檔案目錄
./
+- src
+- test
| +- org
| +- ruoxue
| +- java_147
| +- stream
| +- sort
| +- StreamSortedTest.java
單元測試
Stream Sorted Java 提供不同類型的資料進行排序，使用預設或自定義比較器，排序集合中的元素。
sort
Stream Sorted Java 建立一個 List ，增加三個元素，使用 Stream sorted 方法，依照預設比較器，由小到大，升序對集合進行排序。
@Test
public void sort() {
List<String> list = Arrays.asList("Lichee", "Coconut", "Plum");
System.out.println(list);
ArrayList<String> result = list.stream().sorted().collect(Collectors.toCollection(ArrayList::new));
System.out.println(result);
assertThat(result).containsExactly("Coconut", "Lichee", "Plum");
}[Lichee, Coconut, Plum]
[Coconut, Lichee, Plum]
sortWithReverseOrder
Stream Sorted Java 建立一個 List ，增加三個元素，使用 Stream sorted 方法，依照預設反向比較器，由大到小，降序對集合進行排序。
@Test
public void sortWithReverseOrder() {
List<String> list = Arrays.asList("Lichee", "Coconut", "Plum");
System.out.println(list);
ArrayList<String> result = list.stream().sorted(Comparator.reverseOrder())
.collect(Collectors.toCollection(ArrayList::new));
System.out.println(result);
assertThat(result).containsExactly("Plum", "Lichee", "Coconut");
}[Lichee, Coconut, Plum]
[Plum, Lichee, Coconut]sortWithComparator
Stream Sorted Java 建立一個 List ，增加三個元素，使用 Stream sorted 方法，依照自定義比較器，由小到大，升序對集合進行排序。
@Test
public void sortWithComparator() {
List<String> list = Arrays.asList("Lichee", "Coconut", "Plum");
System.out.println(list);
ArrayList<String> result = list.stream().sorted((s1, s2) -> s1.length() - s2.length())
.collect(Collectors.toCollection(ArrayList::new));
System.out.println(result);
assertThat(result).containsExactly("Plum", "Lichee", "Coconut");
}[Lichee, Coconut, Plum]
[Plum, Lichee, Coconut]sortWithComparing
Stream Sorted Example 建立一個 List ，增加三個元素，使用 Stream sorted 方法，依照自定義比較器，由小到大，升序對集合進行排序。
@Test
public void sortWithComparing() {
List<String> list = Arrays.asList("Lichee", "Coconut", "Plum");
System.out.println(list);
ArrayList<String> result = list.stream().sorted(Comparator.comparing(String::length))
.collect(Collectors.toCollection(ArrayList::new));
System.out.println(result);
assertThat(result).containsExactly("Plum", "Lichee", "Coconut");
}[Lichee, Coconut, Plum]
[Plum, Lichee, Coconut]sortWithMultipleConditions
Stream Sorted Example 建立一個 List ，增加三個元素，使用 Stream sorted 方法，依照多個自定義比較器，由小到大，升序對集合進行排序。
protected static Comparator<String> nameComparator = new Comparator<String>() {
@Override
public int compare(String s1, String s2) {
return s1.compareTo(s2);
}
};
protected static Comparator<String> lengthComparator = (s1, s2) -> Integer.compare(s1.length(), s2.length());
@Test
public void sortWithMultipleConditions() {
List<String> list = Arrays.asList("Coconut", "Plum", "Lichee");
System.out.println(list);
ArrayList<String> result = list.stream().sorted(nameComparator.thenComparing(lengthComparator))
.collect(Collectors.toCollection(ArrayList::new));
System.out.println(result);
assertThat(result).containsExactly("Coconut", "Lichee", "Plum");
}[Coconut, Plum, Lichee]
[Coconut, Lichee, Plum]sortWithNull
建立一個 List ，增加三個元素及一個 null 元素，使用 Stream sorted 方法，依照自定義比較器， null 元素放在第一個，由小到大，升序對集合進行排序。
@Test
public void sortWithNull() {
List<String> list = Arrays.asList("Coconut", "Plum", "Lichee", null);
System.out.println(list);
ArrayList<String> result = list.stream().sorted((s1, s2) -> {
if (s1 == null) {
return s2 == null ? 0 : -1;
} else if (s2 == null) {
return 1;
}
return s1.compareTo(s2);
}).collect(Collectors.toCollection(ArrayList::new));
System.out.println(result);
assertThat(result).containsExactly(null, "Coconut", "Lichee", "Plum");
}[Coconut, Plum, Lichee, null]
[null, Coconut, Lichee, Plum]sortWithNullsFirst
建立一個 List ，增加三個元素及一個 null 元素，使用 Stream sorted 方法，依照 nullsFirst 比較器， null 元素放在第一個，由小到大，升序對集合進行排序。
@Test
public void sortWithNullsFirst() {
List<String> list = Arrays.asList("Coconut", "Plum", "Lichee", null);
System.out.println(list);
ArrayList<String> result = list.stream().sorted(Comparator.nullsFirst(Comparator.comparing(s -> s)))
.collect(Collectors.toCollection(ArrayList::new));
System.out.println(result);
assertThat(result).containsExactly(null, "Coconut", "Lichee", "Plum");
}[Coconut, Plum, Lichee, null]
[null, Coconut, Lichee, Plum]sortWithNullsLast
建立一個 List ，增加三個元素及一個 null 元素，使用 Stream sorted 方法，依照 nullsLast 比較器， null 元素放在最後個，由小到大，升序對集合進行排序。
@Test
public void sortWithNullsLast() {
List<String> list = Arrays.asList("Coconut", "Plum", "Lichee", null);
System.out.println(list);
ArrayList<String> result = list.stream().sorted(Comparator.nullsLast(Comparator.comparing(s -> s)))
.collect(Collectors.toCollection(ArrayList::new));
System.out.println(result);
assertThat(result).containsExactly("Coconut", "Lichee", "Plum", null);
}
[Coconut, Plum, Lichee, null]
[Coconut, Lichee, Plum, null]StreamSortedTest.java
Stream Sorted Example 新增單元測試，驗證是否符合預期。
package org.ruoxue.java_147.stream.sort;
import static org.assertj.core.api.Assertions.assertThat;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Comparator;
import java.util.List;
import java.util.stream.Collectors;
import org.junit.Test;
public class StreamSortedTest {
@Test
public void sort() {
List<String> list = Arrays.asList("Lichee", "Coconut", "Plum");
System.out.println(list);
ArrayList<String> result = list.stream().sorted().collect(Collectors.toCollection(ArrayList::new));
System.out.println(result);
assertThat(result).containsExactly("Coconut", "Lichee", "Plum");
}
@Test
public void sortWithReverseOrder() {
List<String> list = Arrays.asList("Lichee", "Coconut", "Plum");
System.out.println(list);
ArrayList<String> result = list.stream().sorted(Comparator.reverseOrder())
.collect(Collectors.toCollection(ArrayList::new));
System.out.println(result);
assertThat(result).containsExactly("Plum", "Lichee", "Coconut");
}
@Test
public void sortWithComparator() {
List<String> list = Arrays.asList("Lichee", "Coconut", "Plum");
System.out.println(list);
ArrayList<String> result = list.stream().sorted((s1, s2) -> s1.length() - s2.length())
.collect(Collectors.toCollection(ArrayList::new));
System.out.println(result);
assertThat(result).containsExactly("Plum", "Lichee", "Coconut");
}
@Test
public void sortWithComparing() {
List<String> list = Arrays.asList("Lichee", "Coconut", "Plum");
System.out.println(list);
ArrayList<String> result = list.stream().sorted(Comparator.comparing(String::length))
.collect(Collectors.toCollection(ArrayList::new));
System.out.println(result);
assertThat(result).containsExactly("Plum", "Lichee", "Coconut");
}
protected static Comparator<String> nameComparator = new Comparator<String>() {
@Override
public int compare(String s1, String s2) {
return s1.compareTo(s2);
}
};
protected static Comparator<String> lengthComparator = (s1, s2) -> Integer.compare(s1.length(), s2.length());
@Test
public void sortWithMultipleConditions() {
List<String> list = Arrays.asList("Coconut", "Plum", "Lichee");
System.out.println(list);
ArrayList<String> result = list.stream().sorted(nameComparator.thenComparing(lengthComparator))
.collect(Collectors.toCollection(ArrayList::new));
System.out.println(result);
assertThat(result).containsExactly("Coconut", "Lichee", "Plum");
}
@Test
public void sortWithNull() {
List<String> list = Arrays.asList("Coconut", "Plum", "Lichee", null);
System.out.println(list);
ArrayList<String> result = list.stream().sorted((s1, s2) -> {
if (s1 == null) {
return s2 == null ? 0 : -1;
} else if (s2 == null) {
return 1;
}
return s1.compareTo(s2);
}).collect(Collectors.toCollection(ArrayList::new));
System.out.println(result);
assertThat(result).containsExactly(null, "Coconut", "Lichee", "Plum");
}
@Test
public void sortWithNullsFirst() {
List<String> list = Arrays.asList("Coconut", "Plum", "Lichee", null);
System.out.println(list);
ArrayList<String> result = list.stream().sorted(Comparator.nullsFirst(Comparator.comparing(s -> s)))
.collect(Collectors.toCollection(ArrayList::new));
System.out.println(result);
assertThat(result).containsExactly(null, "Coconut", "Lichee", "Plum");
}
@Test
public void sortWithNullsLast() {
List<String> list = Arrays.asList("Coconut", "Plum", "Lichee", null);
System.out.println(list);
ArrayList<String> result = list.stream().sorted(Comparator.nullsLast(Comparator.comparing(s -> s)))
.collect(Collectors.toCollection(ArrayList::new));
System.out.println(result);
assertThat(result).containsExactly("Coconut", "Lichee", "Plum", null);
}
}
心得分享
Java Stream Sorted 實務上經常會使用到資料排序，例如：使用者想要查看按照價格升序排列的所有產品，將能夠首先看到最便宜的產品，進而做出相應的購買決定，提供了幾種 Sort 常見方法的操作範例，在應用上相當廣泛，熟悉 Stream Sorted Example 這些方法的操作，可以快速簡易地實現排序功能。
