为什么int类型方法允许字符类型作为返回值？ [英] Why int type method is allowing character type as return value?

问题描述

这是我的代码但是我没有得到如何 int getValue（）方法接受 char 键入为返回值。它是如何工作的？我可以解释一下这个方法是如何工作的吗？

Here is my code but I am not getting how int getValue() method accepting char type as return value. How is it working? could any body please explain me how this method is working?

public class CharToInteger {
    private static Scanner input;
    public static int getValue(char character){
        return character;       
    }

    public static void main(String[] args) {
        input = new Scanner(System.in);
        System.out.println("Enter a character to get value of it:");
        String inputString=(String) input.next();
        char inputCharacter=inputString.charAt(0);
        System.out.println(getValue(inputCharacter));
    }

}

提前致谢：）

推荐答案

好的，首先要做的事情是：

OK, so first things first:

这是一个加宽的原语类型转换，所以这是合法的。您可以：

This is a widening primitive type conversion, so this is legal. You can:

int foo() { return 'a' /* character constant */ };
long foo() { return 3; /* int constant */ }

但你不能这样做：

char foo() { return 1; /* int constant */ }
int foo() { return 1L; /* long constant */ }

第二：它返回的不是全部的ASCII代码。 Java做Unicode。

Second: what it returns is NOT THE ASCII CODE AT ALL. Java does Unicode.

恰好在创建Java时，Unicode只定义了符合16位的代码点;因此 char 被创建为一个2字节的无符号基本类型（它是Java中唯一的无符号基本类型），与当时称为UCS-2的字符编码匹配（1到1） 1编码和代码点之间的映射。）

It just happens that when Java was created, Unicode only defined code points fitting 16 bits; hence char was created as a 2 byte, unsigned primitive type (it is the only unsigned primitive type in Java) matching the then-called UCS-2 character coding (a 1 to 1 mapping between the coding and code points).

然而，之后，Unicode变为宽，并且出现了BMP之外的代码点（即，大于U + FFFF）;从那时起，UCS-2变成了UTF-16，并且BMP之外的代码点需要两个 char s用于一个代码点（一个主要的代理和一个尾随的代理;在以前的Unicode版本中） ，在Java API中，那些被称为resp。高和低代理）。因此， char 现在是一个UTF-16代码单元。

However, afterwards Unicode went "wide" and code points outside the BMP (ie, greater than U+FFFF) appeared; since then UCS-2 became UTF-16, and code points outside the BMP require two chars for one code point (a leading surrogate and a trailing surrogate; in previous Unicode versions, and in the Java API, those were called resp. high and low surrogate). A char is therefore now a UTF-16 code unit.

然而，对于代码来说仍然如此BMP中的点， char 值与代码点完全匹配。

It is still true, however, that for code points in the BMP, the char value exactly matches the code point.

现在，为了修复你的程序以准确显示每个可能条目的字符值，即代码点，你会这样做（Java 8）：

Now, in order to "fix" your program to accurately display the "character value", ie the code point, for each possible entry, you would do that (Java 8):

public static void main(String[] args) {
    final Scanner input = new Scanner(System.in);
    System.out.println("Enter a character to get value of it:");
    String inputString =  input.next();
    // Print -1 on an empty input
    final OptionalInt codepoint = inputString.codePoints().findFirst();
    System.out.println(codepoint.isPresent() ? codepoint.get() : -1);
}

这也将处理BMP之外的代码点。

This will also handle code points outside the BMP.

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