Java9 @SafeVarargs Annotation

Java 9 introduced the @SafeVarargs annotation, which allows developers to suppress compiler warnings when using varargs with generic types.

What is a Varargs?

Varargs, short for "variable-length arguments", allows a method to accept an indefinite number of arguments. The syntax for varargs is to add an ellipsis (three dots) after the type declaration of the parameter, like so:

public void myMethod(String... args){ ... }

What is the problem with Varargs and Generics?

Before Java 9, there was an issue when using varargs with generics. Consider the following example:

public static <T> List<T> asList(T... args) {
    return Arrays.asList(args); 
}

List<Integer> numbers = asList(1, 2, 3);

This code compiles without errors or warnings. However, if we try to add a List of Integers to the varargs, like this:

List<Integer> numbers = asList(1, 2, 3, Arrays.asList(4, 5, 6));

The code will compile but produce a warning message that states:

Type safety: Potential heap pollution via varargs parameter args

Heap pollution refers to a situation where a variable of a parameterized type references an object that is not of that parameterized type.

What is @SafeVarargs Annotation?

@SafeVarargs is an annotation that can be applied to a method or constructor that takes a varargs parameter of a generic type. If the method satisfies certain conditions, the annotation tells the compiler not to generate warnings for potential heap pollution.

The conditions that must be satisfied are:

1. The method or constructor is final or static.
2. The method or constructor does not perform any unsafe operations on the arguments it receives.
3. The method or constructor does not expose the contents of its varargs parameter to the outside world.

Here is an example of using the @SafeVarargs annotation:

public static <T> List<T> asList(@SafeVarargs T... args) {
    List<T> list = new ArrayList<>();
    for (T arg : args) {
        list.add(arg);
    }
    return list;
}

Now we can safely use varargs with generic types without any warnings:

List<Integer> numbers = asList(1, 2, 3, Arrays.asList(4, 5, 6));

Conclusion

The @SafeVarargs annotation is a useful addition to Java 9 that allows developers to suppress warnings when using varargs with generic types. However, it is important to use this feature responsibly and only apply it to methods or constructors that meet the conditions outlined above.