Question

我正在为牛顿迭代做一个JAVA项目，但我仍然坚持如何让程序不断询问用户是否要继续进行。以及如何确保当用户输入负数时，结果未显示根是用户输入的负数。

import java.util.Scanner;

/**
* Computes estimate of square root of x to within relative error 0.01%.
* 
* @param x
*            positive number to compute square root of
* @return estimate of square root
*/

public final class Newton1 {
    private static double sqrt(double x) {
        if (x < 0) {
            System.out.println("Please enter a postive number.");
            return x;
        }
        double Error = 0.0001;
        double t = x;
        while (Math.abs(t - x / t) > Error * t) {
            t = (x / t + t) / 2.0;
        }
        return t;
    }

    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        System.out.println("Please enter a number: ");
        double x = in.nextDouble();
        System.out.println("Square root is " + sqrt(x));
        System.out.println("Do you want to continue? (Enter Y or N): ");
        String f = in.nextLine();
        while (true) {
            if (f.equalsIgnoreCase("Y")) {
            } else {
                break;
            }
        }
    }
}

Answer 1

您的Exception [EclipseLink-41] (Eclipse Persistence Services - 2.5.2.v20140319-9ad6abd): org.eclipse.persistence.exceptions.DescriptorException Exception Description: A non-read-only mapping must be defined for the sequence number field. Descriptor: RelationalDescriptor(model.Exercises --> [DatabaseTable(weighttrack.exercises)]) Runtime Exceptions: --------------------------------------------------------- at org.eclipse.persistence.internal.jpa.EntityManagerSetupImpl.createDeployFailedPersistenceException(EntityManagerSetupImpl.java:820) at org.eclipse.persistence.internal.jpa.EntityManagerSetupImpl.deploy(EntityManagerSetupImpl.java:760) at org.eclipse.persistence.internal.jpa.EntityManagerFactoryDelegate.getAbstractSession(EntityManagerFactoryDelegate.java:204) at org.eclipse.persistence.internal.jpa.EntityManagerFactoryDelegate.createEntityManagerImpl(EntityManagerFactoryDelegate.java:304) at org.eclipse.persistence.internal.jpa.EntityManagerFactoryImpl.createEntityManagerImpl(EntityManagerFactoryImpl.java:336) at org.eclipse.persistence.internal.jpa.EntityManagerFactoryImpl.createEntityManager(EntityManagerFactoryImpl.java:302) at model.User.initComponents(User.java:39) at model.User.<init>(User.java:23) at model.Dashboard.initComponents(Dashboard.java:35) at model.Dashboard.<init>(Dashboard.java:18) at model.Dashboard$3.run(Dashboard.java:153) at java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:311) at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:756) at java.awt.EventQueue.access$500(EventQueue.java:97) at java.awt.EventQueue$3.run(EventQueue.java:709) at java.awt.EventQueue$3.run(EventQueue.java:703) at java.security.AccessController.doPrivileged(Native Method) at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:80) at java.awt.EventQueue.dispatchEvent(EventQueue.java:726) at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:201) at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:116) at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:105) at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:101) at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:93) at java.awt.EventDispatchThread.run(EventDispatchThread.java:82) Caused by: Exception [EclipseLink-28019] (Eclipse Persistence Services - 2.5.2.v20140319-9ad6abd): org.eclipse.persistence.exceptions.EntityManagerSetupException Exception Description: Deployment of PersistenceUnit [weighttrack?zeroDateTimeBehavior=convertToNullPU] failed. Close all factories for this PersistenceUnit. Internal Exception: Exception [EclipseLink-0] (Eclipse Persistence Services - 2.5.2.v20140319-9ad6abd): org.eclipse.persistence.exceptions.IntegrityException Descriptor Exceptions: --------------------------------------------------------- Exception [EclipseLink-46] (Eclipse Persistence Services - 2.5.2.v20140319-9ad6abd): org.eclipse.persistence.exceptions.DescriptorException Exception Description: There should be one non-read-only mapping defined for the primary key field [weighttrack.exercises.exercise_id]. Descriptor: RelationalDescriptor(model.Exercises --> [DatabaseTable(weighttrack.exercises)]) Exception [EclipseLink-41] (Eclipse Persistence Services - 2.5.2.v20140319-9ad6abd): org.eclipse.persistence.exceptions.DescriptorException Exception Description: A non-read-only mapping must be defined for the sequence number field. Descriptor: RelationalDescriptor(model.Exercises --> [DatabaseTable(weighttrack.exercises)])声明似乎位于错误的位置。试试这个：

while

请注意，在用户输入非负数之前，系统会继续提示他们输入非负数。

Java：如何反复询问用户Y或N.

1 个答案: