問題描述

看下面的三行代碼.

  float f = 1;

  float g = 1.1;

  float h = 1.1f;

第二行有編譯錯誤，而其他行沒有編譯錯誤.第一行在沒有后綴 f 的情況下工作正常，第三行在后綴 f 下工作.這是為什么呢?

Second line has compilation errors, while the other lines do not have compilation errors. First line is working fine without suffix f and third line is working with suffix f. Why is this?

推薦答案

Java 中的浮點(diǎn)字面量默認(rèn)是 double 值.

Floating point literals in Java is a double value by default.

如果浮點(diǎn)文字以 ASCII 字母 F 或 f 為后綴，則為 float 類型；否則它的類型是 double 并且可以選擇以 ASCII 字母 D 或 d 為后綴.

JLS 3.10.2 Floating-Point Literals

A floating-point literal is of type float if it is suffixed with an ASCII letter F or f; otherwise its type is double and it can optionally be suffixed with an ASCII letter D or d.

如果沒有明確的縮小轉(zhuǎn)換，您不能將 double 值分配給 float.因此，您有兩種選擇:

You can't assign a double value to a float without an explicit narrowing conversion. You therefore have two options:

• 對于文字，使用后綴 f 或 F 來表示 float 值
• 對于非文字，使用顯式轉(zhuǎn)換 (float)

• For literals, use the suffix f or F to denote a float value
• For non-literals, use an explicit cast (float)

后者的一個例子是:

double d = 1.1;
float f = (float) d; // compiles fine!

關(guān)于擴(kuò)大轉(zhuǎn)化率

這樣編譯的原因:

On widening conversions

The reason why this compiles:

float f = 1;

是因?yàn)閺?int 到 float 的擴(kuò)大轉(zhuǎn)換可以在賦值的上下文中隱式完成.

is because the widening conversion from int to float can be done implicitly in the context of an assignment.

賦值轉(zhuǎn)換發(fā)生在將表達(dá)式的值賦值給變量時:必須將表達(dá)式的類型轉(zhuǎn)換為變量的類型.賦值上下文允許使用以下之一:

JLS 5.2 Assignment Conversion

Assignment conversion occurs when the value of an expression is assigned 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:

• 擴(kuò)大的原始轉(zhuǎn)換(§5.1.2)
• [...]

以下 19 種基本類型的特定轉(zhuǎn)換稱為加寬基本類型轉(zhuǎn)換:

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

• int 到 long、float 或 double
• [...]

文字的其他數(shù)據(jù)類型后綴

如上所述，double 也有 D 或 d 后綴.以這個片段為例:

Other data type suffix for literals

As mentioned above, there's also the D or d suffix for double. Consider this snippet for example:

static void f(int i) {
    System.out.println("(int)");
}
static void f(double d) {
    System.out.println("(double)");
}

//...
f(1);   // prints "(int)"
f(1D);  // prints "(double)"

long 文字還有一個后綴，即 L 或 l(小寫字母).強(qiáng)烈建議您使用大寫變體.

There's also a suffix for long literals, which is L or l (lowercase letter). It is highly recommended that you use the uppercase variant.

如果整數(shù)文字以 ASCII 字母 L 或 l (elllong>);否則它是 int 類型.后綴 L 是首選，因?yàn)樽帜?l (ell) 通常很難與數(shù)字 1 區(qū)分開來(一個).

JLS 3.10.1 Integer Literals

An integer literal is of type long if it is suffixed with an ASCII letter L or l (ell); otherwise it is of type int. The suffix L is preferred, because the letter l (ell) is often hard to distinguish from the digit 1 (one).

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