想要連接所有陣列或集合元素以獲得一個字串，可以使用 Collectors joining 方法快速完成此操作，在 Collectors 類中定義，屬於 java.util.stream ，joining 可以使用分隔、前綴、後綴符號，將元素連接成字串， Collectors Joining in Java 介紹常見的 joining 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 Collector<CharSequence, ?, String> joining() {
}
public static Collector<CharSequence, ?, String> joining(CharSequence delimiter) {
}
public static Collector<CharSequence, ?, String> joining(CharSequence delimiter, CharSequence prefix,
CharSequence suffix) {
}檔案目錄
./
+- src
+- test
| +- org
| +- ruoxue
| +- java_147
| +- collector
| +- JoiningExamplesTest.java 單元測試
Joining Collectors Java 提供對陣列或集合中的元素連接成字串。
Fruit
建立 Fruit 類別，覆寫 toString ，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();
}
}joining
Joining Collectors Java 建立一個 List ，增加三個元素，將元素屬性連接成字串。
@Test
public void joining() {
List<Fruit> list = Arrays.asList(new Fruit("Blueberry", Double.MAX_VALUE, 1), new Fruit("Melon", -1, 3),
new Fruit("Fig", 3, 1));
String result = list.stream().map(Fruit::getName).collect(Collectors.joining());
System.out.println(result);
assertThat(result).isEqualTo("BlueberryMelonFig");
result = list.stream().map(e -> e.getName().toUpperCase()).collect(Collectors.joining());
System.out.println(result);
assertThat(result).isEqualTo("BLUEBERRYMELONFIG");
}BlueberryMelonFig
BLUEBERRYMELONFIGjoiningWithDelimiter
Joining Collectors Java 建立一個 List ，增加三個元素，使用分隔符號 “, ” ，將元素屬性連接成字串。
@Test
public void joiningWithDelimiter() {
List<Fruit> list = Arrays.asList(new Fruit("Blueberry", Double.MAX_VALUE, 1), new Fruit("Melon", -1, 3),
new Fruit("Fig", 3, 1));
String result = list.stream().map(Fruit::getName).collect(Collectors.joining(", "));
System.out.println(result);
assertThat(result).isEqualTo("Blueberry, Melon, Fig");
result = list.stream().map(e -> e.getName().toUpperCase()).collect(Collectors.joining(", "));
System.out.println(result);
assertThat(result).isEqualTo("BLUEBERRY, MELON, FIG");
}Blueberry, Melon, Fig
BLUEBERRY, MELON, FIG
joiningWithPrefixSuffix
Joining Collectors Java 建立一個 List ，增加三個元素，使用分隔 “, ” 、前綴 “[” 、後綴 “]” 符號，將元素連接成字串。
@Test
public void joiningWithPrefixSuffix() {
List<Fruit> list = Arrays.asList(new Fruit("Blueberry", Double.MAX_VALUE, 1), new Fruit("Melon", -1, 3),
new Fruit("Fig", 3, 1));
String result = list.stream().map(Fruit::getName).collect(Collectors.joining(", ", "[", "]"));
System.out.println(result);
assertThat(result).isEqualTo("[Blueberry, Melon, Fig]");
result = list.stream().map(e -> e.getName().toUpperCase()).collect(Collectors.joining(", ", "[", "]"));
System.out.println(result);
assertThat(result).isEqualTo("[BLUEBERRY, MELON, FIG]");
}[Blueberry, Melon, Fig]
[BLUEBERRY, MELON, FIG]JoiningExamplesTest.java
Joining Collectors 新增單元測試，驗證 Collectors Joining 是否符合預期。
package org.ruoxue.java_147.collector;
import static org.assertj.core.api.Assertions.assertThat;
import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;
import org.apache.commons.lang3.builder.ToStringBuilder;
import org.apache.commons.lang3.builder.ToStringStyle;
import org.junit.Test;
import lombok.Builder;
import lombok.Getter;
import lombok.NoArgsConstructor;
import lombok.Setter;
public class JoiningExamplesTest {
@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();
}
}
@Test
public void joining() {
List<Fruit> list = Arrays.asList(new Fruit("Blueberry", Double.MAX_VALUE, 1), new Fruit("Melon", -1, 3),
new Fruit("Fig", 3, 1));
String result = list.stream().map(Fruit::getName).collect(Collectors.joining());
System.out.println(result);
assertThat(result).isEqualTo("BlueberryMelonFig");
result = list.stream().map(e -> e.getName().toUpperCase()).collect(Collectors.joining());
System.out.println(result);
assertThat(result).isEqualTo("BLUEBERRYMELONFIG");
}
@Test
public void joiningWithDelimiter() {
List<Fruit> list = Arrays.asList(new Fruit("Blueberry", Double.MAX_VALUE, 1), new Fruit("Melon", -1, 3),
new Fruit("Fig", 3, 1));
String result = list.stream().map(Fruit::getName).collect(Collectors.joining(", "));
System.out.println(result);
assertThat(result).isEqualTo("Blueberry, Melon, Fig");
result = list.stream().map(e -> e.getName().toUpperCase()).collect(Collectors.joining(", "));
System.out.println(result);
assertThat(result).isEqualTo("BLUEBERRY, MELON, FIG");
}
@Test
public void joiningWithPrefixSuffix() {
List<Fruit> list = Arrays.asList(new Fruit("Blueberry", Double.MAX_VALUE, 1), new Fruit("Melon", -1, 3),
new Fruit("Fig", 3, 1));
String result = list.stream().map(Fruit::getName).collect(Collectors.joining(", ", "[", "]"));
System.out.println(result);
assertThat(result).isEqualTo("[Blueberry, Melon, Fig]");
result = list.stream().map(e -> e.getName().toUpperCase()).collect(Collectors.joining(", ", "[", "]"));
System.out.println(result);
assertThat(result).isEqualTo("[BLUEBERRY, MELON, FIG]");
}
}心得分享
Collectors Joining 方法是將陣列或集合用作 Stream 來源，然後呼叫 collect 方法，提供將字元或字串陣列或集合中的各個元素，連接成單一字串物件，熟悉 Joining Collectors 這些方法的操作，例如： joining 等方法，可以讓程式碼更加地簡潔及容易維護。
