Java lambda和stream

ldcaws 2022-09-29 我要评论

前言

在日常开发中，对数据排序是非常常见的一种需求，一般通过如下两种方式：

存储系统：通过SQL、NoSQL的排序功能，查询的结果是完成排序的结果；
内存：通过在内存中进行排序，查询的结果是无序的结果；

下面聊聊通过Java中的lambda和stream实现在内存中对数据进行排序。

1、定义一个基础类

@Data
@NoArgsConstructor
@AllArgsConstructor
public class Student {

    private String name;
    private int age;

    @Override
    public boolean equals(Object o) {
        if (this == o) {
            return true;
        }
        if (o == null || getClass() != o.getClass()) {
            return false;
        }
        Student student = (Student) o;
        return age == student.age && Objects.equals(name, student.name);
    }

    @Override
    public int hashCode() {
        return Objects.hash(name, age);
    }
}

2、使用Comparator排序

	@Test
    void test() {
        List<Student> students = Lists.newArrayList(
                new Student("caocao", 21),
                new Student("sunquan", 20)
        );

        Collections.sort(students, new Comparator<Student>() {
            @Override
            public int compare(Student o1, Student o2) {
                return o1.getName().compareTo(o2.getName());
            }
        });
        Assertions.assertEquals(students.get(0), new Student("caocao", 21));
    }

在定义的Comparator中使用name字段排序，string类型的排序是通过ASCII码顺序进行判断。

3、使用lambda排序

	@Test
    void test() {
        List<Student> students = Lists.newArrayList(
                new Student("caocao", 21),
                new Student("sunquan", 20)
        );

        students.sort((o1, o2) -> o1.getName().compareTo(o2.getName()));
        Assertions.assertEquals(students.get(0), new Student("caocao", 21));
    }

将内部类Comparator替换为lambda表达式，使代码更简洁。

4、使用Comparator的comparing方法排序

	@Test
    void test() {
        List<Student> students = Lists.newArrayList(
                new Student("caocao", 21),
                new Student("sunquan", 20)
        );

        students.sort(Comparator.comparing(Student::getName));
        Assertions.assertEquals(students.get(0), new Student("caocao", 21));
    }

5、自定义比对方法

在Student类中自定义比对方法

	public static int compareByNameThenAge(Student s1, Student s2) {
        if (s1.name.equals(s2.name)) {
            return Integer.compare(s1.age, s2.age);
        } else {
            return s1.name.compareTo(s2.name);
        }
    }

先比对name，再比对age

	@Test
    void test() {
        List<Student> students = Lists.newArrayList(
                new Student("caocao", 21),
                new Student("sunquan", 20)
        );

        students.sort((o1,o2) -> Student.compareByNameThenAge(o1,o2));
        Assertions.assertEquals(students.get(0), new Student("caocao", 21));
    }

6、使用stream排序

在流式计算时进行排序

	@Test
    void test() {
        List<Student> students = Lists.newArrayList(
                new Student("caocao", 21),
                new Student("sunquan", 20)
        );

        List<Student> result = students.stream().sorted(new Comparator<Student>() {
            @Override
            public int compare(Student o1, Student o2) {
                return o1.getName().compareTo(o2.getName());
            }
        }).collect(Collectors.toList());
        
        Assertions.assertEquals(result.get(0), new Student("caocao", 21));
    }

7、null值判断

若列表中元素是null或列表中元素参与排序的字段是null，会出现NullPointException异常，即 NPE，简单演示一下：

	@Test
    void sortedNullGotNPE() {
        List<Student> students = Lists.newArrayList(
                null,
                new Student("liubei", 12)
        );
        students.sort(Comparator.comparing(Student::getName));
    }

修改为：

	@Test
    void sortedNullGotNPE() {
        List<Student> students = Lists.newArrayList(
                null,
                new Student("liubei", 12)
        );
        //students.sort(Comparator.comparing(Student::getName));

        Assertions.assertThrows(NullPointerException.class,
                () -> students.sort(Comparator.comparing(Student::getName)));
    }