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)));
}到此这篇关于Java中的lambda和stream实现排序的文章就介绍到这了,更多相关Java lambda和stream 内容请搜索Devmax以前的文章或继续浏览下面的相关文章希望大家以后多多支持Devmax！