Java Secret: More uses for varargs

Overview

Varargs have many uses from simplifying code. However, they are not always used as often as they could be.

Use in reflections

Calling a method via reflections is fairly ugly without varargs.

ClassLoader cl = Thread.currentThread().getContextClassLoader();
Method defineClass = cl.getClass().getDeclaredMethod("defineClass", 
    new Class[] { String.class, byte[].class, int.class, int.class});
defineClass.setAccessible(true);
defineClass.invoke(cl, 
    new Object[] { myNewClassName, myByteCode, 0, myByteCode.length });

With varargs, the code appears cleaner.

ClassLoader cl = Thread.currentThread().getContextClassLoader();
Method defineClass = cl.getClass().getDeclaredMethod(
    "defineClass", String.class, byte[].class, int.class, int.class);
defineClass.setAccessible(true);
defineClass.invoke(cl, myNewClassName, myByteCode, 0, myByteCode.length);

Use of varargs for optional arguments

System.out.printf use varargs which might be optional.

System.out.printf("Hello%n"); // no args to the varargs.
System.out.printf("Hello %s%n", name); // one argument to the varargs.
System.out.printf("Hello %s you are %d years old.%n", name, age); // two arguments

When varargs must have a minimum of one entry

The minimum number of entries for a varargs is 0, so if you need to have at least 1, you can add a parameter.

public static int average(int num, int... nums) {
    long total = num;
    for(int n: nums) total += n;
    return total/(nums.length+1);
}

This method will fail at runtime when no arguments are provided.

int a = average(); // fails to compile, instead of failing at runtime.

Use of varargs to build a List

A commonly used method is Arrays.asList(T...) The first example uses a varargs to build an array, the second example takes and array and passes it one as expected.

List<Integer> ints = Arrays.asList(1,2,3,4,5);
List<String> strings = Arrays.asList("one,two,three,four,five".split(","));

Use of varargs to build a Map

You can also use varargs to help build a map.

public static <K, V> Map<K, V> mapOf(K key, V value, Object... alteratingsKeysAndValues) {
    Map<K, V> map = new LinkedHashMap<K, V>();
    map.put(key, value);
    for(int i = 0; i < alteratingsKeysAndValues.length; i += 2)
        map.put((K) alteratingsKeysAndValues[i], 
                (V) alteratingsKeysAndValues[i+1]);
    return map;
}

Map<String, Integer> smallNumbers = mapOf("zero", 0, "one", 1, "two", 2);