未初始化的int与整数 [英] Uninitialized int vs Integer

问题描述

我正在研究我的Java以准备考试，我遇到了一些未初始化的int / Integer值的问题。

I was just studying up on my Java in preparation for an exam and I ran into a sort of problem with uninitialized int/Integer values.

class A
    {
       int x;
       Integer y;
       static int z;
       static Integer z2;
       public A(){}   
    }

让我说我初始化一个对象A类=新A（）;

Lets say I initialize an object of Class A. A a = new A();

我在编译器中尝试了这个并得到了结果

I've tried this in a compiler and got the results

a.x == 0; true
a.x == null; Static Error: Bad type in comparison expression
a.y == 0; java.lang.NullPointerException
a.y == null; true
a.z == 0; true 
a.z == null; Static Error: Bad type in comparison expression
a.z2 == 0; NullPointerException
a.z2 == null; true

此外，我在交互窗格中尝试了一些未初始化的int / Interger比较，看看我是否愿意如果我的x，y不是上面的类实例变量，则得到不同的结果。

Furthermore , I tried some more uninitialized int/Interger comparisons in an interactions pane to see if I would get different results if my x, y were not class instance variables as they are above.

int x;
Integer y;
x == 0; true
x == null; Static Error: Bad type in comparison expression
y == 0; java.lang.NullPointerException
y == null; true

然而，我的教授在一次演讲中声称价值观应如下：

However, my professor claims in a lecture that the values should be as follows:

x == 0; Uninitialized
x == null; Undefined
y == 0; java.lang.NullPointerException
y == null; Uninitialized

现在我不想怀疑写考试的人，但是x == 0和y == null真值是否正确？关于为什么会非常感谢的解释，谢谢。

Now I don't want to doubt the one who writes the exam, but which x == 0 and y == null truth value is correct? An explanation on why would be very much appreciated, thank you.

推荐答案

• ax == 0 - 是的，因为ax的默认值为0.


• ax == null - 如上所述，这是一个编译时错误。这是从§15.21.3 ：如果无法通过转换转换（§5.5）将任一操作数的类型转换为另一个操作数的类型，则会发生编译时错误。 null类型不能转换为数字。


• ay == 0 - 这会尝试取消装箱 ay，为null，因此抛出NullPointerException。与上面的（有一个文字null）不同，编译器不会试图在编译时弄清楚 ay 将为null。


• ay == null - 同样，为真，因为 ay 初始化为null


• az == 0 - 与 ax 相同（静态除外）


• az == null - 与 ax 相同（静态除外）


• a.z2 == 0 - 与相同（静态除外）


• a.z2 == null - 与 ay 相同（静态除外）


• a.x == 0 - True because a.x has a default value of 0.
• a.x == null - As noted, this is a compile-time error. This follows from §15.21.3: "A compile-time error occurs if it is impossible to convert the type of either operand to the type of the other by a casting conversion (§5.5)." The null type isn't convertible to a number.
• a.y == 0 - This tries to unbox a.y, which is null, so it throws a NullPointerException. Unlike the above (which has a literal null), the compiler doesn't try to figure out at compile-time that a.y will be null.
• a.y == null - Again, true because a.y is initialized to null
• a.z == 0 - Same as a.x (except static)
• a.z == null - Same as a.x (except static)
• a.z2 == 0 - Same as a.y (except static)
• a.z2 == null - Same as a.y (except static)

交互窗格的问题在于IDE如何实现它。如果x和y是本地（未初始化）变量，则所有四个最后比较都将无法编译。

The problem with the interactions pane is that it's up to the IDE how to implement it. If x and y are local (uninitialized) variables, all four of your last comparisons will fail to compile.

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