英文:
Java: Why does "Character newChar = 'c' " work if Character is a wrapper class? How come it doesn't need a constructor?
问题
Example 1:
Character newCharacter = 'c';
Example 2:
Character newCharacterOther = new Character('c');
But what are the differences?
In the first example, how does the Character class know to set its value to "c" without needing a constructor for that?
Does it use a factory method behind the scenes? Can someone please explain to me how the compiler knows what to do?
I have provided an image of the constructor of the Character.class in java.lang.
It even says that it's deprecated and that it shouldn't be accessed like that, but I am still confused a bit.
英文:
These are both fine for me:
Example 1:
Character newCharacter = 'c';
Example 2:
Character newCharacterOther = new Character('c');
But what are the differences?
In the first example, how does the Character class know to set its value to "c" without needing a constructor for that?
Does it use a factory method behind the scenes? Can someone please explain to me how the compiler knows what to do?
I have a provided an image of the constructor of the Character.class in java.lang.
It even says that it's deprecated and that it shouldn't be accessed like that, but I am still confused a bit.
答案1
得分: 6
I'll provide the translated portions:
如语言规范中所述：
> 赋值上下文允许使用以下之一：
>
> * ...
> * 自动装箱转换（§5.1.7）
> * ...
参考第5.1.7节：
> 具体来说，以下九种转换被称为装箱转换：
>
> * ...
> * 从 char 类型到 Character 类型
> * ...
>
> 在运行时，装箱转换如下进行：
>
> * ...
> * 如果 p 是 char 类型的值，则装箱转换将 p 转换为类和类型为 Character 的引用 r，使得 r.charValue() == p
> * ...
所以，你的代码等效于：
Character newCharacter = Character.valueOf('c');
> 但它们有什么区别？
new Anything 保证创建一个新的 Anything 实例（或者会引发异常）。因此，new Character('c') == new Character('c') 总是为 false。
另一方面，Character.valueOf('c') == Character.valueOf('c') 可能 为 true，因为方法调用不要求返回一个新的实例。实际上，规范保证在这两次调用中 不会 获得新实例。
这允许您重复使用现有实例，避免不必要的分配，从而节省内存。
英文:
As described in the language spec:
> Assignment contexts allow the use of one of the following:
>
> * ...
> * a boxing conversion (§5.1.7)
> * ...
Referring to section 5.1.7:
> Specifically, the following nine conversions are called the boxing conversions:
>
> * ...
> * From type char to type Character
> * ...
>
> At run time, boxing conversion proceeds as follows:
>
> * ...
> * If p is a value of type char, then boxing conversion converts p into a reference r of class and type Character, such that r.charValue() == p
> * ...
So, your line is equivalent to:
Character newCharacter = Character.valueOf('c');
Indeed, if you decompile your class, you will see that's exactly what gets invoked.
> But what are the differences?
new Anything is guaranteed to create a new instance of Anything (or fail with an exception). So, new Character('c') == new Character('c') is always false.
On the other hand, Character.valueOf('c') == Character.valueOf('c') may be true, because there is no requirement for a method invocation to return a new instance. Indeed, it is guaranteed by specification that you won't get a new instance on these two invocations.
This allows you to reuse existing instances, avoiding unnecessary allocations, thus saving memory.
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