documentations error:
this method returns void.
But in doc. it is mentioned as:

1. The return value of this method may be examined to determine
    whether an invocation had any effect.
2. If the named class is not a top-level class, this call will have no effect on the actual assertion status of any class, and its return value is undefined.
Please see api below as given in jdk1.4 build41 doc.
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public void setClassAssertionStatus(String className,
                                    boolean enabled)

    Sets the assertion status for the named top-level class in this class loader and
    any nested classes contained therein. This setting takes precedence over the
    class loader's default assertion status, and over any applicable per-package
    default. This method has no effect if the named class has already been loaded
    by this class loader. (Once a class is loaded, its assertion status cannot
    change.) The return value of this method may be examined to determine
    whether an invocation had any effect.

    If the named class is not a top-level class, this call will have no effect on the
    actual assertion status of any class, and its return value is undefined.
    Parameters:
       className - the fully qualified class name of the top-level class whose
       assertion status is to be set.
       enabled - true if the named class is to have assertions enabled when (and
       if) it is loaded by this class loader, false if the class is to have assertions
       disabled.