Why does `System.out.println(null);` give “The met

问题:

I am using the code:

System.out.println(null);

It is showing the error:

The method println(char[]) is ambiguous for the type PrintStream

Why doesn't null represent Object?

回答1:

There are 3 println methods in PrintStream that accept a reference type - println(char x[]), println(String x), println(Object x).

When you pass null, all 3 are applicable. The method overloading rules prefer the method with the most specific argument types, so println(Object x) is not chosen.

Then the compiler can't choose between the first two - println(char x[]) & println(String x) - since String is not more specific than char[] and vice versa.

If you want a specific method to be chosen, cast the null to the required type.

For example :

System.out.println((String)null);

回答2:

If you call System.println(null) there are multiple candidate methods (with char [], String and Object argument) and the compiler can't decide which one to take.

Fix
Add an explicit cast to null.

System.out.println((Object)null);

Or use null object pattern.

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Why does't null represent Object?

From JLS 4.1 The Kinds of Types and Values

There is also a special null type, the type of the expression null, which has no name.
Because the null type has no name, it is impossible to declare a variable of the null type or to cast to the null type.
The null reference is the only possible value of an expression of null type. The null reference can always be cast to any reference type.
The null reference can always be assigned or cast to any reference type

^{In practice, the programmer can ignore the null type and just pretend that null is merely a special literal that can be of any reference type}

So the type of null is not Object, though it can be converted to (read-as assigned to) an Object.