Java 简单计算返回空指针异常。。。为什么?
(我是Java新手,请原谅我的无知) 我有一个非常简单的课程:Java 简单计算返回空指针异常。。。为什么?,java,nullpointerexception,Java,Nullpointerexception,(我是Java新手,请原谅我的无知) 我有一个非常简单的课程: public class Bill { private String Total; private int Tip; private int People; Bill() { Total = "0"; Tip=0; People=0; } Bill (String total, int people, int tip) {
public class Bill {
private String Total;
private int Tip;
private int People;
Bill() {
Total = "0";
Tip=0;
People=0;
}
Bill (String total, int people, int tip) {
Total = total;
Tip = tip;
People = people;
}
//the problem is here somewhere.
private Double Split = (Double.parseDouble(Total) * ((double)Tip / 100)) / (double)People;
public String getTotal() {
return Total;
}
public double getSplit() {
return Split;
}
public double getTip() {
return Tip;
}
}
当我调用“getSplit()”方法时,运行时会崩溃,出现NullPointer异常:
09-03 19:37:02.609: E/AndroidRuntime(11325): Caused by: java.lang.NullPointerException
09-03 19:37:02.609: E/AndroidRuntime(11325): at java.lang.StringToReal.parseDouble(StringToReal.java:244)
09-03 19:37:02.609: E/AndroidRuntime(11325): at java.lang.Double.parseDouble(Double.java:295)
09-03 19:37:02.609: E/AndroidRuntime(11325): at williamgill.de.helloworld.Bill.<init>(Bill.java:21)
09-03 19:37:02.609: E/AndroidRuntime(11325): at williamgill.de.helloworld.DisplayMessageActivity.onCreate(DisplayMessageActivity.java:38)
09-0319:37:02.609:E/AndroidRuntime(11325):由以下原因引起:java.lang.NullPointerException
09-03 19:37:02.609:E/AndroidRuntime(11325):位于java.lang.StringToReal.parseDouble(StringToReal.java:244)
09-03 19:37:02.609:E/AndroidRuntime(11325):在java.lang.Double.parseDouble(Double.java:295)
09-03 19:37:02.609:E/AndroidRuntime(11325):在williamgill.de.helloworld.Bill。(Bill.java:21)
09-03 19:37:02.609:E/AndroidRuntime(11325):在williamgill.de.helloworld.DisplayMessageActivity.onCreate(DisplayMessageActivity.java:38)
我似乎在类型转换方面犯了一些明显的错误,但我一辈子都想不出是什么原因。在这一行执行时
private Double Split = (Double.parseDouble(Total) * ((double)Tip / 100)) / (double)People;
构造函数尚未初始化字段Total
和其他字段,其默认值为null
parseDouble()
必须抛出NullPointerException
如果其参数为null
(我无法验证这一点,它也可能来自解引用)
声明该字段
private Double Split;
并在构造函数中进行赋值
Bill (String total, int people, int tip) {
Total = total;
Tip = tip;
People = people;
Split = (Double.parseDouble(Total) * ((double)Tip / 100)) / (double)People;
}
建议:Java非常量实例变量应以小写字母开头。执行此行时
private Double Split = (Double.parseDouble(Total) * ((double)Tip / 100)) / (double)People;
构造函数尚未初始化字段Total
和其他字段,其默认值为null
parseDouble()
必须抛出NullPointerException
如果其参数为null
(我无法验证这一点,它也可能来自解引用)
声明该字段
private Double Split;
并在构造函数中进行赋值
Bill (String total, int people, int tip) {
Total = total;
Tip = tip;
People = people;
Split = (Double.parseDouble(Total) * ((double)Tip / 100)) / (double)People;
}
建议:Java非常量实例变量应该以小写字母开头。另一个建议:构造函数应该是
public
。根据Java惯例,变量和方法一样,应该以小写字母开头。谢谢-这解决了我的问题。一旦你这样说就完全有道理了……:)再次感谢@威尔吉尔:不客气。请注意注释中给出的建议并遵循Java惯例。说到大写字母,Double和Double之间有区别吗?另一个建议:构造函数应该是public
。变量,比如方法,根据Java约定,应该以小写字母开头。谢谢-这解决了我的问题。一旦你这样说就完全有道理了……:)再次感谢@威尔吉尔:不客气。请注意注释中给出的建议并遵循Java惯例。说到大写字母,Double和Double之间有区别吗?