java - 带有继承和子类的 Nullpointerexcption 和突然的 IOStream 闭包

Question

之前有一个简短的背景，以便我们可以在相同的波长上进行交流。我有大约 8-10 门大学课程，从数据结构到所有语言的编程课程，再到特定的课程，如 java 和 c++。我有点生疏，因为我通常会从编码中休息 2-3 个月。这是我两年前开始考虑的个人项目。

好的，具体到细节和一个具体问题，我的 mutator 函数有问题。似乎是我试图错误地访问私有变量。问题是，我是否过多地嵌套了我的类并试图以不正确的方式改变基类变量。如果是这样，请指出正确的文献，或者确认这是我的问题，以便我可以重新研究这些信息。谢谢

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;
    }
};

那显然是 Date 类，接下来是另一个基类 Location。

package GroceryReceiptProgram;

    import java.io.*;
    import java.util.Vector;

    public class Location {

        String streetName, state, city, country;
        int zipCode, address;

        Location() {
            try {
                BufferedReader keyboard = new BufferedReader(new InputStreamReader(System.in));
                System.out.println("What is the street name");
                streetName = keyboard.readLine();
                System.out.println("Which state?");
                state = keyboard.readLine();
                System.out.println("Which city?");
                city = keyboard.readLine();
                System.out.println("Which country?");
                country = keyboard.readLine();
                System.out.println("Which zipcode?");//if not u.s. continue around this step
                zipCode = Integer.parseInt(keyboard.readLine());
                System.out.println("What address?");
                address = Integer.parseInt(keyboard.readLine());
            } catch (IOException e) {
                System.out.println(e.toString());
            }
        }

        public void setZipCode(int zipCode) {
            this.zipCode = zipCode;
        }

        public void setZipCode() {
            try {
                BufferedReader keyboard = new BufferedReader(new InputStreamReader(System.in));
                System.out.println("What zipCode?");
                this.zipCode = 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 void set() {
            setAddress();
            setCity();
            setCountry();
            setState();
            setStreetName();
            setZipCode();
        }

        public int getZipCode() {
            return zipCode;
        }

        public void setAddress(int address) {
            this.address = address;
        }

        public void setAddress() {
            try {
                BufferedReader keyboard = new BufferedReader(new InputStreamReader(System.in));
                System.out.println("What minute?");
                this.address = 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 getAddress() {
            return address;
        }

        public void setStreetName(String streetName) {
            this.streetName = streetName;
        }

        public void setStreetName() {
            try {
                BufferedReader keyboard = new BufferedReader(new InputStreamReader(System.in));
                System.out.println("What minute?");
                this.streetName = keyboard.readLine();
                keyboard.close();
            } catch (IOException e) {
                System.out.println(e.toString());
            }
        }

        public String getStreetName() {
            return streetName;
        }

        public void setState(String state) {
            this.state = state;
        }

        public void setState() {
            try {
                BufferedReader keyboard = new BufferedReader(new InputStreamReader(System.in));
                System.out.println("What minute?");
                this.state = keyboard.readLine();
                keyboard.close();
            } catch (IOException e) {
                System.out.println(e.toString());
            }
        }

        public String getState() {
            return state;
        }

        public void setCity(String city) {
            this.city = city;
        }

        public void setCity() {
            try {
                BufferedReader keyboard = new BufferedReader(new InputStreamReader(System.in));
                System.out.println("What minute?");
                this.city = keyboard.readLine();
                keyboard.close();
            } catch (IOException e) {
                System.out.println(e.toString());
            }
        }

        public String getCity() {
            return city;
        }

        public void setCountry(String country) {
            this.country = country;
        }

        public void setCountry() {
            try {
                BufferedReader keyboard = new BufferedReader(new InputStreamReader(System.in));
                System.out.println("What minute?");
                this.country = keyboard.readLine();
                keyboard.close();
            } catch (IOException e) {
                System.out.println(e.toString());
            }
        }

        public String getCountry() {
            return country;
        }
    };

他们的父母（什么是正确的名字？）类

package GroceryReceiptProgram;

    import java.io.*;

    public class FoodGroup {

        private int price, count;
        private Date purchaseDate, expirationDate;
        private Location location;
        private String name;

        public FoodGroup() {
            try {
                setPrice();
                setCount();
                expirationDate.set();
                purchaseDate.set();
                location.set();
            } catch (NullPointerException e) {
                System.out.println(e.toString() + ": in the constructor of the FoodGroup class");
            }
        }

        public void setPrice(int price) {
            this.price = price;
        }

        public void setPrice() {
            try (BufferedReader keyboard = new BufferedReader(new InputStreamReader(System.in))) {
                System.out.println("What Price?");
                price = 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() + ": in the FoodGroup class, setPrice function");
            } catch (NullPointerException e) {
                System.out.println(e.toString() + ": in FoodGroup class. SetPrice()");
            }
        }

        public int getPrice() {
            return price;
        }

        public void setCount(int count) {
            this.count = count;
        }

        public void setCount() {
            try (BufferedReader keyboard = new BufferedReader(new InputStreamReader(System.in))) {
                System.out.println("What count?");
                count = 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() + ": in the FoodGroup class, setCount()");
            } catch (NullPointerException e) {
                System.out.println(e.toString() + ": in FoodGroup class, setCount");
            }
        }

        public int getCount() {
            return count;
        }

        public void setName(String name) {
            this.name = name;
        }

        public void setName() {
            try {
                BufferedReader keyboard = new BufferedReader(new InputStreamReader(System.in));
                System.out.println("What minute?");
                this.name = keyboard.readLine();

            } catch (IOException e) {
                System.out.println(e.toString());
            }
        }

        public String getName() {
            return name;
        }

        public void setLocation(Location location) {
            this.location = location;
        }

        public Location getLocation() {
            return location;
        }

        public void setPurchaseDate(Date purchaseDate) {
            this.purchaseDate = purchaseDate;
        }

        public void setPurchaseDate() {
            this.purchaseDate.set();
        }

        public Date getPurchaseDate() {
            return purchaseDate;
        }

        public void setExpirationDate(Date expirationDate) {
            this.expirationDate = expirationDate;
        }

        public void setExpirationDate() {
            this.expirationDate.set();
        }

        public Date getExpirationDate() {
            return expirationDate;
        }
    }

最后是主类，所以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();
        }
    }

如果有人有进一步的兴趣，这里是 UML 的链接。

https://www.dropbox.com/s/1weigjnxih70tbv/GRP.dia

score 2 · Accepted Answer

你的构造函数有问题FoodGroup，你没有初始化一些实例变量，所以它们取null默认值，给你带来问题：

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它们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
    //...
}

对这个和未来项目的一些建议：

不要创建与 Java API 中的类具有相似名称的类。例如，您的Date课程很容易被java.util.Date.
不要使用Vector类，而是使用List<YourClass> list = new ArrayList<YourClass>(). 更多信息：ArrayList 和 Vector 之间有什么区别？为什么 Java Vector类被认为已过时或不推荐使用？（这个来自Jon Skeet）。
尝试为您的类使用公共构造函数。在您的情况下，您的程序将编译和运行只是因为您的所有类都在同一个包中，但是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default可见性并且只能被以下类看到同一个包。请注意：控制对类成员的访问
FoodGroup不是父类，它是 1)FoodGroup和Date, 2)FoodGroup和之间的“具有”关系Location。它可以被看作是一个控制两个类实例的行为和生存的控制器。
When you post code here or other sites, please post a SSCCE instead of your whole code. In that way, it will be easier for anyone to help you.

score 0 · Accepted Answer

    public FoodGroup() {
        try {
            setPrice();
            setCount();
            expirationDate.set(); // expiration date is null here

您需要在使用它们之前实例化您的字段。

java - 带有继承和子类的 Nullpointerexcption 和突然的 IOStream 闭包

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