Module jdk.dynalink
Package jdk.dynalink

Class NamespaceOperation

  • All Implemented Interfaces:
    Operation

    public final class NamespaceOperation
extends Object
implements Operation
    Describes an operation that operates on at least one Namespace of an object. E.g. a property getter would be described as 
     Operation propertyGetter = new NamespaceOperation(
     StandardOperation.GET,
     StandardNamespace.PROPERTY);
    They are often combined with NamedOperation, e.g. to express a property getter for a property named "color", you would construct: 
     Operation colorPropertyGetter = new NamedOperation(
     new NamespaceOperation(
         StandardOperation.GET,
         StandardNamespace.PROPERTY),
     "color");

    While NamespaceOperation can be constructed directly, it is often convenient to use the Operation.withNamespace(Namespace) and Operation.withNamespaces(Namespace...) factory methods instead, e.g.: 

     Operation getElementOrPropertyEmpty =
     StandardOperation.GET
         .withNamespace(StandardNamespace.PROPERTY)
         .named("color");

    Operations on multiple namespaces

    If multiple namespaces are specified, the namespaces are treated as alternatives to each other in order of preference. The semantics of such operation is "first applicable". That is, a composite of GET:PROPERTY|ELEMENT:color should be interpreted as get the property named "color" on the object, but if the property does not exist, then get the collection element named "color" instead.

    Operations with multiple namespaces are helpful in implementation of languages that don't distinguish between one or more of the namespaces, or when expressing operations against objects that can be considered both ordinary objects and collections, e.g. Java Map objects. A GET:PROPERTY|ELEMENT:empty operation against a Java map will always match the Map.isEmpty() property, but GET:ELEMENT|PROPERTY:empty will actually match a map element with key "empty" if the map contains that key, and only fall back to the isEmpty() property getter if the map does not contain the key. If the source language mandates this semantics, it can be easily achieved using operations on multiple namespaces.

    Even if the language itself doesn't distinguish between some of the namespaces, it can be helpful to map different syntaxes to different namespace orderings. E.g. the source expression obj.color could map to GET:PROPERTY|ELEMENT|METHOD:color, but a different source expression that looks like collection element access obj[key] could be expressed instead as GET:ELEMENT|PROPERTY|METHOD in order to favor the element semantics. Finally, if the retrieved value is subsequently called, then it makes sense to bring METHOD to the front of the namespace list: the getter part of the source expression obj.color() could be GET:METHOD|PROPERTY|ELEMENT:color and the one for obj[key]() could be GET:METHOD|ELEMENT|PROPERTY.

    The base operation of a namespace operation can not itself be a namespace or named operation, but rather one of simple operations such are elements of StandardOperation. A namespace operation itself can serve as the base operation of a named operation, though; a typical way to construct e.g. the GET:ELEMENT|PROPERTY:empty from above would be: 

     Operation getElementOrPropertyEmpty = StandardOperation.GET
     .withNamespaces(
         StandardNamespace.ELEMENT,
         StandardNamespace.PROPERTY)
     .named("empty");

    • Constructor Detail

      • NamespaceOperation

        public NamespaceOperation​(Operation baseOperation,
                          Namespace... namespaces)
        Constructs a new namespace operation.
        Parameters:
        baseOperation - the base operation that operates on one or more namespaces.
        namespaces - one or more namespaces this operation operates on.
        Throws:
        IllegalArgumentException - if less than one namespace is specified, or the base operation is itself a NamespaceOperation or a NamedOperation.
        NullPointerException - if either the namespaces array or any of its elements are null, or if baseOperation is null.

    • Method Detail

      • getBaseOperation

        public Operation getBaseOperation​()
        Returns the base operation of this named operation.
        Returns:
        the base operation of this named operation.

      • getNamespaces

        public Namespace[] getNamespaces​()
        Returns the namespaces in this namespace operation. The returned array is a copy and changes to it don't have effect on this object.
        Returns:
        the namespaces in this namespace operation.

      • getNamespaceCount

        public int getNamespaceCount​()
        Returns the number of namespaces in this namespace operation.
        Returns:
        the number of namespaces in this namespace operation.

      • getNamespace

        public Namespace getNamespace​(int i)
        Returns the i-th namespace in this namespace operation.
        Parameters:
        i - the namespace index
        Returns:
        the i-th namespace in this namespace operation.
        Throws:
        IndexOutOfBoundsException - if the index is out of range.

      • contains

        public boolean contains​(Namespace namespace)
        Returns true if this namespace operation contains a namespace equal to the specified namespace.
        Parameters:
        namespace - the namespace being searched for. Must not be null.
        Returns:
        true if the if this namespace operation contains a namespace equal to the specified namespace.

      • equals

        public boolean equals​(Object obj)
        Returns true if the other object is also a namespace operation and their base operation and namespaces are equal.
        Overrides:
        equals in class Object
        Parameters:
        obj - the object to compare to
        Returns:
        true if this object is equal to the other one, false otherwise.
        See Also:
        Object.hashCode(), HashMap

      • toString

        public String toString​()
        Returns the string representation of this namespace operation. Defined to be the toString of its base operation, followed by a colon character, followed with the list of its namespaces separated with the vertical line character (e.g. "GET:PROPERTY|ELEMENT").
        Overrides:
        toString in class Object
        Returns:
        the string representation of this namespace operation.

      • getBaseOperation

        public static Operation getBaseOperation​(Operation op)
        If the passed operation is a namespace operation, returns its getBaseOperation(), otherwise returns the operation as is.
        Parameters:
        op - the operation
        Returns:
        the base operation of the passed operation.

      • getNamespaces

        public static Namespace[] getNamespaces​(Operation op)
        If the passed operation is a namespace operation, returns its getNamespaces(), otherwise returns an empty array.
        Parameters:
        op - the operation
        Returns:
        the namespaces of the passed operation.

      • contains

        public static boolean contains​(Operation op,
                               Operation baseOperation,
                               Namespace namespace)
        Returns true if the specified operation is a NamespaceOperation and its base operation is equal to the specified operation, and it contains the specified namespace. If it is not a NamespaceOperation, then it returns false.
        Parameters:
        op - the operation. Must not be null.
        baseOperation - the base operation being searched for. Must not be null.
        namespace - the namespace being searched for. Must not be null.
        Returns:
        true if the if the passed operation is a NamespaceOperation, its base operation equals the searched base operation, and contains a namespace equal to the searched namespace.