Java I'；我在扫描字符串时遇到困难

我希望用户键入一个单词，以便将其分配给

String day

。输出是

DaysOfTheWeek.main（DaysOfTheWeek.java:15）处线程“main”java.lang.NullPointerException中的异常

---

您没有初始化

scan

对象，因此会出现空指针异常。你应该按照这里的说明去做

您尚未初始化扫描仪

此外，不需要将此扫描仪用作静态扫描仪。最好用这样的主方法初始化扫描仪

import java.util.*;
public class DaysOfTheWeek {

public static void main(String[] args) {



    System.out.println("What day is it today?");
    Scanner scan = new Scanner(System.in);
    String day = scan.next();
    int dayNumber = 0;

    switch (day.toLowerCase()) {
        case "sunday":
            dayNumber = 0;
            break;
        case "monday":
                dayNumber = 1;
            break;
        case "tuesday":
                dayNumber = 2;
            break;
        case "wednesday":
                dayNumber = 3;
            break;
        case "thursday":
                dayNumber = 4;
            break;
        case "friday":
                dayNumber = 5;
            break;
        case "saturday":
                dayNumber = 6;
            break;

    }//switch

    import java.util.*;
public class DaysOfTheWeek {

public static void main(String[] args) {



    System.out.println("What day is it today?");
    String day = new Scanner(System.in).next();


    int dayNumber = 0;

    switch (day.toLowerCase()) {
        case "sunday":
            dayNumber = 0;
            break;
        case "monday":
                dayNumber = 1;
            break;
        case "tuesday":
                dayNumber = 2;
            break;
        case "wednesday":
                dayNumber = 3;
            break;
        case "thursday":
                dayNumber = 4;
            break;
        case "friday":
                dayNumber = 5;
            break;
        case "saturday":
                dayNumber = 6;
            break;

    }//switch

记住，如果你想使用任何像扫描仪这样的物体一次，像这样使用会更好更容易

import java.util.*;
public class DaysOfTheWeek {

public static void main(String[] args) {



    System.out.println("What day is it today?");
    Scanner scan = new Scanner(System.in);
    String day = scan.next();
    int dayNumber = 0;

    switch (day.toLowerCase()) {
        case "sunday":
            dayNumber = 0;
            break;
        case "monday":
                dayNumber = 1;
            break;
        case "tuesday":
                dayNumber = 2;
            break;
        case "wednesday":
                dayNumber = 3;
            break;
        case "thursday":
                dayNumber = 4;
            break;
        case "friday":
                dayNumber = 5;
            break;
        case "saturday":
                dayNumber = 6;
            break;

    }//switch

    import java.util.*;
public class DaysOfTheWeek {

public static void main(String[] args) {



    System.out.println("What day is it today?");
    String day = new Scanner(System.in).next();


    int dayNumber = 0;

    switch (day.toLowerCase()) {
        case "sunday":
            dayNumber = 0;
            break;
        case "monday":
                dayNumber = 1;
            break;
        case "tuesday":
                dayNumber = 2;
            break;
        case "wednesday":
                dayNumber = 3;
            break;
        case "thursday":
                dayNumber = 4;
            break;
        case "friday":
                dayNumber = 5;
            break;
        case "saturday":
                dayNumber = 6;
            break;

    }//switch

因此，main方法中的代码也执行相同的操作

String day = new Scanner(System.in).next();