Java 记录 StackOverflow RuntimeException

Question

我正在学习 Java 记录、预览功能，并且在运行以下代码时遇到 StackOverflow 异常。

    import java.util.ArrayList;
    import java.util.List;
    import java.util.stream.Collectors;
    public class Example {
        public record Course(String name, Student topper) { }
        public record Student(String name, List<Course> enrolled) {}

        public static void main(String[] args) {
            Student john = new Student("John", new ArrayList<>());
            john.enrolled().add(new Course("Enrolled for Math", john));
            john.enrolled().add(new Course("Enrolled for History", john));
            System.out.println(john);
        }
    }


Below is the exception trace :


 java --enable-preview Example
 Exception in thread "main" java.lang.StackOverflowError
    at Example$Course.toString(Example.java:6)
    at java.base/java.lang.String.valueOf(String.java:3388)
    at java.base/java.lang.StringBuilder.append(StringBuilder.java:167)
    at java.base/java.util.AbstractCollection.toString(AbstractCollection.java:457)

从异常中我意识到它与 toString() 有关，当我在记录中覆盖 toString() 如下代码时，我没有看到 Exception 。

// code with implementation of toString()
public class Example {

    public record Course(String name, Student topper) {
    public String toString()
        {
            return name;
        }
   }
   public record Student(String name, List<Course> enrolled) {
   public String toString()
        {
             return this.name+" : "+enrolled.stream().map(s->s.toString()).collect(Collectors.joining(","));
        }
   }
   public static void main(String... args) {
       Student john = new Student("John", new ArrayList<>());
       john.enrolled().add(new Course("Enrolled for Math", john));
       john.enrolled().add(new Course("Enrolled for History", john));
       System.out.println(john);
   }
}

此代码打印 John : Enrolled for Math,Enrolled for History 。有人可以解释为什么如果我不覆盖 toString() 会得到 StackOverflow？当我打印 john.hashCode() 时，我也看到 StackOverflow

Answer 1

与Record#toString无关特别是。仔细看看你的声明:

public record Course(String name, Student topper) {}
public record Student(String name, List<Course> enrolled) {}

Course#toString需求 Student#toString和 Student#toString需求 Course#toString .因此，如果您尝试打印 Student那么全部报名 Course将一起打印。那些 Course s 需求 Student所以他们会打印 Student里面。这个循环会一直持续到你得到 StackOverflow异常(exception)。

为避免这种情况，您应该重新排列代码，使它们不相互依赖。您可以为每个实例创建一个 ID。一个例子是:

public record Course(String name, String studentId) {}
public record Student(String name, String studentId, List<Course> courses) {}