使用所提供的靜態方法，傳入 2 個 Function 參數，設定鍵與值轉換成 Map ， ToConcurrentMap Java Collectors 介紹常見的 toConcurrentMap 等方法，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, U> Collector<T, ?, ConcurrentMap<K, U>> toConcurrentMap(
Function<? super T, ? extends K> keyMapper, Function<? super T, ? extends U> valueMapper) {
}
檔案目錄
./
+- src
+- test
| +- org
| +- ruoxue
| +- java_147
| +- collector
| +- CollectorsToConcurrentMapTest.java 單元測試
Collectors ToConcurrentMap Java 提供設定鍵與值轉換成 Map。
toConcurrentMap
Collectors ToConcurrentMap Java 建立一個 List ，增加三個元素，轉換成 Map 。
@Test
public void toConcurrentMap() {
int expectedSize = 3;
List<String> list = Arrays.asList("Blueberry", "Melon", "Fig");
Function<String, String> key = s -> s.toUpperCase();
Function<String, Integer> length = s -> s.length();
Map<String, Integer> result = list.stream().collect(Collectors.toConcurrentMap(key, length));
System.out.println(result);
assertEquals(expectedSize, result.size());
result = list.stream().collect(Collectors.toConcurrentMap(Function.identity(), String::length));
System.out.println(result);
assertEquals(expectedSize, result.size());
Function<Integer, Integer> twice = i -> i * i;
result = list.stream().collect(Collectors.toConcurrentMap(key, length.andThen(twice)));
System.out.println(result);
assertEquals(expectedSize, result.size());
}{FIG=3, BLUEBERRY=9, MELON=5}
{Fig=3, Blueberry=9, Melon=5}
{FIG=9, BLUEBERRY=81, MELON=25}toConcurrentMapThrowException
Java Collectors ToConcurrentMap Duplicate Key 建立一個 List ，增加五個元素，轉換成 Map ，當有重複鍵時，會拋出例外。
@Test(expected = IllegalStateException.class)
public void toConcurrentMapThrowException() {
int expectedSize = 3;
List<String> list = Arrays.asList("Blueberry", "Melon", "Fig", "Blueberry", "Melon");
Function<String, String> key = s -> s.toUpperCase();
Function<String, Integer> length = s -> s.length();
Map<String, Integer> result = list.stream().collect(Collectors.toConcurrentMap(key, length));
System.out.println(result);
assertEquals(expectedSize, result.size());
}
java.lang.IllegalStateException: Duplicate key 9
at java.util.stream.Collectors.lambda$throwingMerger$0(Collectors.java:133)
at java.util.concurrent.ConcurrentHashMap.merge(ConcurrentHashMap.java:1990)
at java.util.stream.Collectors.lambda$toConcurrentMap$59(Collectors.java:1472)
at java.util.stream.ReduceOps$3ReducingSink.accept(ReduceOps.java:169)
at java.util.Spliterators$ArraySpliterator.forEachRemaining(Spliterators.java:948)
at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:482)
at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:472)
at java.util.stream.ReduceOps$ReduceOp.evaluateSequential(ReduceOps.java:708)
at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234)
at java.util.stream.ReferencePipeline.collect(ReferencePipeline.java:499)
at org.ruoxue.java_147.collector.CollectorsToConcurrentMapTest.toConcurrentMapThrowException(CollectorsToConcurrentMapTest.java:42)
toConcurrentMapWithDuplicateKey
Java Collectors ToConcurrentMap Duplicate Key 建立一個 List ，增加五個元素，轉換成 Map ，當有重複鍵時，傳入 BinaryOperator 參數，解決重複鍵拋出例外的問題。
@Test
public void toConcurrentMapWithDuplicateKey() {
int expectedSize = 3;
List<String> list = Arrays.asList("Blueberry", "Melon", "Fig", "Blueberry", "Melon");
Function<String, String> key = s -> s.toUpperCase();
Function<String, Integer> length = s -> s.length();
Map<String, Integer> result = list.stream()
.collect(Collectors.toConcurrentMap(key, length, (oldValue, newValue) -> oldValue));
System.out.println(result);
assertEquals(expectedSize, result.size());
result = list.stream()
.collect(Collectors.toConcurrentMap(Function.identity(), String::length, (oldValue, newValue) -> oldValue));
System.out.println(result);
assertEquals(expectedSize, result.size());
}
{FIG=3, BLUEBERRY=9, MELON=5}
{Fig=3, Blueberry=9, Melon=5}toConcurrentMapConcurrentHashMap
Collectors ToConcurrentMap Java 建立一個 List ，增加五個元素，指定轉換成 TreeMap 。
@Test
public void toConcurrentMapConcurrentHashMap() {
int expectedSize = 3;
List<String> list = Arrays.asList("Blueberry", "Melon", "Fig", "Blueberry", "Melon");
Function<String, String> key = s -> s.toLowerCase();
Function<String, Integer> length = s -> s.length();
Function<Integer, Integer> twice = i -> i * i;
Map<String, Integer> result = list.stream().collect(Collectors.toConcurrentMap(key, length.andThen(twice),
(oldValue, newValue) -> oldValue, ConcurrentHashMap::new));
System.out.println(result);
assertEquals(expectedSize, result.size());
}
{blueberry=81, fig=9, melon=25}CollectorsToConcurrentMapTest.java
Collectors ToConcurrentMap Example 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.concurrent.ConcurrentHashMap;
import java.util.function.Function;
import java.util.stream.Collectors;
import org.junit.Test;
public class CollectorsToConcurrentMapTest {
@Test
public void toConcurrentMap() {
int expectedSize = 3;
List<String> list = Arrays.asList("Blueberry", "Melon", "Fig");
Function<String, String> key = s -> s.toUpperCase();
Function<String, Integer> length = s -> s.length();
Map<String, Integer> result = list.stream().collect(Collectors.toConcurrentMap(key, length));
System.out.println(result);
assertEquals(expectedSize, result.size());
result = list.stream().collect(Collectors.toConcurrentMap(Function.identity(), String::length));
System.out.println(result);
assertEquals(expectedSize, result.size());
Function<Integer, Integer> twice = i -> i * i;
result = list.stream().collect(Collectors.toConcurrentMap(key, length.andThen(twice)));
System.out.println(result);
assertEquals(expectedSize, result.size());
}
@Test(expected = IllegalStateException.class)
public void toConcurrentMapThrowException() {
int expectedSize = 3;
List<String> list = Arrays.asList("Blueberry", "Melon", "Fig", "Blueberry", "Melon");
Function<String, String> key = s -> s.toUpperCase();
Function<String, Integer> length = s -> s.length();
Map<String, Integer> result = list.stream().collect(Collectors.toConcurrentMap(key, length));
System.out.println(result);
assertEquals(expectedSize, result.size());
}
@Test
public void toConcurrentMapWithDuplicateKey() {
int expectedSize = 3;
List<String> list = Arrays.asList("Blueberry", "Melon", "Fig", "Blueberry", "Melon");
Function<String, String> key = s -> s.toUpperCase();
Function<String, Integer> length = s -> s.length();
Map<String, Integer> result = list.stream()
.collect(Collectors.toConcurrentMap(key, length, (oldValue, newValue) -> oldValue));
System.out.println(result);
assertEquals(expectedSize, result.size());
result = list.stream()
.collect(Collectors.toConcurrentMap(Function.identity(), String::length, (oldValue, newValue) -> oldValue));
System.out.println(result);
assertEquals(expectedSize, result.size());
}
@Test
public void toConcurrentMapConcurrentHashMap() {
int expectedSize = 3;
List<String> list = Arrays.asList("Blueberry", "Melon", "Fig", "Blueberry", "Melon");
Function<String, String> key = s -> s.toLowerCase();
Function<String, Integer> length = s -> s.length();
Function<Integer, Integer> twice = i -> i * i;
Map<String, Integer> result = list.stream().collect(Collectors.toConcurrentMap(key, length.andThen(twice),
(oldValue, newValue) -> oldValue, ConcurrentHashMap::new));
System.out.println(result);
assertEquals(expectedSize, result.size());
}
}心得分享
Collectors ToConcurrentMap Java 提供了幾種 Collectors 常見方法的操作範例，在應用上相當廣泛，熟悉 Collectors ToConcurrentMap Example 這些方法的操作，例如： toConcurrentMap 等方法，可以讓程式碼更加地簡潔及容易維護。
