Java NullPointerException&;具有继承性和子类的IOStream突然闭包
之前有一个简短的背景,这样我们就可以在相同的波长上进行交流。我已经有大约8-10个大学编程课程,从数据结构到所有语言,到java和C++等特定的课程。我有点生疏了,因为我通常从编码开始休息2-3个月。这是我两年前开始考虑的一个个人项目 好的,具体到细节,还有一个特定的问题,我的变异函数有问题。似乎我试图错误地访问私有变量。问题是,我是否嵌套了太多的类,并试图以错误的方式变异基类变量。如果是这样,请为我指出正确的文献,或确认这是我的问题,以便我可以重新研究这些信息。谢谢Java NullPointerException&;具有继承性和子类的IOStream突然闭包,java,io,nullpointerexception,class-hierarchy,Java,Io,Nullpointerexception,Class Hierarchy,之前有一个简短的背景,这样我们就可以在相同的波长上进行交流。我已经有大约8-10个大学编程课程,从数据结构到所有语言,到java和C++等特定的课程。我有点生疏了,因为我通常从编码开始休息2-3个月。这是我两年前开始考虑的一个个人项目 好的,具体到细节,还有一个特定的问题,我的变异函数有问题。似乎我试图错误地访问私有变量。问题是,我是否嵌套了太多的类,并试图以错误的方式变异基类变量。如果是这样,请为我指出正确的文献,或确认这是我的问题,以便我可以重新研究这些信息。谢谢 package Groce
package GroceryReceiptProgram;
import java.io.*;
import java.util.Vector;
public class Date {
private int hour, minute, day, month, year;
Date() {
try {
BufferedReader keyboard = new BufferedReader(new InputStreamReader(System.in));
System.out.println("What's the hour? (Use 1-24 military notation");
hour = Integer.parseInt(keyboard.readLine());
System.out.println("what's the minute? ");
minute = Integer.parseInt(keyboard.readLine());
System.out.println("What's the day of the month?");
day = Integer.parseInt(keyboard.readLine());
System.out.println("Which month of the year is it, use an integer");
month = Integer.parseInt(keyboard.readLine());
System.out.println("What year is it?");
year = Integer.parseInt(keyboard.readLine());
keyboard.close();
} catch (IOException e) {
System.out.println("Yo houston we have a problem");
}
}
public void setHour(int hour) {
this.hour = hour;
}
public void setHour() {
try {
BufferedReader keyboard = new BufferedReader(new InputStreamReader(System.in));
System.out.println("What hour, use military notation?");
this.hour = Integer.parseInt(keyboard.readLine());
keyboard.close();
} catch (NumberFormatException e) {
System.out.println(e.toString() + ":doesnt seem to be a number");
} catch (IOException e) {
System.out.println(e.toString());
}
}
public int getHour() {
return hour;
}
public void setMinute(int minute) {
this.minute = minute;
}
public void setMinute() {
try (BufferedReader keyboard = new BufferedReader(new InputStreamReader(System.in))) {
System.out.println("What minute?");
this.minute = Integer.parseInt(keyboard.readLine());
} catch (NumberFormatException e) {
System.out.println(e.toString() + ": doesnt seem to be a number");
} catch (IOException e) {
System.out.println(e.toString() + ": minute shall not cooperate");
} catch (NullPointerException e) {
System.out.println(e.toString() + ": in the setMinute function of the Date class");
}
}
public int getMinute() {
return minute;
}
public void setDay(int day) {
this.day = day;
}
public void setDay() {
try {
BufferedReader keyboard = new BufferedReader(new InputStreamReader(System.in));
System.out.println("What day 0-6?");
this.day = Integer.parseInt(keyboard.readLine());
keyboard.close();
} catch (NumberFormatException e) {
System.out.println(e.toString() + ":doesnt seem to be a number");
} catch (IOException e) {
System.out.println(e.toString());
}
}
public int getDay() {
return day;
}
public void setMonth(int month) {
this.month = month;
}
public void setMonth() {
try {
BufferedReader keyboard = new BufferedReader(new InputStreamReader(System.in));
System.out.println("What month 0-11?");
this.month = Integer.parseInt(keyboard.readLine());
keyboard.close();
} catch (NumberFormatException e) {
System.out.println(e.toString() + ":doesnt seem to be a number");
} catch (IOException e) {
System.out.println(e.toString());
}
}
public int getMonth() {
return month;
}
public void setYear(int year) {
this.year = year;
}
public void setYear() {
try {
BufferedReader keyboard = new BufferedReader(new InputStreamReader(System.in));
System.out.println("What year?");
this.year = Integer.parseInt(keyboard.readLine());
keyboard.close();
} catch (NumberFormatException e) {
System.out.println(e.toString() + ":doesnt seem to be a number");
} catch (IOException e) {
System.out.println(e.toString());
}
}
public int getYear() {
return year;
}
public void set() {
setMinute();
setHour();
setDay();
setMonth();
setYear();
}
public Vector<Integer> get() {
Vector<Integer> holder = new Vector<Integer>(5);
holder.add(hour);
holder.add(minute);
holder.add(month);
holder.add(day);
holder.add(year);
return holder;
}
};
他们的父类(正确的名称是什么?)
最后是主类,这样我可以访问所有这些工作
package GroceryReceiptProgram;
public class NewMain {
public static void main(String[] args) {
FoodGroup test = new FoodGroup();
}
}
如果有人对此感兴趣,这里有一个UML链接
在使用字段之前,您需要实例化它们。您的
FoodGroup
构造函数中有一个问题,您尚未初始化一些实例变量,因此它们采用null
默认值,这会给您带来问题:
public FoodGroup() {
//...
//null pointer exception here!
expirationDate.set();
//null pointer exception here!
purchaseDate.set();
//null pointer exception here!
location.set();
//...
}
这个问题可以通过在使用变量之前创建这些变量来解决:
public FoodGroup() {
//...
expirationDate = new Date();
//not null pointer exception!
expirationDate.set();
//similar to the other cases
//...
}
对本项目和未来项目的一些建议:
- 不要创建名称与JavaAPI中的类相似的类。例如,您的
类很容易被误解为Date
java.util.Date
- 不要使用
类,而是使用Vector
。更多信息请点击这里:和(这是来自)List List=new ArrayList()
- 尝试为类使用公共构造函数。在您的情况下,您的程序将编译并运行,因为您的所有类都在同一个包中,但如果它们在不同的包中,则会出现问题,因为几乎所有构造函数都具有
默认的
可见性,并且只能由相同包的类看到。请注意:
不是父类,它是1)FoodGroup
和FoodGroup
,2)Date
和FoodGroup
之间的“has-a”关系。它可以被看作是控制两个类实例的行为和活动的控制器Location
- 当您在此处或其他站点发布代码时,请发布一个而不是整个代码。这样,任何人都会更容易帮助你
public FoodGroup() {
try {
setPrice();
setCount();
expirationDate.set(); // expiration date is null here
public FoodGroup() {
//...
//null pointer exception here!
expirationDate.set();
//null pointer exception here!
purchaseDate.set();
//null pointer exception here!
location.set();
//...
}
public FoodGroup() {
//...
expirationDate = new Date();
//not null pointer exception!
expirationDate.set();
//similar to the other cases
//...
}