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In this noncompliant code example, the @SuppressWarnings annotation's scope encompasses the whole class. This is dangerous because all unchecked warnings within the class will be suppressed. Oversights of this nature can lead to a ClassCastException at runtime. 

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

Compliant Solution

Limit the scope of the @SuppressWarnings annotation to the nearest unchecked warning-generating code. In this case, it may be used in the declaration for the Set.

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This noncompliant code example is from the implementation of java.util.ArrayList. When the class is compiled, it emits an unchecked cast warning, as shown. Because the return statement is not a declaration, the Java Language Specification [JLS 2011] makes it impossible to suppress the warning trivially. Consequently, the @SuppressWarnings is used over method scope. This can cause issues when some functionality that performs type-unsafe operations is added to the method at a later date [Bloch 2008]. 

@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());

Because the return statement is not a declaration, the Java Language Specification [JLS 2011] makes it impossible to suppress the warning trivially by using @SuppressWarnings at that statement. Consequently, the @SuppressWarnings is used over method scope where it is allowed. This can cause issues when some functionality that performs type-unsafe operations is added to the method at a later date [Bloch 2008].

Compliant Solution

When it is impossible to use the @SuppressWarnings annotation, as in the preceding noncompliant code example, declare a new variable to hold the return value and adorn it with the @SuppressWarnings annotation.

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

Bibliography

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