Question

我有一个问题，我发送字符串变量，从主题，消息和附件到服务器，当我把它们放在列表中时，消息变量总是为空！我已经输出了变量消息，它提出了它应该的但是，但是只要我把它放在列表中。它显示为null。

private void doSend(String name)
{
    String to = input.nextLine();
    String from = input.nextLine();
    String subject = input.nextLine();
    String message = input.nextLine();
    String attachment = input.nextLine();        

    System.out.println(to);
    System.out.println(from);
    System.out.println(subject);
    System.out.println(message);
    System.out.println(attachment);        

    // stores the message, but not into the mailbox        
    MultiEchoServer.MailBox.add(new Email(to, from,subject, message, attachment));

    System.out.println(MailBox);

    System.out.println("Message Sent to: " + to);
    System.out.println(message);
}

示例输出

pj     // this is the to variable

dsds   // this is the from variable

subject  // this is the subject variable

message  // this is the message variable

[pj dsds subject null]  //this is the Mailbox List

Message Sent to: pj //not part of the error

message // this is the message variable being outputted again to see it it changed

我甚至不确定是否有人可以帮助我，但如果您需要查看更多代码，请告诉我们！

电子邮件类

class Email
{
    private String to, from, subject,  message, attachment;
    int id;        

    public Email(String to ,String from ,String subject, String message, String attachment)
    {
        this.to = to;
        this.from = from;
        this.subject = subject;
        this.message = message;
        this.message = attachment;
    }

    public int id()
    {   
        return(id);
    }

    public String to()
    {
        return(to);
    }

    public String from()
    {   
        return(from);
    }

    public String subject()
    {
        return(subject);
    }

    public String message()
    {
        return(message);
    }
    public String attachment()
    {
        return(attachment);
    }

    public String toString()
    {
        return(to + " " + from + " " + subject + " " + message + "" + attachment);
    }
}

Answer 1

Email班constructor存在问题。您使用message和message两次分配attachment字段。

Answer 2

有两件事正在发生。正如@Dilip指出的那样，构造函数初始化中存在错误。

此外，self.firstField.placeholder = @"First"; self.firstField.textAlignment = NSTextAlignmentCenter; self.firstField.font = [UIFont fontWithName:@"Helvetica Neue Light" size:50.0]; self.secondField.placeholder = @"Second"; self.secondField.textAlignment = NSTextAlignmentCenter; self.secondField.font = [UIFont fontWithName:@"Helvetica Neue Light" size:50.0]; self.thirdField.placeholder = @"Third"; self.thirdField.textAlignment = NSTextAlignmentCenter; self.thirdField.font = [UIFont fontWithName:@"Helvetica Neue Light" size:50.0]; [self.scrollView addSubview:self.firstField]; [self.firstField autoPinEdgesToSuperviewEdgesWithInsets:UIEdgeInsetsZero excludingEdge:ALEdgeRight]; [self.firstField autoMatchDimension:ALDimensionWidth toDimension:ALDimensionWidth ofView:self.scrollView]; [self.firstField autoMatchDimension:ALDimensionHeight toDimension:ALDimensionHeight ofView:self.scrollView]; [self.scrollView addSubview:self.secondField]; [self.secondField autoPinEdge:ALEdgeLeft toEdge:ALEdgeRight ofView:self.firstField]; [self.secondField autoMatchDimension:ALDimensionWidth toDimension:ALDimensionWidth ofView:self.scrollView]; [self.secondField autoMatchDimension:ALDimensionHeight toDimension:ALDimensionHeight ofView:self.scrollView]; [self.scrollView addSubview:self.thirdField]; [self.thirdField autoPinEdge:ALEdgeLeft toEdge:ALEdgeRight ofView:self.secondField]; [self.thirdField autoPinEdgesToSuperviewEdgesWithInsets:UIEdgeInsetsZero excludingEdge:ALEdgeLeft]; [self.thirdField autoMatchDimension:ALDimensionWidth toDimension:ALDimensionWidth ofView:self.scrollView]; [self.thirdField autoMatchDimension:ALDimensionHeight toDimension:ALDimensionHeight ofView:self.scrollView]; self.automaticallyAdjustsScrollViewInsets = NO;显然是一个空字符串。

因此，在ctor中，字符串字段attachment被分配一个空字符串，而字符串字段message未被初始化，因此attachment。

在Java中，null打印空值作为字符串文字“null”。

“”打印为局部变量toString()的值，“null”打印为局部变量message的值。

这说明了为什么对局部变量，函数参数和字段使用相同的名称可能是危险的。

此外，根据您的实现，使用String作为附件之类的东西，可能是也可能不是String，对于这种抽象可能是一个糟糕的选择。

不添加到List的字符串 - Java

2 个答案: