英文:
Why does the following code autoboxes to wrong types and compiles?
问题
class Test {
public static <T> boolean test(T a, T b) {
return a.equals(b);
}
public static void main(String[] args) {
int i = 0;
long j = 0;
if (!test(i, j)) {
throw new RuntimeException("i is not equal to j");
}
}
}
在上面的代码片段中，我期望以下两种情况之一发生：
1）会有编译错误，因为 i 被自动装箱为 Integer，而 j 被自动装箱为 Long，而方法 test 的声明要求其两个参数具有相同的类型。
2）i 和 j 都会被自动装箱为 Long，代码会编译并运行，显示 i 和 j 相等。
但实际发生的是，i 被自动装箱为 Integer，j 被自动装箱为 Long，代码编译没有错误。这不是与 test 方法的声明相矛盾吗？允许这样的代码的原因是什么？
英文:
class Test {
public static <T> boolean test(T a, T b) {
return a.equals(b);
}
public static void main(String[] args) {
int i = 0;
long j = 0;
if (!test(i, j)) {
throw new RuntimeException("i is not equal to j");
}
}
}
In the code snippet above I am expecting one of the following two things to happen:
-
There will be a compiler error, because
iis autoboxed toIntegerandjis autoboxed toLongand the declaration of the methodtestrequires that both of its arguments are of the same type. -
Both
iandjto be autoboxed to Long and the code to compile and run showing thatiandjare equal.
But what happens actually is i is autoboxed to Integer and j is autoboxed to Long and the code compiles without error. Doesn't this contradict with the declaration of test? What is the reason to allow such code?
答案1
得分: 3
如果将 i 装箱为 Integer，并且将 j 装箱为 Long，仍然可以合法地调用名为 test 的方法，其中假定其泛型类型为 java.lang.Number，它是 Integer 和 Long 的超类型。
实际上，您可以使用 任意 两个对象调用您的 test 方法，因为 T 可以被视为基本类型 Object。方法上的泛型不限制其参数。
英文:
If i is boxed to an Integer and j is boxed to a Long, it's still legal to call the method test with its generic type assumed to be java.lang.Number, which is a supertype of both Integer and Long.
In fact, you can call your test method with any two objects, because T can be taken as the base type Object. The generics on the method do not restrict its arguments at all.
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