Question

我正在为我正在上的一门计算机科学课程做作业，我试图在一行中声明3个String数组，但是它一直给我这个错误消息

java.lang.NullPointerException

我弄乱了一点，并通过更改

解决了问题。

public static String[] offence,name,date = new String[8];

到

public static String[] offence,name = new String[8];

public static String[] date = new String[8];

为什么只有当我这样做时它才起作用？

PS，我是新手，所以请放轻松。

    package pDatabaseApp;

    import java.io.*;

    public class PDatabaseMenu {

    public static String[] offence,name = new String[8];
    public static String[] date = new String[8];

    public static void main(String[] args) throws IOException {

        String line;

        BufferedReader in;
        in = new BufferedReader(new InputStreamReader(System.in));

        boolean finnish = false, reenter = false;

        while(finnish == false) {

            int i;

            for(i = 0; i <= 7; i++) {

                System.out.println("enter a name");

                name[i] = in.readLine();

                System.out.println("enter a day");

                String day = in.readLine();

                System.out.println("enter a month");

                String month = in.readLine();

                System.out.println("enter a year");

                String year = in.readLine();

                date[i] = day + "/" + month + "/" + year;

                String offenceEnter[] = new String[3];
                String offenceType[] = {"Assault","Arson","Theft"};

                int l;
                for(l = 0; l <= 2; l++) {

                    System.out.println("is there offence " + offenceType[l]);
                    offenceEnter[l] = in.readLine();

                    if(offenceEnter[l] == "yes") {

                        offence[i] = offenceType[l];

                    }
                }

            }

        }

        in.close();
    }
}

Answer 1

记住代码

public static String[] offence,name,date = new String[8];

将仅初始化日期。因此，其他变量保持未初始化状态。结果，您得到了java.lang.NullPointerException。

您需要类似

 public static String[] offence= new String[8],name= new String[8],date = new String[8];

Answer 2

按照上面的代码编写方式，数组攻击最终为null。如果要初始化所有三个变量，则应具有此变量。

public static String[] offence = new String[8],name = new String[8],date = new String[8];

Answer 3

使用冒犯和姓名之间的逗号，

public static String[] offence,name = new String[8];

是以下操作的快捷方式：

public static String[] offence;
public static String[] name = new String[8];

这个问题非常密切相关： Initializing multiple variables to the same value in Java

Answer 4

public static String[] offence,name,date = new String[8];

上面的代码暗示您将试图声明和初始化一个大小为8的String数组，并带有三个指向它的指针：犯罪，姓名和日期。对于可变对象（如数组），通常更有意义的是分别初始化每个对象。

话虽这么说，但我不确定为什么您的修改后的代码行得通，但是我认为它不会按照您希望的方式运行。我建议执行以下操作：

public static String[] offence, name, date;
offence = new String[8];
name = new String[8];
date = new String[8];

希望我能提供帮助！

Answer 5

package pDatabaseApp;

import java.io.*;

public class PDatabaseMenu {

public static String[] offence = new String[8];
public static String[] name = new String[8];
public static String[] date = new String[8];

public static void main(String[] args) throws IOException {

    String line;

    BufferedReader in;
    in = new BufferedReader(new InputStreamReader(System.in));

    boolean finnish = false, reenter = false;

    while(finnish == false) {

        int i;

        for(i = 0; i < 8; i++) {

            System.out.println("enter a name");

            name[i] = in.readLine();

            System.out.println("enter a day");

            String day = in.readLine();

            System.out.println("enter a month");

            String month = in.readLine();

            System.out.println("enter a year");

            String year = in.readLine();

            date[i] = day + "/" + month + "/" + year;

            String offenceEnter[] = new String[3];
            String offenceType[] = {"Assault","Arson","Theft"};

            int l;
            for(l = 0; l < 3; l++) {

                System.out.println("is there offence " + offenceType[l]);
                offenceEnter[l] = in.readLine();

                if(offenceEnter[l].Equals("yes"))//A for each is better but idk if you did that

{

                    offence[i] = offenceType[l];

                }
            }

        }

    }

    in.close();
}
}

不能在一行中声明超过2个数组

5 个答案: