Item 42 - Use varargs judiciously

From Effective Java 2/e by Joshua Bloch

Varargs stands for variable arity method

Simple use of varargs

static int sum(int... args) {
   int sum = 0;
   for (int arg : args)
      sum += arg;
   return sum;

Use varargs to pass one or more arguments

static int min(int firstArg, int... remainingArgs) {
   int min = firstArg;
   for (int arg : remainingArgs)
      if (arg < min)
         min = arg;
   return min;
  • Don’t retrofit every method that has a final array parameter; use varargs only when a call really operates on a variable-length sequence of values.
// Consider the case of Arrays.asList
// This method was never designed to gather multiple arguments into a list
// but it seemed like a good idea to retrofit it to do so when varargs were added to the platform

// As a result, it became possible to do things like this
List<String> homophones = Arrays.asList("to", "too", "two");

// Obsolete idiom to print an array!
// It prints [Ljava.lang.Integer;@3e25a5.

// The right way to print an array

// If you accidentally tried it on an array of primitives, the program wouldn’t compile
// Error: asList(Object[]) in Arrays can't be applied to (int[])
public static void main(String[] args) {
   int[] digits = { 3, 1, 4, 1, 5, 9, 2, 6, 5, 4 };
  • Exercise care when using the varargs facility in performance-critical situations. Every invocation of a varargs method causes an array allocation and initialization.

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