Java中是否有数字的默认类型 [英] Is there a default type for numbers in Java

问题描述

如果我写这样的东西

System.out.println(18);

哪种类型有18"?是int 还是byte?或者它还没有类型?

Which type has the '18'? Is it int or byte? Or doesn't it have a type yet?

它不能是 int，因为这样的东西是正确的:

It can't be int, because something like this is correct:

byte b = 3;

这是不正确的:

int i = 3;
byte bb = i; //error!

我想我在 赋值转换 :

常量的编译时收缩意味着代码如下:

The compile-time narrowing of constants means that code such as:

byte theAnswer = 42;

byte theAnswer = 42;

是允许的.如果没有缩小，整数文字 42 的类型为 int 的事实意味着需要强制转换为字节:

is allowed. Without the narrowing, the fact that the integer literal 42 has type int would mean that a cast to byte would be required:

byte theAnswer = (byte) 42;//强制转换是允许的，但不是必需的

byte theAnswer = (byte) 42; // cast is permitted but not required

推荐答案

这个

18

被称为 整数文字.有各种各样的文字、浮点数、String、字符等

is known as an integer literal. There are all sorts of literals, floating point, String, character, etc.

以下，

byte b = 3;

文字 3 是一个整数文字.这也是一个常量表达式.由于 Java 可以判断 3 适合 byte，因此它可以安全地应用 缩小原始转换 并将结果存储在 byte 变量中.

the literal 3 is an integer literal. It's also a constant expression. And since Java can tell that 3 fits in a byte, it can safely apply a narrowing primitive conversion and store the result in a byte variable.

在这个

int i = 3;
byte bb = i; //error!

文字 3 是一个常量表达式，但变量 i 不是.编译器只是简单地决定 i 不是一个常量表达式，因此不会特意计算它的值，转换为 byte 可能会丢失信息(如何将 12345 转换为 byte?)，因此不应被允许.您可以通过使 i 成为常量变量

the literal 3 is a constant expression, but the variable i is not. The compiler simply decides that i is not a constant expression and therefore doesn't go out of its way to figure out its value, a conversion to byte may lose information (how to convert 12345 to a byte?) and should therefore not be allowed. You can override this behavior by making i a constant variable

final int i = 3;
byte bb = i; // no error!

或通过指定显式转换

int i = 3;
byte bb = (byte) i; // no error!

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