对下界通配符使用增强的for循环

Question

I have this for loop:

for (int j = 0; j < list.size(); j++) {
    System.out.print(list.get(j) + " ");
}

the signature of the method is:

public static void addNumbers(List<? super Number> anoList)

the list is declared as:

static List<? super Number> list2;

or

static List list2;

for that matter.

All good so far. But then Netbeans gives a hint to convert the for loop to an enhanced for loop. When i apply that i get:

    for (? super Number anoList1 : anoList) {
        System.out.print(anoList1 + " ");
    }

which is not working and produces lots of errors. For one: cannot find anoList1 and . expected . expected , : expected , ; expected ; expected .

I have tried several things to get rid of the errors, but none of them is working.

What is the correct way for this enhanced for loop to be written?

From other topics i have read, the advice is not to use an enhanced for loop at all and leave it like it is.

Answer 1

What is the correct way for this enhanced for loop to be written?

for (Object obj : anyList) {...}

Since a List<? super Number> List<? super Number> is a List of elements which could be any supertype of Number (including Object ), the only guarantee we have about the actual type of the List is Object .

Answer 2

Here ? super Number ? super Number means that the list may contain elements of any type which is supertype of Number . Practically this means that nothing can be guaranteed about the list elements type except that they are descendants of Object type. Such method signature actually look useless. Probably you wanted to use

public static void addNumbers(List<? extends Number> anoList)

Which would mean that list element type is some subtype of Number like List<Integer> or List<Double> .

Answer 3

I'm riding on the answer given by Tagir. You'll see that this won't compile

public static void addNumbers(List<? super Number> anoList){
    for(Number n : anoList){ // Fail

    }
}

Fails with

Error:(29, 24) java: incompatible types: capture#1 of ? super java.lang.Number cannot be converted to java.lang.Number

But this is good

public static void addNumbers(List<? extends Number> anoList){
    for(Number n : anoList){

    }
}

So as Tagir mentioned, you probably want to use extends instead of super . However, if you really do need ? super Number ? super Number , then Radiodef's answer is your only option for the enhanced for loop.