问题

我使用JDK 11.0编译了以下代码：

```java
import java.util.Scanner;
public class inputWithScanner {
    public static void main(String[] args) {
    Scanner input = new Scanner(System.in);
    System.out.println("Enter your name and roll number");
    String name = input.next();
    Integer rollNumber = Integer.parseInt(input.next());
    input.close();
    if (name.isEmpty())
        System.out.println("oops! You didnt enter your name");
    if (rollNumber < 0)
        System.out.println("Enter a valid roll number");
    System.out.println("Hello "+name+" Roll Number   "+rollNumber);
}

并使用以下输入测试了该程序（对于rollNumber变量的混合输入是故意的）：

Enter your name and roll number
Manraj 2018CSE1023

编译器引发了一个异常：NumberFormatException，堆栈跟踪如下：

Exception in thread "main" java.lang.NumberFormatException: For input string: "2018CSE1023"
        at java.base/java.lang.NumberFormatException.forInputString(NumberFormatException.java:65)
        at java.base/java.lang.Integer.parseInt(Integer.java:652)
        at java.base/java.lang.Integer.parseInt(Integer.java:770)
        at inputWithScanner.main(inputWithScanner.java:7)

问题是：
为什么会重复调用parseInt()方法？

我尝试了相同的代码，使用valueOf()方法，并使用相同的输入执行了该程序。编译器引发了相同的异常，堆栈跟踪如下：

Exception in thread "main" java.lang.NumberFormatException: For input string: "2018CSE1023"
        at java.base/java.lang.NumberFormatException.forInputString(NumberFormatException.java:65)
        at java.base/java.lang.Integer.parseInt(Integer.java:652)
        at java.base/java.lang.Integer.valueOf(Integer.java:983)
        at inputWithScanner.main(inputWithScanner.java:7)

这表明valueOf()方法调用了parseInt()方法。但为什么parseInt()调用了自身？

<details>
<summary>英文:</summary>

I compiled the following code with JDK 11.0:

import java.util.Scanner;
public class inputWithScanner {
public static void main(String[] args) {
Scanner input = new Scanner(System.in);
System.out.println("Enter your name and roll number");
String name = input.next();
Integer rollNumber = Integer.parseInt(input.next());
input.close();
if (name.isEmpty())
System.out.println("oops! You didnt enter your name");
if (rollNumber < 0)
System.out.println("Enter a valid roll number");
System.out.println("Hello "+name+" Roll Number "+rollNumber);

and tested the program with the following input (mixed input for the rollNumber variable is deliberate):

Enter your name and roll number
Manraj 2018CSE1023

the compiler raises an exception: NumberFormatException with the stack trace as follows:
```Exception in thread "main" java.lang.NumberFormatException: For input string: "2018CSE1023"
        at java.base/java.lang.NumberFormatException.forInputString(NumberFormatException.java:65)
        at java.base/java.lang.Integer.parseInt(Integer.java:652)
        at java.base/java.lang.Integer.parseInt(Integer.java:770)
        at inputWithScanner.main(inputWithScanner.java:7)

The question is:
Why is there a repeated call for parseInt() method?

I tried same code with valueOf() method and executed the program with same input. The compiler raises the same exception, and stack trace is as follows:

Exception in thread "main" java.lang.NumberFormatException: For input string: "2018CSE1023"
        at java.base/java.lang.NumberFormatException.forInputString(NumberFormatException.java:65)
        at java.base/java.lang.Integer.parseInt(Integer.java:652)
        at java.base/java.lang.Integer.valueOf(Integer.java:983)
        at inputWithScanner.main(inputWithScanner.java:7)

It shows that the valueOf() method calls parseInt() method. But Why parseInt() calls itself?

答案1

得分: 8

parseInt 被定义在这个链接中：

public static int parseInt(String s) throws NumberFormatException {
    return parseInt(s, 10); // 这是当前的第 770 行
}

然后在第 652 行（在内部调用parseInt的地方），它抛出一个异常。此时堆栈跟踪中包括了这两个不同参数的parseInt方法。

简而言之，这两个是不同的方法，一个是带一个参数的，它调用了带两个参数的方法。

valueOf 的定义在这个链接中：

public static Integer valueOf(String s, int radix) throws NumberFormatException {
    return Integer.valueOf(parseInt(s, radix));
}

它直接调用了带两个参数的版本。

public static int parseInt(String s) throws NumberFormatException {
    return parseInt(s,10); // this is line 770 currently
}

public static Integer valueOf(String s, int radix) throws NumberFormatException {
    return Integer.valueOf(parseInt(s,radix));
}