Package com.google.appengine.api.datastore

Class Key

java.lang.Object
com.google.appengine.api.datastore.Key
All Implemented Interfaces:
Serializable, Comparable<Key>
public final class Key extends Object implements Serializable, Comparable<Key>
The primary key for a datastore entity.

A datastore GUID. A Key instance uniquely identifies an entity across all apps, and includes all information necessary to fetch the entity from the datastore with DatastoreService.get(Key).

You can create Key objects directly by using KeyFactory.createKey(java.lang.String, long) or getChild(java.lang.String, long).

You can also retrieve the Key automatically created when you create a new Entity, or serialize Key objects, or use KeyFactory to convert them to and from websafe String values.

See Also:

  • Method Summary

    Modifier and Type
    Method
    Description
    int
    compareTo(Key other)
    Compares two Key objects.
    boolean
    equals(@Nullable Object object)
    Compares two Key objects by comparing ids, kinds, parent and appIdNamespace.
    String
    getAppId()
    Returns the appId for this Key.
    Key
    getChild(String kind, long id)
    Creates a new key having this as parent and the given numeric identifier.
    Key
    getChild(String kind, String name)
    Creates a new key having this as parent and the given name.
    long
    getId()
    Returns the numeric identifier of this Key.
    String
    getKind()
    Returns the kind of the Entity represented by this Key.
    @Nullable String
    getName()
    Returns the name of this Key.
    String
    getNamespace()
    Returns the namespace for this Key.
    @Nullable Key
    getParent()
    If this Key has a parent, return a Key that represents it.
    int
    hashCode()
     
    boolean
    isComplete()
    Returns true if this Key has a name specified or has been assigned an identifier.
    String
    toString()
     

    Methods inherited from class java.lang.Object

    getClass, notify, notifyAll, wait, wait, wait

  • Method Details

    • getKind

      public String getKind()
      Returns the kind of the Entity represented by this Key.

    • getParent

      public @Nullable Key getParent()
      If this Key has a parent, return a Key that represents it. If not, simply return null.

    • hashCode

      public int hashCode()
      Overrides:
      hashCode in class Object

    • toString

      public String toString()
      Overrides:
      toString in class Object

    • equals

      public boolean equals(@Nullable Object object)
      Compares two Key objects by comparing ids, kinds, parent and appIdNamespace. If both keys are assigned names rather than ids, compares names instead of ids. If neither key has an id or a name, the keys are only equal if they reference the same object.
      Overrides:
      equals in class Object

    • getAppId

      public String getAppId()
      Returns the appId for this Key.

    • getNamespace

      public String getNamespace()
      Returns the namespace for this Key.

    • getId

      public long getId()
      Returns the numeric identifier of this Key.

    • getName

      public @Nullable String getName()
      Returns the name of this Key.

    • getChild

      public Key getChild(String kind, long id)
      Creates a new key having this as parent and the given numeric identifier. The parent key must be complete.
      Parameters:
      kind - the kind of the child key to create
      id - the numeric identifier of the key in kind, unique for this parent

    • getChild

      public Key getChild(String kind, String name)
      Creates a new key having this as parent and the given name. The parent key must be complete.
      Parameters:
      kind - the kind of the child key to create
      name - the name of the key in kind, as an arbitrary string unique for this parent

    • isComplete

      public boolean isComplete()
      Returns true if this Key has a name specified or has been assigned an identifier.

    • compareTo

      public int compareTo(Key other)
      Compares two Key objects. The algorithm proceeds as follows: Turn each Key into an iterator where the first element returned is the top-most ancestor, the next element is the child of the previous element, and so on. The last element will be the Key we started with. Once we have assembled these two iterators (one for 'this' and one for the Key we're comparing to), consume them in parallel, comparing the next element from each iterator. If at any point the comparison of these two elements yields a non-zero result, return that as the result of the overall comparison. If we exhaust the iterator built from 'this' before we exhaust the iterator built from the other Key, we return less than. An example:

      app1.type1.4.app1.type2.9 < app1.type1.4.app1.type2.9.app1.type3.2

      If we exhaust the iterator built from the other Key before we exhaust the iterator built from 'this', we return greater than. An example:

      app1.type1.4.app1.type2.9.app1.type3.2 > app1.type1.4.app1.type2.9

      The relationship between individual Key Keys is performed by comparing app followed by kind followed by id. If both keys are assigned names rather than ids, compares names instead of ids. If neither key has an id or a name we return an arbitrary but consistent result. Assuming all other components are equal, all ids are less than all names.

      Specified by:
      compareTo in interface Comparable<Key>