为什么Scanner.next()和Scanner.nextLine()会抛出NullPointerException?

`Scanner.next()` 和 `Scanner.nextLine()` 方法可能会抛出 `NullPointerException` 是因为在使用它们之前，如果没有正确初始化 `Scanner` 对象，或者尝试从空的输入流（如关闭了的文件、网络连接中断等）读取数据时，就会发生这种情况。

具体来说：

1. **未初始化Scanner对象**：如果你忘记调用 `Scanner scanner = new Scanner(System.in);` 或其他适当的输入源来创建一个 `Scanner` 实例，然后直接调用 `next()` 或 `nextLine()`，你会得到一个 `NullPointerException`，因为这些方法是在实例化后的对象上调用的。

2. **空流**：如果 `Scanner` 的输入源被设置为 `null` 或者已经关闭（例如，当你关闭了 `System.in` 或其他输入流），那么再次尝试从中读取数据也会抛出异常。

为了避免这个错误，你应该确保在使用 `Scanner` 方法前已经正确地初始化了它，并检查输入流是否有效。例如：

Scanner scanner = new Scanner(System.in);
if (scanner.hasNext()) {
    String input = scanner.nextLine();
} else {
    System.out.println("Input stream is empty or null.");
}

相关问题

public class Demo { public static void main(String[] args) { Scanner sc = new Scanner(System.in); String s = sc.nextLine(); Map<Character, Integer> m = new HashMap<>(); for (int i = 0; i < s.length() - 1; i++) { char c = s.charAt(i); if (m.get(c) != null) { int value = m.get(c); m.put(c, (value + 1)); } else { m.put(c, 1); } } Set<Map.Entry<Character,Integer>> entries=m.entrySet(); for(Map.Entry<Character,Integer> map1:entries){ System.out.println(map1.getKey()+"出现了"+map1.getValue()+"次"); } } }这个代码有什么问题

这段代码有个潜在的问题就是只遍历了字符串中的前s.length()-1个字符，因此会漏掉最后一个字符。应该将for循环的条件改为 i < s.length()：

for (int i = 0; i < s.length(); i++) {
    // ...
}

此外，代码中没有对输入的字符串进行空指针判断，如果输入的字符串为null，将会抛出NullPointerException异常。可以在读取输入时进行判断：

String s = sc.nextLine();
if (s == null) {
    // 处理空输入的情况
}

package demo; import java.util.HashMap; import java.util.Scanner; public class exercise333 { private static Scanner scanner; /** * 学生信息集合 */ private static HashMap<String,Student> map; public static void main(String[] args) { // TODO Auto-generated method stub select(); } private static void select() { System.out.println("请输入学号："); Scanner sc = new Scanner(System.in); String nub = sc.nextLine(); Student student = map.get(nub); if(student == null) System.out.println("没有查询到该学生！"); else System.out.println(student.toString()); } } class Student{ private String sno; /** * 学号 */ private String name; /** * 姓名 */ private String major; /** * 专业 */ private String score; /** * 成绩 */ public String toString() { return "学生信息{"+ "学号='" + sno + '\'' + "姓名='" + name + '\'' + "专业='" + major + '\'' + "成绩='" + score + '\'' + '}'; } public String getSno() { return sno; } public void setSno(String sno) { this.sno = sno; } public String getName() { return name ; } public void setName(String name) { this.name = name; } public String getMajor() { return major; } public void setMajor(String major) { this.major = major; } public String getScore() { return score; } public void setScore(String score) { this.score =score; } public Student(String sno,String name,String major,String score) { this.sno = sno; this.name = name; this.major = major; this.score = score; } }

There are a few issues with the provided code:

1. The `map` object is not initialized, which will cause a `NullPointerException` when using the `get` method. To fix this, initialize the map object before using it:

private static HashMap<String,Student> map = new HashMap<String,Student>();

2. The `toString` method in the `Student` class is missing some plus signs to concatenate the strings. Fix it by adding the plus signs:

public String toString() {
    return "学生信息{" +
            "学号='" + sno + '\'' +
            ", 姓名='" + name + '\'' +
            ", 专业='" + major + '\'' +
            ", 成绩='" + score + '\'' +
            '}';
}

3. The `score` field is declared as `String`, which is not a suitable type for storing scores. Change it to a numeric type, such as `int` or `double`.

4. The `setScore` method has a typo and should be `this.score = score;`

5. It's a good practice to use `try-catch` blocks when reading from input, as the input could be incorrect and cause exceptions. For example, using `Integer.parseInt()` to get an integer from input could throw a `NumberFormatException`, and using `scanner.nextLine()` could throw a `NoSuchElementException` if there is no more input available.

Here is the fixed code with these issues addressed:

package demo;

import java.util.HashMap;
import java.util.Scanner;

public class Exercise333 {
    private static Scanner scanner;
    /** 学生信息集合 */
    private static HashMap<String, Student> map = new HashMap<String, Student>();

    public static void main(String[] args) {
        // TODO Auto-generated method stub
        select();
    }

    private static void select() {
        System.out.println("请输入学号：");
        Scanner sc = new Scanner(System.in);
        String nub = sc.nextLine();

        try {
            Student student = map.get(nub);
            if (student == null)
                System.out.println("没有查询到该学生！");
            else
                System.out.println(student.toString());
        } catch (Exception e) {
            System.out.println("输入有误！");
        } finally {
            sc.close();
        }
    }
}

class Student {
    private String sno; /** 学号 */
    private String name; /** 姓名 */
    private String major; /** 专业 */
    private int score; /** 成绩 */

    public String toString() {
        return "学生信息{" +
                "学号='" + sno + '\'' +
                ", 姓名='" + name + '\'' +
                ", 专业='" + major + '\'' +
                ", 成绩='" + score + '\'' +
                '}';
    }

    public String getSno() {
        return sno;
    }

    public void setSno(String sno) {
        this.sno = sno;
    }

    public String getName() {
        return name;
    }

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

    public String getMajor() {
        return major;
    }

    public void setMajor(String major) {
        this.major = major;
    }

    public int getScore() {
        return score;
    }

    public void setScore(int score) {
        this.score = score;
    }

    public Student(String sno, String name, String major, int score) {
        this.sno = sno;
        this.name = name;
        this.major = major;
        this.score = score;
    }
}