Table of Contents

Java List sort Method

提供 List sort 方法操作排序，當資料需要按特定順序排列，需實作 Comparator 接口，內有一個 compare 方法，利用 Lambda 表達式或傳統方式來實作其功能， Sort Java List 提供了預設比較器和建立自定義比較器，對資料進行排序，本篇增加了範例，並透過單元測試來驗證產出結果。

default void sort(Comparator<? super E> c) {
}

檔案目錄

./
   +- src
       +- test
       |   +- org
       |       +- ruoxue
       |           +- java_147
       |               +- list
       |                   +- sort
       |                       +- ListSortTest.java

單元測試

List Sort 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

List Sort Java 建立一個 List ，增加三個元素，使用 List sort 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("Banana", "Apple", "Cherry");
		System.out.println(list);

		list.sort(null);
		System.out.println(list);
		assertThat(list).containsExactly("Apple", "Banana", "Cherry");
	}

[Banana, Apple, Cherry]
[Apple, Banana, Cherry]

sortWithReverseOrder

List Sort Java 建立一個 List ，增加三個元素，使用 List sort 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("Banana", "Apple", "Cherry");
		System.out.println(list);

		list.sort(Comparator.reverseOrder());
		System.out.println(list);
		assertThat(list).containsExactly("Cherry", "Banana", "Apple");
	}

[Banana, Apple, Cherry]
[Cherry, Banana, Apple]

sortWithComparator

List Sort Java 建立一個 List ，增加三個元素，使用 List sort 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("Banana", "Apple", "Cherry");
		System.out.println(list);

		list.sort((s1, s2) -> s1.length() - s2.length());
		System.out.println(list);
		assertThat(list).containsExactly("Apple", "Banana", "Cherry");
	}

[Banana, Apple, Cherry]
[Apple, Banana, Cherry]

sortWithComparing

List Sort Example 建立一個 List ，增加三個元素，使用 List sort 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("Banana", "Apple", "Cherry");
		System.out.println(list);

		list.sort(Comparator.comparing(String::length));
		System.out.println(list);
		assertThat(list).containsExactly("Apple", "Banana", "Cherry");
	}

[Banana, Apple, Cherry]
[Apple, Banana, Cherry]

sortWithMultipleConditions

List Sort Example 建立一個 List ，增加三個元素，使用 List sort 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("Apple", "Cherry", "Banana");
		System.out.println(list);

		list.sort(nameComparator.thenComparing(lengthComparator));
		System.out.println(list);
		assertThat(list).containsExactly("Apple", "Banana", "Cherry");
	}

[Apple, Cherry, Banana]
[Apple, Banana, Cherry]

sortWithNull

建立一個 List ，增加三個元素及一個 null 元素，使用 List sort 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("Apple", "Cherry", "Banana", null);
		System.out.println(list);

		list.sort((s1, s2) -> {
			if (s1 == null) {
				return s2 == null ? 0 : -1;
			} else if (s2 == null) {
				return 1;
			}
			return s1.compareTo(s2);
		});
		System.out.println(list);
		assertThat(list).containsExactly(null, "Apple", "Banana", "Cherry");
	}

[Apple, Cherry, Banana, null]
[null, Apple, Banana, Cherry]

sortWithNullsFirst

建立一個 List ，增加三個元素及一個 null 元素，使用 List sort 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("Apple", "Cherry", "Banana", null);
		System.out.println(list);

		list.sort(Comparator.nullsFirst(Comparator.comparing(s -> s)));
		System.out.println(list);
		assertThat(list).containsExactly(null, "Apple", "Banana", "Cherry");
	}

[Apple, Cherry, Banana, null]
[null, Apple, Banana, Cherry]

sortWithNullsLast

建立一個 List ，增加三個元素及一個 null 元素，使用 List sort 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("Apple", "Cherry", "Banana", null);
		System.out.println(list);

		list.sort(Comparator.nullsLast(Comparator.comparing(s -> s)));
		System.out.println(list);
		assertThat(list).containsExactly("Apple", "Banana", "Cherry", null);
	}

[Apple, Cherry, Banana, null]
[Apple, Banana, Cherry, null]

ListSortTest.java

List Sort Example 新增單元測試，驗證是否符合預期。

package org.ruoxue.java_147.list.sort;

import static org.assertj.core.api.Assertions.assertThat;

import java.util.Arrays;
import java.util.Comparator;
import java.util.List;

import org.junit.Test;

public class ListSortTest {

	@Test
	public void sort() {
		List<String> list = Arrays.asList("Banana", "Apple", "Cherry");
		System.out.println(list);

		list.sort(null);
		System.out.println(list);
		assertThat(list).containsExactly("Apple", "Banana", "Cherry");
	}

	@Test
	public void sortWithReverseOrder() {
		List<String> list = Arrays.asList("Banana", "Apple", "Cherry");
		System.out.println(list);

		list.sort(Comparator.reverseOrder());
		System.out.println(list);
		assertThat(list).containsExactly("Cherry", "Banana", "Apple");
	}

	@Test
	public void sortWithComparator() {
		List<String> list = Arrays.asList("Banana", "Apple", "Cherry");
		System.out.println(list);

		list.sort((s1, s2) -> s1.length() - s2.length());
		System.out.println(list);
		assertThat(list).containsExactly("Apple", "Banana", "Cherry");
	}

	@Test
	public void sortWithComparing() {
		List<String> list = Arrays.asList("Banana", "Apple", "Cherry");
		System.out.println(list);

		list.sort(Comparator.comparing(String::length));
		System.out.println(list);
		assertThat(list).containsExactly("Apple", "Banana", "Cherry");
	}

	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("Apple", "Cherry", "Banana");
		System.out.println(list);

		list.sort(nameComparator.thenComparing(lengthComparator));
		System.out.println(list);
		assertThat(list).containsExactly("Apple", "Banana", "Cherry");
	}

	@Test
	public void sortWithNull() {
		List<String> list = Arrays.asList("Apple", "Cherry", "Banana", null);
		System.out.println(list);

		list.sort((s1, s2) -> {
			if (s1 == null) {
				return s2 == null ? 0 : -1;
			} else if (s2 == null) {
				return 1;
			}
			return s1.compareTo(s2);
		});
		System.out.println(list);
		assertThat(list).containsExactly(null, "Apple", "Banana", "Cherry");
	}

	@Test
	public void sortWithNullsFirst() {
		List<String> list = Arrays.asList("Apple", "Cherry", "Banana", null);
		System.out.println(list);

		list.sort(Comparator.nullsFirst(Comparator.comparing(s -> s)));
		System.out.println(list);
		assertThat(list).containsExactly(null, "Apple", "Banana", "Cherry");
	}

	@Test
	public void sortWithNullsLast() {
		List<String> list = Arrays.asList("Apple", "Cherry", "Banana", null);
		System.out.println(list);

		list.sort(Comparator.nullsLast(Comparator.comparing(s -> s)));
		System.out.println(list);
		assertThat(list).containsExactly("Apple", "Banana", "Cherry", null);
	}
}