805 
 806     /**
 807      * Returns the set of the modules in this layer.
 808      *
 809      * @return A possibly-empty unmodifiable set of the modules in this layer
 810      */
 811     public Set<Module> modules() {
 812         Set<Module> modules = this.modules;
 813         if (modules == null) {
 814             this.modules = modules =
 815                 Collections.unmodifiableSet(new HashSet<>(nameToModule.values()));
 816         }
 817         return modules;
 818     }
 819 
 820     private volatile Set<Module> modules;
 821 
 822 
 823     /**
 824      * Returns the module with the given name in this layer, or if not in this
 825      * layer, the {@linkplain #parents parent} layers. Finding a module in
 826      * parent layers is equivalent to invoking {@code findModule} on each
 827      * parent, in search order, until the module is found or all parents have
 828      * been searched. In a <em>tree of layers</em>  then this is equivalent to
 829      * a depth-first search.
 830      *
 831      * @param  name
 832      *         The name of the module to find
 833      *
 834      * @return The module with the given name or an empty {@code Optional}
 835      *         if there isn't a module with this name in this layer or any
 836      *         parent layer
 837      */
 838     public Optional<Module> findModule(String name) {
 839         Objects.requireNonNull(name);
 840         if (this == EMPTY_LAYER)
 841             return Optional.empty();
 842         Module m = nameToModule.get(name);
 843         if (m != null)
 844             return Optional.of(m);
 845 
 846         return layers()
 847                 .skip(1)  // skip this layer
 848                 .map(l -> l.nameToModule)
 849                 .filter(map -> map.containsKey(name))
 850                 .map(map -> map.get(name))
 851                 .findAny();
 852     }
 853 
 854 
 855     /**
 856      * Returns the {@code ClassLoader} for the module with the given name. If
 857      * a module of the given name is not in this layer then the {@link #parents
 858      * parent} layers are searched in the manner specified by {@link
 859      * #findModule(String) findModule}.
 860      *
 861      * <p> If there is a security manager then its {@code checkPermission}
 862      * method is called with a {@code RuntimePermission("getClassLoader")}
 863      * permission to check that the caller is allowed to get access to the
 864      * class loader. </p>
 865      *
 866      * @apiNote This method does not return an {@code Optional<ClassLoader>}
 867      * because `null` must be used to represent the bootstrap class loader.
 868      *
 869      * @param  name
 870      *         The name of the module to find
 871      *
 872      * @return The ClassLoader that the module is defined to
 873      *
 874      * @throws IllegalArgumentException if a module of the given name is not
 875      *         defined in this layer or any parent of this layer
 876      *
 877      * @throws SecurityException if denied by the security manager

 805 
 806     /**
 807      * Returns the set of the modules in this layer.
 808      *
 809      * @return A possibly-empty unmodifiable set of the modules in this layer
 810      */
 811     public Set<Module> modules() {
 812         Set<Module> modules = this.modules;
 813         if (modules == null) {
 814             this.modules = modules =
 815                 Collections.unmodifiableSet(new HashSet<>(nameToModule.values()));
 816         }
 817         return modules;
 818     }
 819 
 820     private volatile Set<Module> modules;
 821 
 822 
 823     /**
 824      * Returns the module with the given name in this layer, or if not in this
 825      * layer, the {@linkplain #parents() parent} layers. Finding a module in
 826      * parent layers is equivalent to invoking {@code findModule} on each
 827      * parent, in search order, until the module is found or all parents have
 828      * been searched. In a <em>tree of layers</em>  then this is equivalent to
 829      * a depth-first search.
 830      *
 831      * @param  name
 832      *         The name of the module to find
 833      *
 834      * @return The module with the given name or an empty {@code Optional}
 835      *         if there isn't a module with this name in this layer or any
 836      *         parent layer
 837      */
 838     public Optional<Module> findModule(String name) {
 839         Objects.requireNonNull(name);
 840         if (this == EMPTY_LAYER)
 841             return Optional.empty();
 842         Module m = nameToModule.get(name);
 843         if (m != null)
 844             return Optional.of(m);
 845 
 846         return layers()
 847                 .skip(1)  // skip this layer
 848                 .map(l -> l.nameToModule)
 849                 .filter(map -> map.containsKey(name))
 850                 .map(map -> map.get(name))
 851                 .findAny();
 852     }
 853 
 854 
 855     /**
 856      * Returns the {@code ClassLoader} for the module with the given name. If
 857      * a module of the given name is not in this layer then the {@link #parents()
 858      * parent} layers are searched in the manner specified by {@link
 859      * #findModule(String) findModule}.
 860      *
 861      * <p> If there is a security manager then its {@code checkPermission}
 862      * method is called with a {@code RuntimePermission("getClassLoader")}
 863      * permission to check that the caller is allowed to get access to the
 864      * class loader. </p>
 865      *
 866      * @apiNote This method does not return an {@code Optional<ClassLoader>}
 867      * because `null` must be used to represent the bootstrap class loader.
 868      *
 869      * @param  name
 870      *         The name of the module to find
 871      *
 872      * @return The ClassLoader that the module is defined to
 873      *
 874      * @throws IllegalArgumentException if a module of the given name is not
 875      *         defined in this layer or any parent of this layer
 876      *
 877      * @throws SecurityException if denied by the security manager