Add type parameter to Lookup::accessClass and Lookup::ensureInitialized

Summary

Generically parameterize Lookup::accessClass and Lookup::ensureInitialized, and document this type parameter.

Problem

When a class instance is passed to these two methods, the return value loses its type argument, render it infeasible to use in chained calls like Objects::requireNonNull.

Solution

Add a type parameter to these two methods; and use this type parameter as the type argument to Class passed and returned. Indicate in the specification that the targetClass argument passed is returned.

Specification

@@ -2810,6 +2810,7 @@ assertEquals("[x, y, z]", pb.command().toString());
          * This method returns when {@code targetClass} is fully initialized, or
          * when {@code targetClass} is being initialized by the current thread.
          *
+         * @param <T> the type of the class to be initialized
          * @param targetClass the class to be initialized
          * @return {@code targetClass} that has been initialized, or that is being
          *         initialized by the current thread.
@@ -2825,7 +2826,7 @@ assertEquals("[x, y, z]", pb.command().toString());
          * @since 15
          * @jvms 5.5 Initialization
          */
-        public Class<?> ensureInitialized(Class<?> targetClass) throws IllegalAccessException {
+        public <T> Class<T> ensureInitialized(Class<T> targetClass) throws IllegalAccessException {
             if (targetClass.isPrimitive())
                 throw new IllegalArgumentException(targetClass + " is a primitive class");
             if (targetClass.isArray())
@@ -2925,8 +2926,9 @@ assertEquals("[x, y, z]", pb.command().toString());
          * <p>
          * Otherwise, {@code targetClass} is not accessible.
          *
+         * @param <T> the type of the class to be access-checked
          * @param targetClass the class to be access-checked
-         * @return the class that has been access-checked
+         * @return {@code targetClass} that has been access-checked
          * @throws IllegalAccessException if the class is not accessible from the lookup class
          * and previous lookup class, if present, using the allowed access modes.
          * @throws SecurityException if a security manager is present and it
@@ -2935,7 +2937,7 @@ assertEquals("[x, y, z]", pb.command().toString());
          * @since 9
          * @see <a href="#cross-module-lookup">Cross-module lookups</a>
          */
-        public Class<?> accessClass(Class<?> targetClass) throws IllegalAccessException {
+        public <T> Class<T> accessClass(Class<T> targetClass) throws IllegalAccessException {
             if (!isClassAccessible(targetClass)) {
                 throw makeAccessException(targetClass);
             }