Page History

The compiler issues unchecked warnings when it When the compiler detects potential type-safety issues arising from mixing raw types with generic code. This includes , it issues unchecked warnings, including unchecked cast warnings, unchecked method invocation warnings, unchecked generic array creation warnings, and unchecked conversion warnings [Bloch 2008]. It is permissible to use the @SuppressWarnings("unchecked") annotation to suppress unchecked warnings when, and only when, the warning-emitting code is guaranteed to be type safe. A common use case is mixing legacy code with new client code. The perils of ignoring unchecked warnings are discussed extensively in OBJ03-J. Do not mix generic with nongeneric raw types in new code.

According to the Java API [API 2011], Annotation Type SuppressWarnings documentation [API 2011],

As a matter of style, programmers should always use this annotation on the most deeply nested element where it is effective. If you want to suppress a warning in a particular method, you should annotate that method rather than its class.

...

In this noncompliant code example, the @SuppressWarnings annotation's scope encompasses the whole class. :

@SuppressWarnings("unchecked")
class Legacy {
  Set s = new HashSet();
  public final void doLogic(int a, char c) {
    s.add(a);
    s.add(c); // Type -unsafe operation, ignored
  }
}

This code is dangerous because all unchecked warnings within the class will be suppressed. Oversights of this nature can lead to a ClassCastException at runtime.

...

This noncompliant code example is from the implementation of java.util.ArrayList. :

@SuppressWarnings("unchecked")
public <T> T[] toArray(T[] a) {
  if (a.length < size)
    return (T[]) Arrays.copyOf(elements, size, a.getClass()); // Produces unchecked warning
 // ...
}

When the class is compiled, it emits an unchecked cast warning.:

// Unchecked cast warning
ArrayList.java:305: warning: [unchecked] unchecked cast found : Object[], required: T[]
return (T[]) Arrays.copyOf(elements, size, a.getClass());

...

This rule cannot be statically enforced in full generality; static analysis could be possible for some special cases.

Bibliography

...

...

...