Table of Contents
ToggleJava Convert List to Map
將 List 轉換成 Map , List 是屬於 Java Collections 框架的接口,物件的形式有序存儲元素,並保留插入順序,允許存儲重複值, Map 將資料存儲在鍵、值對中,可以通過另一種類型的索引訪問,插入重複鍵,會替換相應鍵的元素, Java List Map 任務是將該 List 轉換為 Map ,本篇增加了範例,並透過單元測試來驗證產出結果。
檔案目錄
./
+- src
+- test
| +- org
| +- ruoxue
| +- java_147
| +- conversion
| +- list
| +- ListToMapTest.java
單元測試
Java List to Map 提供 List 轉換成 Map 等操作。
Fruit
建立 Fruit 物件,覆寫 equals 、 hashCode ,定義屬性和方法,用來建立一個物件。
@NoArgsConstructor
@Getter
@Setter
@Builder
public static class Fruit {
private String name;
private double quantity;
private int type;
public Fruit(String name, double quantity, int type) {
this.name = name;
this.quantity = quantity;
this.type = type;
}
public String toString() {
ToStringBuilder builder = new ToStringBuilder(this, ToStringStyle.JSON_STYLE);
builder.appendSuper(super.toString());
builder.append("name", name);
builder.append("quantity", quantity);
builder.append("type", type);
return builder.toString();
}
public boolean equals(Object object) {
if (!(object instanceof Fruit)) {
return false;
}
if (this == object) {
return true;
}
Fruit other = (Fruit) object;
return new EqualsBuilder().append(getName(), other.getName()).isEquals();
}
public int hashCode() {
return new HashCodeBuilder().append(getName()).toHashCode();
}
}
traditional
Java List to Map 使用傳統方式轉換成 Map 。
@Test
public void traditional() {
List<Fruit> list = Arrays.asList(new Fruit("Apple", 1, 1), new Fruit("Banana", 2, 1),
new Fruit("Cherry", 3, 1));
Map<String, Fruit> result = new HashMap<>();
for (Fruit e : list) {
result.put(e.getName(), e);
}
System.out.println(result);
assertThat(result.values()).containsAll(list);
}
{Apple={"name":"Apple","quantity":1.0,"type":1}, Cherry={"name":"Cherry","quantity":3.0,"type":1}, Banana={"name":"Banana","quantity":2.0,"type":1}}
toMap
Java List to Map 使用 toMap 轉換成 Map 。
@Test
public void toMap() {
List<Fruit> list = Arrays.asList(new Fruit("Apple", 1, 1), new Fruit("Banana", 2, 1),
new Fruit("Cherry", 3, 1));
Map<String, Fruit> result = list.stream().collect(Collectors.toMap(Fruit::getName, Function.identity()));
System.out.println(result);
assertThat(result.values()).containsAll(list);
}
{Apple={"name":"Apple","quantity":1.0,"type":1}, Cherry={"name":"Cherry","quantity":3.0,"type":1}, Banana={"name":"Banana","quantity":2.0,"type":1}}
toMapThrowException
Java List to Map 使用 toMap 轉換成 Map ,當 List 有重複元素,會拋出例外。
@Test(expected = IllegalStateException.class)
public void toMapThrowException() {
List<Fruit> list = Arrays.asList(new Fruit("Apple", 1, 1), new Fruit("Banana", 2, 1), new Fruit("Banana", 2, 1),
new Fruit("Cherry", 3, 1));
Map<String, Fruit> result = list.stream().collect(Collectors.toMap(Fruit::getName, Function.identity()));
System.out.println(result);
assertThat(result.values()).containsAll(list);
}
java.lang.IllegalStateException: Duplicate key {"name":"Banana","quantity":2.0,"type":1}
at java.util.stream.Collectors.lambda$throwingMerger$0(Collectors.java:133)
at java.util.HashMap.merge(HashMap.java:1255)
at java.util.stream.Collectors.lambda$toMap$58(Collectors.java:1320)
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.conversion.ListToMapTest.toMapThrowException(ListToMapTest.java:93)
toMapWithDuplicateKey
Java List to Map 使用 toMap 轉換成 Map ,當 List 有重複元素,實作 mergeFunction ,合併鍵值。
@Test
public void toMapWithDuplicateKey() {
List<Fruit> list = Arrays.asList(new Fruit("Apple", 1, 1), new Fruit("Banana", 2, 1), new Fruit("Banana", 2, 1),
new Fruit("Cherry", 3, 1));
Map<String, Fruit> result = list.stream()
.collect(Collectors.toMap(Fruit::getName, Function.identity(), (oldValue, newValue) -> oldValue));
System.out.println(result);
assertThat(result.values()).containsAll(list);
}
{Apple={"name":"Apple","quantity":1.0,"type":1}, Cherry={"name":"Cherry","quantity":3.0,"type":1}, Banana={"name":"Banana","quantity":2.0,"type":1}}
MapUtils_populateMap
Convert List to Map in Java 使用 MapUtils.populateMap 轉換成 Map 。
@Test
public void MapUtils_populateMap() {
List<Fruit> list = Arrays.asList(new Fruit("Apple", 1, 1), new Fruit("Banana", 2, 1), new Fruit("Banana", 2, 1),
new Fruit("Cherry", 3, 1));
Map<String, Fruit> result = new HashMap<>();
MapUtils.populateMap(result, list, Fruit::getName);
System.out.println(result);
assertThat(result.values()).containsAll(list);
}
{Apple={"name":"Apple","quantity":1.0,"type":1}, Cherry={"name":"Cherry","quantity":3.0,"type":1}, Banana={"name":"Banana","quantity":2.0,"type":1}}
Maps_uniqueIndex
Convert List to Map in Java 使用 Maps.uniqueIndex 轉換成 Map 。
@Test
public void Maps_uniqueIndex() {
List<Fruit> list = Arrays.asList(new Fruit("Apple", 1, 1), new Fruit("Banana", 2, 1),
new Fruit("Cherry", 3, 1));
Map<String, Fruit> result = Maps.uniqueIndex(list, Fruit::getName);
System.out.println(result);
assertThat(result.values()).containsAll(list);
}
{Apple={"name":"Apple","quantity":1.0,"type":1}, Banana={"name":"Banana","quantity":2.0,"type":1}, Cherry={"name":"Cherry","quantity":3.0,"type":1}}
ListToMapTest.java
Convert List to Map in Java 新增單元測試,驗證是否符合預期。
package org.ruoxue.java_147.conversion.list;
import static org.assertj.core.api.Assertions.assertThat;
import java.util.Arrays;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.function.Function;
import java.util.stream.Collectors;
import org.apache.commons.collections4.MapUtils;
import org.apache.commons.lang3.builder.EqualsBuilder;
import org.apache.commons.lang3.builder.HashCodeBuilder;
import org.apache.commons.lang3.builder.ToStringBuilder;
import org.apache.commons.lang3.builder.ToStringStyle;
import org.junit.Test;
import com.google.common.collect.Maps;
import lombok.Builder;
import lombok.Getter;
import lombok.NoArgsConstructor;
import lombok.Setter;
public class ListToMapTest {
@NoArgsConstructor
@Getter
@Setter
@Builder
public static class Fruit {
private String name;
private double quantity;
private int type;
public Fruit(String name, double quantity, int type) {
this.name = name;
this.quantity = quantity;
this.type = type;
}
public String toString() {
ToStringBuilder builder = new ToStringBuilder(this, ToStringStyle.JSON_STYLE);
builder.appendSuper(super.toString());
builder.append("name", name);
builder.append("quantity", quantity);
builder.append("type", type);
return builder.toString();
}
public boolean equals(Object object) {
if (!(object instanceof Fruit)) {
return false;
}
if (this == object) {
return true;
}
Fruit other = (Fruit) object;
return new EqualsBuilder().append(getName(), other.getName()).isEquals();
}
public int hashCode() {
return new HashCodeBuilder().append(getName()).toHashCode();
}
}
@Test
public void traditional() {
List<Fruit> list = Arrays.asList(new Fruit("Apple", 1, 1), new Fruit("Banana", 2, 1),
new Fruit("Cherry", 3, 1));
Map<String, Fruit> result = new HashMap<>();
for (Fruit e : list) {
result.put(e.getName(), e);
}
System.out.println(result);
assertThat(result.values()).containsAll(list);
}
@Test
public void toMap() {
List<Fruit> list = Arrays.asList(new Fruit("Apple", 1, 1), new Fruit("Banana", 2, 1),
new Fruit("Cherry", 3, 1));
Map<String, Fruit> result = list.stream().collect(Collectors.toMap(Fruit::getName, Function.identity()));
System.out.println(result);
assertThat(result.values()).containsAll(list);
}
@Test
public void toMapThrowException() {
List<Fruit> list = Arrays.asList(new Fruit("Apple", 1, 1), new Fruit("Banana", 2, 1), new Fruit("Banana", 2, 1),
new Fruit("Cherry", 3, 1));
Map<String, Fruit> result = list.stream().collect(Collectors.toMap(Fruit::getName, Function.identity()));
System.out.println(result);
assertThat(result.values()).containsAll(list);
}
@Test
public void toMapWithDuplicateKey() {
List<Fruit> list = Arrays.asList(new Fruit("Apple", 1, 1), new Fruit("Banana", 2, 1), new Fruit("Banana", 2, 1),
new Fruit("Cherry", 3, 1));
Map<String, Fruit> result = list.stream()
.collect(Collectors.toMap(Fruit::getName, Function.identity(), (oldValue, newValue) -> oldValue));
System.out.println(result);
assertThat(result.values()).containsAll(list);
}
@Test
public void MapUtils_populateMap() {
List<Fruit> list = Arrays.asList(new Fruit("Apple", 1, 1), new Fruit("Banana", 2, 1), new Fruit("Banana", 2, 1),
new Fruit("Cherry", 3, 1));
Map<String, Fruit> result = new HashMap<>();
MapUtils.populateMap(result, list, Fruit::getName);
System.out.println(result);
assertThat(result.values()).containsAll(list);
}
@Test
public void Maps_uniqueIndex() {
List<Fruit> list = Arrays.asList(new Fruit("Apple", 1, 1), new Fruit("Banana", 2, 1),
new Fruit("Cherry", 3, 1));
Map<String, Fruit> result = Maps.uniqueIndex(list, Fruit::getName);
System.out.println(result);
assertThat(result.values()).containsAll(list);
}
}
心得分享
List to Map in Java 將 List 轉換為 Map 實例的多種方法,也有其他選擇使用外部程式庫來轉換成 Map ,像是 Apache Commons Collections 、 Guava 等, Convert List to Map in Java 提供將 List 實例轉換為 Map 實例的多種方法。