map<T, U> toMap(Collection<E> collection, Function<E, T> key, Function<E, U> value) {
if (CollUtil.isEmpty(collection)) {
return Collections.EMPTY_MAP;
}
return collection.stream().collect(Collectors.toMap(key, value));
}
/**
* 将collection按照规则(比如有相同的班级id)分类成map
* <br></br>
* <B>{@code Collection<E> -------> Map<T,List<E>> } </B>
*
* @param collection 需要分类的集合
* @param key 分类的规则
* @param <E> collection中的泛型
* @param <T> map中的key类型
* @return 分类后的map
*/
public static <E, T> Map<T, List<E>> groupByKey(Collection<E> collection, Function<E, T> key) {
if (CollUtil.isEmpty(collection)) {
return Collections.EMPTY_MAP;
}
return collection
.stream()
.collect(Collectors.groupingBy(key, Collectors.toList()));
}
/**
* 将collection按照两个规则(比如有相同的年级id,班级id)分类成双层map
* <br></br>
* <B>{@code Collection<E> ---> Map<T,Map<U,List<E>>> } </B>
*
* @param collection 需要分类的集合
* @param key1 第一个分类的规则
* @param key2 第二个分类的规则
* @param <E> 第一个map中的key类型
* @param <T> 第二个map中的key类型
* @return 分类后的map
*/
public static <E, T, U> Map<T, Map<U, List<E>>> groupBy2Key(Collection<E> collection, Function<E, T> key1, Function<E, U> key2) {
if (CollUtil.isEmpty(collection)) {
return Collections.EMPTY_MAP;
}
return collection
.stream()
.collect(Collectors.groupingBy(key1, Collectors.groupingBy(key2, Collectors.toList())));
}
/**
* 将collection按照两个规则(比如有相同的年级id,班级id)分类成双层map
* <br></br>
* <B>{@code Collection<E> ---> Map<T,Map<U,E>> } </B>
*
* @param collection 需要分类的集合
* @param key1 第一个分类的规则
* @param key2 第二个分类的规则
* @param <T> 第一个map中的key类型
* @param <U> 第二个map中的key类型
* @param <E> collection中的泛型
* @return 分类后的map
*/
public static <E, T, U> Map<T, Map<U, E>> group2Map(Collection<E> collection, Function<E, T> key1, Function<E, U> key2) {
if (CollUtil.isEmpty(collection) || key1 == null || key2 == null) {
return Collections.EMPTY_MAP;
}
return collection
.stream()
.collect(Collectors.groupingBy(key1, Collectors.toMap(key2, Function.identity())));
}
/**
* 将collection转化为List集合，但是两者的泛型不同
* <br></br>
* <B>{@code Collection<E> ------> List<T> } </B>
*
* @param collection 需要转化的集合
* @param function collection中的泛型转化为list泛型的lambda表达式
* @param <E> collection中的泛型
* @param <T> List中的泛型
* @return 转化后的list
*/
public static <E, T> List<T> translate2List(Collection<E> collection, Function<E, T> function) {
if (CollUtil.isEmpty(collection)) {
return Collections.EMPTY_LIST;
}
return collection
.stream()
.map(function)
.filter(Objects::nonNull)
.collect(Collectors.toList());
}
/**
* 将collection转化为Set集合，但是两者的泛型不同
* <br></br>
* <B>{@code Collection<E> ------> Set<T> } </B>
*
* @param collection 需要转化的集合
* @param function collection中的泛型转化为set泛型的lambda表达式
* @param <E> collection中的泛型
* @param <T> Set中的泛型
* @return 转化后的Set
*/
public static <E, T> Set<T> translate2Set(Collection<E> collection, Function<E, T> function) {
if (CollUtil.isEmpty(collection) || function == null) {
return Collections.EMPTY_SET;
}
return collection
.stream()
.map(function)
.filter(Objects::nonNull)
.collect(Collectors.toSet());
}
/**
* 合并两个相同key类型的map
*
* @param map1 第一个需要合并的 map
* @param map2 第二个需要合并的 map
* @param merge 合并的lambda，将key value1 value2合并成最终的类型,注意value可能为空的情况
* @param <T> map中的key类型
* @param <X> 第一个 map的value类型
* @param <Y> 第二个 map的value类型
* @param <Z> 最终map的value类型
* @return 合并后的map
*/
public static <T, X, Y, Z> Map<T, Z> merge(Map<T, X> map1, Map<T, Y> map2, BiFunction<X, Y, Z> merge) {
if (MapUtil.isEmpty(map1) && MapUtil.isEmpty(map2)) {
return Collections.EMPTY_MAP;
} else if (MapUtil.isEmpty(map1)) {
map1 = Collections.EMPTY_MAP;
} else if (MapUtil.isEmpty(map2)) {
map2 = Collections.EMPTY_MAP;
}
Set<T> key = new HashSet<>();
key.addAll(map1.keySet());
key.addAll(map2.keySet());
Map<T, Z> map = new HashMap<>();
for (T t : key) {
X x = map1.get(t);
Y y = map2.get(t);
Z z = merge.apply(x, y);
if (z != null) {
map.put(t, z);
}
}
return map;
}
}

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hutool-core/src/test/java/cn/hutool/core/collection/CollectionStreamTest.java Normal file
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package cn.hutool.core.collection;
import org.junit.Assert;
import org.junit.Test;
import java.util.*;
/**
* CollectionStream测试方法
*/
public class CollectionStreamTest {
@Test
public void testToIdentityMap() {
Map<Long, Student> map = CollectionStream.toIdentityMap(null, Student::getStudentId);
Assert.assertEquals(map, Collections.EMPTY_MAP);
List<Student> list = new ArrayList<>();
map = CollectionStream.toIdentityMap(list, Student::getStudentId);
Assert.assertEquals(map, Collections.EMPTY_MAP);
list.add(new Student(1, 1, 1, "张三"));
list.add(new Student(1, 1, 2, "李四"));
list.add(new Student(1, 1, 3, "王五"));
map = CollectionStream.toIdentityMap(list, Student::getStudentId);
Assert.assertEquals(map.get(1L).getName(), "张三");
Assert.assertEquals(map.get(2L).getName(), "李四");
Assert.assertEquals(map.get(3L).getName(), "王五");
Assert.assertEquals(map.get(4L), null);
}
@Test
public void testToMap() {
Map<Long, String> map = CollectionStream.toMap(null, Student::getStudentId, Student::getName);
Assert.assertEquals(map, Collections.EMPTY_MAP);
List<Student> list = new ArrayList<>();
map = CollectionStream.toMap(list, Student::getStudentId, Student::getName);
Assert.assertEquals(map, Collections.EMPTY_MAP);
list.add(new Student(1, 1, 1, "张三"));
list.add(new Student(1, 1, 2, "李四"));
list.add(new Student(1, 1, 3, "王五"));
map = CollectionStream.toMap(list, Student::getStudentId, Student::getName);
Assert.assertEquals(map.get(1L), "张三");
Assert.assertEquals(map.get(2L), "李四");
Assert.assertEquals(map.get(3L), "王五");
Assert.assertEquals(map.get(4L), null);
}
@Test
public void testGroupByKey() {
Map<Long, List<Student>> map = CollectionStream.groupByKey(null, Student::getClassId);
Assert.assertEquals(map, Collections.EMPTY_MAP);
List<Student> list = new ArrayList<>();
map = CollectionStream.groupByKey(list, Student::getClassId);
Assert.assertEquals(map, Collections.EMPTY_MAP);
list.add(new Student(1, 1, 1, "张三"));
list.add(new Student(1, 2, 2, "李四"));
list.add(new Student(2, 1, 1, "擎天柱"));
list.add(new Student(2, 2, 2, "威震天"));
list.add(new Student(2, 3, 2, "霸天虎"));
map = CollectionStream.groupByKey(list, Student::getClassId);
Map<Long, List<Student>> compare = new HashMap<>();
List<Student> class1 = new ArrayList<>();
class1.add(new Student(1, 1, 1, "张三"));
class1.add(new Student(2, 1, 1, "擎天柱"));
compare.put(1L, class1);
List<Student> class2 = new ArrayList<>();
class2.add(new Student(1, 2, 2, "李四"));
class2.add(new Student(2, 2, 2, "威震天"));
compare.put(2L, class2);
List<Student> class3 = new ArrayList<>();
class3.add(new Student(2, 3, 2, "霸天虎"));
compare.put(3L, class3);
Assert.assertEquals(true, map.equals(compare));
}
public void testGroupBy2Key() {
Map<Long, Map<Long, List<Student>>> map = CollectionStream.groupBy2Key(null, Student::getTermId, Student::getClassId);
Assert.assertEquals(map, Collections.EMPTY_MAP);
List<Student> list = new ArrayList<>();
map = CollectionStream.groupBy2Key(list, Student::getTermId, Student::getClassId);
Assert.assertEquals(map, Collections.EMPTY_MAP);
list.add(new Student(1, 1, 1, "张三"));
list.add(new Student(1, 2, 2, "李四"));
list.add(new Student(1, 2, 3, "王五"));
list.add(new Student(2, 1, 1, "擎天柱"));
list.add(new Student(2, 2, 2, "威震天"));
list.add(new Student(2, 2, 3, "霸天虎"));
map = CollectionStream.groupBy2Key(list, Student::getTermId, Student::getClassId);
Map<Long, Map<Long, List<Student>>> compare = new HashMap<>();
Map<Long, List<Student>> map1 = new HashMap<>();
List<Student> list11 = new ArrayList<>();
list11.add(new Student(1, 1, 1, "张三"));
map1.put(1L, list11);
compare.put(1L, map1);
List<Student> list12 = new ArrayList<>();
list12.add(new Student(1, 2, 2, "李四"));
list12.add(new Student(1, 2, 3, "王五"));
map1.put(2L, list12);
compare.put(2L, map1);
Map<Long, List<Student>> map2 = new HashMap<>();
List<Student> list21 = new ArrayList<>();
list21.add(new Student(2, 1, 1, "擎天柱"));
map2.put(1L, list21);
compare.put(2L, map2);
List<Student> list22 = new ArrayList<>();
list22.add(new Student(2, 2, 2, "威震天"));
list22.add(new Student(2, 2, 3, "霸天虎"));
map2.put(2L, list22);
compare.put(2L, map2);
Assert.assertEquals(true, map.equals(compare));
}
@Test
public void testGroup2Map() {
List<Student> list = null;
Map<Long, Map<Long, Student>> map = CollectionStream.group2Map(list, Student::getTermId, Student::getClassId);
Assert.assertEquals(map, Collections.EMPTY_MAP);
list = new ArrayList<>();
map = CollectionStream.group2Map(list, Student::getTermId, Student::getClassId);
Assert.assertEquals(map, Collections.EMPTY_MAP);
list.add(new Student(1, 1, 1, "张三"));
list.add(new Student(1, 2, 1, "李四"));
list.add(new Student(2, 2, 1, "王五"));
map = CollectionStream.group2Map(list, Student::getTermId, Student::getClassId);
Map<Long, Map<Long, Student>> compare = new HashMap<>();
Map<Long, Student> map1 = new HashMap<>();
map1.put(1L, new Student(1, 1, 1, "张三"));
map1.put(2L, new Student(1, 2, 1, "李四"));
compare.put(1L, map1);
Map<Long, Student> map2 = new HashMap<>();
map2.put(2L, new Student(2, 2, 1, "王五"));
compare.put(2L, map2);
Assert.assertEquals(true,compare.equals(map));
}
@Test
public void testTranslate2List() {
List<String> list = CollectionStream.translate2List(null, Student::getName);
Assert.assertEquals(list, Collections.EMPTY_LIST);
List<Student> students = new ArrayList<>();
list = CollectionStream.translate2List(students, Student::getName);
Assert.assertEquals(list, Collections.EMPTY_LIST);
students.add(new Student(1, 1, 1, "张三"));
students.add(new Student(1, 2, 2, "李四"));
students.add(new Student(2, 1, 1, "李四"));
students.add(new Student(2, 2, 2, "李四"));
students.add(new Student(2, 3, 2, "霸天虎"));
list = CollectionStream.translate2List(students, Student::getName);
List<String> compare = new ArrayList<>();
compare.add("张三");
compare.add("李四");
compare.add("李四");
compare.add("李四");
compare.add("霸天虎");
Assert.assertEquals(true, list.equals(compare));
}
@Test
public void testTranslate2Set() {
Set<String> set = CollectionStream.translate2Set(null, Student::getName);
Assert.assertEquals(set, Collections.EMPTY_SET);
List<Student> students = new ArrayList<>();
set = CollectionStream.translate2Set(students, Student::getName);
Assert.assertEquals(set, Collections.EMPTY_SET);
students.add(new Student(1, 1, 1, "张三"));
students.add(new Student(1, 2, 2, "李四"));
students.add(new Student(2, 1, 1, "李四"));
students.add(new Student(2, 2, 2, "李四"));
students.add(new Student(2, 3, 2, "霸天虎"));
set = CollectionStream.translate2Set(students, Student::getName);
Set<String> compare = new HashSet<>();
compare.add("张三");
compare.add("李四");
compare.add("李四");
compare.add("李四");
compare.add("霸天虎");
Assert.assertEquals(true, set.equals(compare));
}
@Test
public void testMerge() {
Map<Long, Student> map1 = null;
Map<Long, Student> map2 = Collections.EMPTY_MAP;
Map<Long, String> map = CollectionStream.merge(map1, map2, (s1, s2) -> s1.getName() + s2.getName());
Assert.assertEquals(map, Collections.EMPTY_MAP);
map1 = new HashMap<>();
map1.put(1L, new Student(1, 1, 1, "张三"));
map = CollectionStream.merge(map1, map2, this::merge);
Map<Long, String> temp = new HashMap<>();
temp.put(1L, "张三");
Assert.assertEquals(map, temp);
map2 = new HashMap<>();
map2.put(1L, new Student(2, 1, 1, "李四"));
map = CollectionStream.merge(map1, map2, this::merge);
Map<Long, String> compare = new HashMap<>();
compare.put(1L, "张三李四");
Assert.assertEquals(true, map.equals(compare));
}
public String merge(Student student1, Student student2) {
if (student1 == null && student2 == null) {
return null;
} else if (student1 == null) {
return student2.getName();
} else if (student2 == null) {
return student1.getName();
} else {
return student1.getName() + student2.getName();
}
}
/**
* 班级类
*/
public static class Student {
private long termId;//学期id
private long classId;//班级id
private long studentId;//班级id
private String name;//学生名称
public Student(long termId, long classId, long studentId, String name) {
this.termId = termId;
this.classId = classId;
this.studentId = studentId;
this.name = name;
}
public long getTermId() {
return termId;
}
public void setTermId(long termId) {
this.termId = termId;
}
public long getClassId() {
return classId;
}
public void setClassId(long classId) {
this.classId = classId;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public long getStudentId() {
return studentId;
}
public void setStudentId(long studentId) {
this.studentId = studentId;
}
@Override
public boolean equals(Object obj) {
return toString().equals(obj.toString());
}
@Override
public String toString() {
return "Student{" +
"termId=" + termId +
", classId=" + classId +
", studentId=" + studentId +
", name='" + name + '\'' +
'}';
}
}
}