为什么在 Java 中声明非十进制浮点数时不需要 'f'？

Question

如果在 Java 中声明浮点数时需要 'f' 后缀，因为双精度是默认值，为什么当浮点数是 a 时不使用 'f' 是可以接受的非十进制？

float myFloat = 123.45 // not ok because double is the default
float myFloat = 123.45f // ok because the float is explicit

float myFloat = 123 // ok, but why? isn't it still a double by default?

Answer 1

123 是一个 int 值，默认情况下作为扩大转换提升为 float。

int i = 123;
float f = i;

反之则不然

float f = 123;
int i = f; // <-- the compile-time error

19 specific conversions on primitive types are called the widening primitive conversions:

...
int to long, float, or double
...

Answer 2

123 字面值默认为 int，不是双倍的。将 int 文字分配给 float 没有问题，因为这需要扩大原始转换。

将表达式的值赋给变量时，允许进行以下赋值：

JLS 5.2. Assignment contexts :

Assignment contexts allow the value of an expression to be assigned (§15.26) to a variable; the type of the expression must be converted to the type of the variable.

Assignment contexts allow the use of one of the following:

an identity conversion (§5.1.1)

a widening primitive conversion (§5.1.2)
...

JLS 5.1.2. Widening primitive Conversion

19 specific conversions on primitive types are called the widening primitive conversions:

byte to short, int, long, float, or double

short to int, long, float, or double

char to int, long, float, or double

int to long, float, or double
...

为什么在 Java 中声明非十进制浮点数时不需要 'f'？

Why isn't 'f' necessary when declaring a non-decimal float in Java?

java

floating-point

double

primitive-types