Java vararg pass lambda and values

Question

I'm trying to unite lambdas and simple values in varag.

public static void Log(String format, Object ... args) {
    final Object[] fmt = new Object[ args.length ];
        for(int i = 0; i < args.length; i++)
            fmt[i] = args[i] instanceof Supplier  ?
                     ( (Supplier) args[i] ).get() :
                     args[i];
    final String s = String.format( format, fmt );
    System.err.println( s );
}

final Supplier
    s = () -> "aaa",
    d = () -> 111;
Log( "%s %d %s %d", "bbb", 222, s, d );    // OK, OUTPUT: bbb 222 aaa 111
Log( "%s %d %s %d", "bbb", 222, () -> "aaa", () -> 111 );  // COMPILE FAIL

ERROR: method Log cannot be applied to given types; REQUIERED String,Object[] found: String,String,int,()->"aaa",()->111 REASON: varargs mismatch; Object is not a functional interface

Is it possible to pass both lambdas and values to vararg?

Answer 1

The problem is in the error message

Object is not a functional interface

You can only create a lambda for a functional interfaces (one with exactly one abstract method) Object is not an interface and it doesn't have any abstract methods so you can't create a lambda of this type. What you can do is

Log( "%s %d %s %d", "bbb", 222, (Supplier) () -> "aaa", (Supplier) () -> 111 );  

This way the compiler knows what sort of lambda you intended to implement.

By comparison you could write the following and this would behave differently in your method.

Log( "%s %d %s %d", "bbb", 222, (Callable) () -> "aaa", (Callable) () -> 111 );