Java8 Stream调用sorted()排序报Student无法转Comparable异常如何解决

使用Java 8 Stream的无参自然排序方法sorted()处理Student实体列表时，IDE控制台抛出类型转换异常：class com.java8.Student cannot be cast to class java.lang.Comparable，相关代码文件StreamStudent.java存放于com.java8包路径下。

完整异常栈信息 #

Exception in thread "main" java.lang.ClassCastException: class com.java8.Student cannot be cast to class java.lang.Comparable (com.java8.Student is in unnamed module of loader 'app'; java.lang.Comparable is in module java.base of loader 'bootstrap')
    at java.base/java.util.Comparators$NaturalOrderComparator.compare(Comparators.java:47)
    at java.base/java.util.TimSort.countRunAndMakeAscending(TimSort.java:355)
    at java.base/java.util.TimSort.sort(TimSort.java:220)
    at java.base/java.util.Arrays.sort(Arrays.java:1307)
    at java.base/java.util.stream.SortedOps$SizedRefSortingSink.end(SortedOps.java:353)
    at java.base/java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:510)
    at java.base/java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:499)
    at java.base/java.util.stream.ReduceOps$ReduceOp.evaluateSequential(ReduceOps.java:921)
    at java.base/java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234)
    at java.base/java.util.stream.ReferencePipeline.collect(ReferencePipeline.java:682)
    at com.java8.StreamStudent.main(StreamStudent.java:72)

问题复现代码 #

package com.java8;

import java.util.*;
import java.util.stream.*;

class Student{
    String firstName;
    String lastName;
    Integer groupId;
    Integer age;

    // constructor and getter, setter

}

public class StreamStudent {
    public static void main(String[] args) {
        List<Student> stdList = new ArrayList<>();
        stdList.add(new Student("Sara", "Mills", 1, 18));
        stdList.add(new Student("Andrew", "Gibson", 2, 21));
        stdList.add(new Student("Craig", "Ellis", 1, 23));
        stdList.add(new Student("Steven", "Cole", 2, 19));
        stdList.add(new Student("Andrew", "Carter", 2, 2));

        
        System.out.println("1. Sort all student firstname by name");
        List<Student> students =  stdList.stream().sorted().collect(Collectors.toList());
        students.forEach((s) -> System.out.println(s.getFirstName() + " " + s.getLastName() + " " + s.getGroupId() + " " + s.getAge()));
    }
}

无参的Stream.sorted()方法默认对元素执行自然排序，要求流中所有元素必须实现java.lang.Comparable接口，通过重写的compareTo()方法定义排序逻辑。
当前自定义的Student类没有实现Comparable接口，排序过程中JVM尝试将Student实例强转为Comparable类型时失败，就会抛出上述ClassCastException。

二选一即可：

• 方案1：修改Student类，实现Comparable<Student>接口，定义类的默认排序规则。比如示例中需要按firstName排序的实现代码：

class Student implements Comparable<Student>{
    String firstName;
    String lastName;
    Integer groupId;
    Integer age;

    // 原有构造方法、getter、setter逻辑保留即可

    @Override
    public int compareTo(Student other) {
        // 此处定义排序规则：按firstName字符串的自然顺序排序
        // 如果需要按其他字段排序、或者多字段组合排序，直接修改该方法的返回逻辑即可
        return this.firstName.compareTo(other.firstName);
    }
}

修改完成后，原有代码中无参的.sorted()调用可以直接正常运行。

• 方案2：不修改Student类定义，调用sorted()的重载方法，传入自定义比较器指定排序规则。比如按firstName排序的写法：

List<Student> students =  stdList.stream()
        // 传入比较器，指定按firstName字段排序
        .sorted(Comparator.comparing(Student::getFirstName))
        .collect(Collectors.toList());

如果需要多字段组合排序，比如先按groupId升序、再按年龄升序，可以直接拼接比较规则：

.sorted(
    Comparator.comparingInt(Student::getGroupId)
    .thenComparingInt(Student::getAge)
)

内容的提问来源于stack exchange，提问作者Faheem azaz Bhanej