這兩種方法 groupingBy 與 partitioningBy ，可能會引起混淆， groupingBy 按給定的特定屬性對集合中的元素進行分組，採用 Function 接口，而 partitioningBy 根據不同條件，對 Stream 進行分區，傳回 Map，採用 Predicate 接口， Difference Between Collectors partitioningBy and groupingBy in Java 本篇增加了範例，並透過單元測試來驗證產出結果。
public static <T, K> Collector<T, ?, Map<K, List<T>>> groupingBy(Function<? super T, ? extends K> classifier) {
}
public static <T, K, A, D> Collector<T, ?, Map<K, D>> groupingBy(Function<? super T, ? extends K> classifier,
Collector<? super T, A, D> downstream) {
}
public static <T, K, D, A, M extends Map<K, D>> Collector<T, ?, M> groupingBy(
Function<? super T, ? extends K> classifier, Supplier<M> mapFactory,
Collector<? super T, A, D> downstream) {
}public static <T> Collector<T, ?, Map<Boolean, List<T>>> partitioningBy(Predicate<? super T> predicate) {
}
public static <T, D, A> Collector<T, ?, Map<Boolean, D>> partitioningBy(Predicate<? super T> predicate,
Collector<? super T, A, D> downstream) {
}檔案目錄
./
+- src
+- test
| +- org
| +- ruoxue
| +- java_147
| +- collectors
| +- DifferenceCollectorsGroupingByPartitioningByTest.java 單元測試
Difference Between Collectors groupingBy and partitioningBy 提供 groupingBy 、 partitioningBy 等操作。
groupingBy
Difference Between Collectors groupingBy and partitioningBy 建立一個 List ，增加五個元素，根據元素屬性進行分組。
@Test
public void groupingBy() {
List<Integer> list = Arrays.asList(1, 2, 3, 4, 5);
Map<Integer, List<Integer>> result = list.stream().collect(Collectors.groupingBy(e -> e % 3));
System.out.println(result);
assertEquals(3, result.size());
}{0=[3], 1=[1, 4], 2=[2, 5]}partitioningBy
Difference Between Collectors groupingBy and partitioningBy 建立一個 List ，增加五個元素，根據指定條件進行分區。
@Test
public void partitioningBy() {
List<Integer> list = Arrays.asList(1, 2, 3, 4, 5);
Map<Boolean, List<Integer>> result = list.stream().collect(Collectors.partitioningBy(e -> e > 3));
System.out.println(result);
assertEquals(3, result.get(Boolean.FALSE).size());
assertEquals(2, result.get(Boolean.TRUE).size());
}{false=[1, 2, 3], true=[4, 5]}groupingByWithMapping
Difference Between Collectors groupingBy and partitioningBy 建立一個 List ，增加五個元素，根據屬性對元素分組，再轉換成元素屬性，傳回 Map。
@Test
public void groupingByWithMapping() {
List<String> list = Arrays.asList("Blueberry", "Melon", "Fig", "Guava", "Kiwifruit");
Map<Integer, Long> result = list.stream().collect(
Collectors.groupingBy(String::length, Collectors.mapping(Function.identity(), Collectors.counting())));
System.out.println(result);
assertEquals(3, result.size());
}
{3=1, 5=2, 9=2}partitioningByWithMapping
Difference Between Collectors groupingBy and partitioningBy 建立一個 List ，增加五個元素，根據指定條件進行分區，再轉換成元素屬性，傳回 Map 。
@Test
public void partitioningByWithMapping() {
List<String> list = Arrays.asList("Blueberry", "Melon", "Fig", "Guava", "Kiwifruit");
Map<Boolean, List<String>> result = list.stream().collect(Collectors.partitioningBy(e -> e.length() > 3,
Collectors.mapping(e -> e.toUpperCase(), Collectors.toList())));
System.out.println(result);
assertEquals(1, result.get(Boolean.FALSE).size());
assertEquals(4, result.get(Boolean.TRUE).size());
}{false=[FIG], true=[BLUEBERRY, MELON, GUAVA, KIWIFRUIT]}groupingByWithSupplier
Difference Between Collectors partitioningBy and groupingBy 建立一個 List ，增加五個元素，根據元素屬性進行分組，轉換成元素屬性。
@Test
public void groupingByWithSupplier() {
List<String> list = Arrays.asList("Blueberry", "Melon", "Fig", "Guava", "Kiwifruit");
Map<Integer, List<String>> result = list.stream().collect(Collectors.groupingBy(String::length, LinkedMap::new,
Collectors.mapping(String::toUpperCase, Collectors.toList())));
System.out.println(result);
assertEquals(1, result.get(3).size());
assertEquals(2, result.get(5).size());
assertEquals(2, result.get(9).size());
}{9=[BLUEBERRY, KIWIFRUIT], 5=[MELON, GUAVA], 3=[FIG]}比較結果
| 方法 | 參數 |
|---|---|
| groupingBy | Function |
| partitioningBy | Predicate |
DifferenceCollectorsGroupingByPartitioningByTest.java
Difference Between Collectors partitioningBy and groupingBy 新增單元測試，驗證是否符合預期。
package org.ruoxue.java_147.collector;
import static org.junit.Assert.*;
import java.util.Arrays;
import java.util.List;
import java.util.Map;
import java.util.function.Function;
import java.util.stream.Collectors;
import org.apache.commons.collections4.map.LinkedMap;
import org.junit.Test;
public class DifferenceCollectorsGroupingByPartitioningByTest {
@Test
public void groupingBy() {
List<Integer> list = Arrays.asList(1, 2, 3, 4, 5);
Map<Integer, List<Integer>> result = list.stream().collect(Collectors.groupingBy(e -> e % 3));
System.out.println(result);
assertEquals(3, result.size());
}
@Test
public void partitioningBy() {
List<Integer> list = Arrays.asList(1, 2, 3, 4, 5);
Map<Boolean, List<Integer>> result = list.stream().collect(Collectors.partitioningBy(e -> e > 3));
System.out.println(result);
assertEquals(3, result.get(Boolean.FALSE).size());
assertEquals(2, result.get(Boolean.TRUE).size());
}
@Test
public void groupingByWithMapping() {
List<String> list = Arrays.asList("Blueberry", "Melon", "Fig", "Guava", "Kiwifruit");
Map<Integer, Long> result = list.stream().collect(
Collectors.groupingBy(String::length, Collectors.mapping(Function.identity(), Collectors.counting())));
System.out.println(result);
assertEquals(3, result.size());
}
@Test
public void partitioningByWithMapping() {
List<String> list = Arrays.asList("Blueberry", "Melon", "Fig", "Guava", "Kiwifruit");
Map<Boolean, List<String>> result = list.stream().collect(Collectors.partitioningBy(e -> e.length() > 3,
Collectors.mapping(e -> e.toUpperCase(), Collectors.toList())));
System.out.println(result);
assertEquals(1, result.get(Boolean.FALSE).size());
assertEquals(4, result.get(Boolean.TRUE).size());
}
@Test
public void groupingByWithSupplier() {
List<String> list = Arrays.asList("Blueberry", "Melon", "Fig", "Guava", "Kiwifruit");
Map<Integer, List<String>> result = list.stream().collect(Collectors.groupingBy(String::length, LinkedMap::new,
Collectors.mapping(String::toUpperCase, Collectors.toList())));
System.out.println(result);
assertEquals(1, result.get(3).size());
assertEquals(2, result.get(5).size());
assertEquals(2, result.get(9).size());
}
}
心得分享
Difference Between Collectors groupingBy and partitioningBy Methods in Java 因為 partitioningBy 使用 Predicate 接口，接受傳入參數 T ，執行 test 方法後傳回 boolean ，同樣地對 groupingBy 而言，卻是使用 Function 接口，接受傳入參數 T ，執行 apply 方法後傳回 R ， 在這兩種情況下， test 和 apply 方法都是提供 lambda 表達式， Difference Between Collectors partitioningBy and groupingBy 提供這兩個方法的區別與測試。
