为什么数组常量只能用于初始值设定项？ [英] Why can array constants only be used in initializers?

问题描述

可能重复：

数组常量只能用于初始值设定项错误

我正在研究数组，我通过这种在一行中声明和初始化数组的简短方法。例如，

I was studying arrays, and I came through this short-cut method of declaring and initializing an array in one line. For example,

int[] a = {1, 2, 3, 4, 5};

但是当我尝试执行以下代码时，我得到了这个编译错误，说数组常量只能是在初始化程序中使用。

But when I tried to do following code, I got this compiler error saying "Array constants can only be used in initializer".

int[] a;
a = {1, 2, 3, 4};

为什么会这样？

推荐答案

这是不允许的，因为 JLS说所以。语法仅在声明和数组创建表达式中允许。

It's not allowed because the JLS says so. The syntax is only permitted in declarations and in array creation expressions.

后者提供了另一种实现相同结果的方法：

The latter provide an alternative way of achieving the same result:

int[] a;
a = new int[]{1, 2, 3, 4};

至于要求新T []的实际根本原因，我的猜测如下。考虑以下数组初始值设定项：

As to the actual underlying reason for requiring the new T[], my guess is as follows. Consider the following array initializer:

{1, 2, 3, 4}

它可用于初始化不同类型的数组：

It can be used to initialize arrays of different types:

new int[]{1, 2, 3, 4};
new float[]{1, 2, 3, 4};
new double[]{1, 2, 3, 4};

如果新T [] 位不是不，我怀疑裸 {1,2,3,4} 可能会在语义分析过程中造成困难。在这里，我正在考虑以下情况：

If the new T[] bit wasn't required, I suspect that the bare {1, 2, 3, 4} could cause difficulties during semantic analysis. Here, I am thinking about cases like:

void f(float[] x) { ... }
void f(double[] x) { ... }
void g() {
  f({1, 2, 3, 4});
}

如果允许这种语法，语言规范必须处理复杂性选择要调用哪个函数。

If this syntax were allowed, the language spec would have to deal with the complexity of choosing which function to call.

同样，还不清楚 {null} 。它可以是 Object [] ， Integer [] ， Serializable [] 依此类推。

In a similar vein, it's not clear what should be the type of {null}. It can be Object[], Integer[], Serializable[] and so on.

最后，空数组 {} 将是最棘手的。在这里，我们甚至无法判断它是一个对象数组还是一组标量。

And finally, the empty array {} would be the trickiest of all. Here, we can't even tell if it's an array of objects or an array of scalars.

而不是处理所有这些复杂性，语言设计师似乎选择了通过要求 new T [] 语法来避免它们。

Instead of dealing with all these complexities, it seems that the language designers chose to avoid them by requiring the new T[] syntax.

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