1. Introduction

Varargs were introduced in Java 5 and provide a short-hand for methods that support an arbitrary number of parameters of one type.

In this article, we'll see how we can use this core Java feature.

2. Before Varargs

Before Java 5, whenever we wanted to pass an arbitrary number of arguments, we had to pass all arguments in an array or implement N methods (one for each additional parameter):

public String format() { ... }

public String format(String value) { ... }

public String format(String val1, String val2) { ... }

3. Use of Varargs

Varargs help us avoid writing boilerplate code by introducing the new syntax that can handle an arbitrary number of parameters automatically – using an array under the hood.

We can define them using a standard type declaration, followed by an ellipsis:

public String formatWithVarArgs(String... values) {
    // ...
}

And now, we can call our method with an arbitrary number of arguments, like:

formatWithVarArgs();

formatWithVarArgs("a", "b", "c", "d");

As mentioned earlier, varargs are arrays so we need to work with them just like we'd work with a normal array.

4. Rules

Varargs are straightforward to use. But there're a few rules we have to keep in mind:

  • Each method can only have one varargs parameter
  • The varargs argument must be the last parameter

5. Heap Pollution

Using varargs can lead to so-called Heap PollutionTo better understand the heap pollution, consider this varargs method:

static String firstOfFirst(List<String>... strings) {
    List<Integer> ints = Collections.singletonList(42);
    Object[] objects = strings;
    objects[0] = ints; // Heap pollution

    return strings[0].get(0); // ClassCastException
}

If we call this strange method in a test:

String one = firstOfFirst(Arrays.asList("one", "two"), Collections.emptyList());

assertEquals("one", one);

We would get a ClassCastException even though we didn't even use any explicit type casts here:

java.lang.ClassCastException: class java.lang.Integer cannot be cast to class java.lang.String

5.1. Safe Usage

Every time we use varargs, the Java compiler creates an array to hold the given parameters. In this case, the compiler creates an array with generic type components to hold the arguments.

When we use varargs with generic types, as there's a potential risk of a fatal runtime exception, the Java compiler warns us about a possible unsafe varargs usage:

warning: [varargs] Possible heap pollution from parameterized vararg type T

The varargs usage is safe if and only if:

  • We don't store anything in the implicitly created array. In this example, we did store a List<Integer> in that array
  • We don't let a reference to the generated array escape the method (more on this later)

If we are sure that the method itself does safely use the varargs, we can use @SafeVarargs to suppress the warning.

Put simply, the varargs usage is safe if we use them to transfer a variable number of arguments from the caller to the method and nothing more!

5.2. Escaping Varargs Reference

Let's consider another unsafe usage of varargs:

static <T> T[] toArray(T... arguments) {
    return arguments;
}

At first, it might seem that the toArray method is completely harmless. However, because it let the varargs array escape to the caller, it violates the second rule of safe varargs.

To see how this method can be dangerous, let's use it in another method:

static <T> T[] returnAsIs(T a, T b) {
    return toArray(a, b);
}

Then if we call this method:

String[] args = returnAsIs("One", "Two");

We would, again, get a ClassCastException. Here's what happens when we call the returnAsIs method:

  • To pass and to the toArray method, Java needs to create an array
  • Since the Object[] can hold items of any type, the compiler creates one
  • The toArray method returns the given Object[] to the caller
  • Since the call site expects a String[], the compiler tries to cast the Object[] to the expected String[], hence the ClassCastException

For a more detailed discussion on heap pollution, it's highly recommended to read item 32 of Effective Java by Joshua Bloch.

6. Conclusion

Varargs can make a lot of boilerplate go away in Java.

And, thanks to their implicit autoboxing to and from Array, they play a role in future-proofing our code.

As always, all code examples from this article can are available in our GitHub repository.

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JoeHx
2 years ago

I remember when I first discovered varargs I used them EVERYWHERE. Ahh, such innocence.

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