String.format() takes an array as a single argument

Question

Why this does work OK?:

String f = "Mi name is %s %s.";
System.out.println(String.format(f, "John", "Connor"));

And this doesnt?:

String f = "Mi name is %s %s.";
System.out.println(String.format(f, (Object)new String[]{"John","Connor"}));

If the method String.format takes a vararg Object?

It compiles OK but when I execute this the String.format() takes the vararg Object as a single an unique argument (the toString() value of the array itself), so it throws a MissingFormatArgumentException because it cannot match with the second string specifier (%s).

How can I make it work? Thanks in advance, any help will be greatly appreciated.

Answer 1

Use this : (I would recommend this way)

String f = "Mi name is %s %s.";
System.out.println(String.format(f, (Object[])new String[]{"John","Connor"}));

OR

String f = "Mi name is %s %s.";
System.out.println(String.format(f, new String[]{"John","Connor"}));

But if you use this way, you will get following warning :

The argument of type String[] should explicitly be cast to Object[] for the invocation of the varargs method format(String, Object...) from type String. It could alternatively be cast to Object for a varargs invocation.

Answer 2

The problem is that after the cast to Object, the compiler doesn't know that you're passing an array. Try casting the second argument to (Object[]) instead of (Object).

System.out.println(String.format(f, (Object[])new String[]{"John","Connor"}));

Or just don't use a cast at all:

System.out.println(String.format(f, new String[]{"John","Connor"}));

(See this answer for a little more info.)