In this tutorial we’re going to show how to eliminate unchecked warnings that occur frequently when mixing Java Generics with legacy code.

Sometime, when working with pre-generics code (for example old Apache Commons), you may encounter the following warnings:
Unchecked assignment: ‘java.util.List’ to ‘java.util.List<java.lang.String>’

It means that you try to assign not type safe object to a type safe variable. If you are make sure that such assignment is type safe, you can disable the warning using @SuppressWarnings annotation, as in the following examples.

private static List namesFromLibrary() {
    return Arrays.asList("Java", "Clojure");
}

public static void main(String[] args) {
    @SuppressWarnings("unchecked")
    List<String> names = namesFromLibrary();
    System.out.println(names);
}

In short methods unchecked warning can be eliminated on method level:

@SuppressWarnings("unchecked")
private static List<String> moreNames() {
    return new ArrayList(Arrays.asList("Bernard", "Witold"));
}

public static void main(String[] args) {
    List<String> moreNames = moreNames();
    System.out.println(moreNames);
}

@SuppressWarnings cannot be used on return statement, but result can be assigned to a local variable, where the warning can be eliminated:

private static List<String> evenMoreNames() {
    @SuppressWarnings("unchecked")
    ArrayList names = new ArrayList(Arrays.asList("Jon", "Snow"));
    return names;
}

public static void main(String[] args) {
    List<String> evenMoreNames = evenMoreNames();
    System.out.println(evenMoreNames);
}

Although it is allowed, this practice is strongly discouraged, because it may eliminate more than one would like to:

@SuppressWarnings("unchecked")
public class UncheckedWarnings {
    // code with unchecked warnings
}

Use @SuppressWarnings on the narrowest possible scope.

• Effective Java (2nd Edition)