An object that maps keys to values. A map cannot contain duplicate keys; each key can map to at most one value.
This interface takes the place of the
Dictionary class, which was a totally abstract class rather than an interface.
Map interface provides three collection views , which allow a map's contents to be viewed as a set of keys, collection of values, or set of key-value mappings. The order of a map is defined as the order in which the iterators on the map's collection views return their elements. Some map implementations, like the
TreeMap class, make specific guarantees as to their order; others, like the
HashMap class, do not.
Note: great care must be exercised if mutable objects are used as map keys. The behavior of a map is not specified if the value of an object is changed in a manner that affects
equals comparisons while the object is a key in the map. A special case of this prohibition is that it is not permissible for a map to contain itself as a key. While it is permissible for a map to contain itself as a value, extreme caution is advised: the
hashCode methods are no longer well defined on such a map.
All general-purpose map implementation classes should provide two "standard" constructors: a void (no arguments) constructor which creates an empty map, and a constructor with a single argument of type
Map, which creates a new map with the same key-value mappings as its argument. In effect, the latter constructor allows the user to copy any map, producing an equivalent map of the desired class. There is no way to enforce this recommendation (as interfaces cannot contain constructors) but all of the general-purpose map implementations in the JDK comply.
The "destructive" methods contained in this interface, that is, the methods that modify the map on which they operate, are specified to throw
UnsupportedOperationException if this map does not support the operation. If this is the case, these methods may, but are not required to, throw an
UnsupportedOperationException if the invocation would have no effect on the map. For example, invoking the
putAll(Map) method on an unmodifiable map may, but is not required to, throw the exception if the map whose mappings are to be "superimposed" is empty.
Some map implementations have restrictions on the keys and values they may contain. For example, some implementations prohibit null keys and values, and some have restrictions on the types of their keys. Attempting to insert an ineligible key or value throws an unchecked exception, typically
ClassCastException. Attempting to query the presence of an ineligible key or value may throw an exception, or it may simply return false; some implementations will exhibit the former behavior and some will exhibit the latter. More generally, attempting an operation on an ineligible key or value whose completion would not result in the insertion of an ineligible element into the map may throw an exception or it may succeed, at the option of the implementation. Such exceptions are marked as "optional" in the specification for this interface.
Many methods in Collections Framework interfaces are defined in terms of the
equals method. For example, the specification for the
containsKey(Object key) method says: "returns
true if and only if this map contains a mapping for a key
k such that
(key==null ? k==null : key.equals(k)) ." This specification should not be construed to imply that invoking
Map.containsKey with a non-null argument
key will cause
key.equals(k) to be invoked for any key
k. Implementations are free to implement optimizations whereby the
equals invocation is avoided, for example, by first comparing the hash codes of the two keys. (The
Object.hashCode() specification guarantees that two objects with unequal hash codes cannot be equal.) More generally, implementations of the various Collections Framework interfaces are free to take advantage of the specified behavior of underlying
Object methods wherever the implementor deems it appropriate.
Some map operations which perform recursive traversal of the map may fail with an exception for self-referential instances where the map directly or indirectly contains itself. This includes the
toString() methods. Implementations may optionally handle the self-referential scenario, however most current implementations do not do so.
Map.copyOf static factory methods provide a convenient way to create unmodifiable maps. The
Map instances created by these methods have the following characteristics:
- They are unmodifiable. Keys and values cannot be added, removed, or updated. Calling any mutator method on the Map will always cause
UnsupportedOperationException to be thrown. However, if the contained keys or values are themselves mutable, this may cause the Map to behave inconsistently or its contents to appear to change.
- They disallow
null keys and values. Attempts to create them with
null keys or values result in
- They are serializable if all keys and values are serializable.
- They reject duplicate keys at creation time. Duplicate keys passed to a static factory method result in
- The iteration order of mappings is unspecified and is subject to change.
- They are value-based. Callers should make no assumptions about the identity of the returned instances. Factories are free to create new instances or reuse existing ones. Therefore, identity-sensitive operations on these instances (reference equality (
==), identity hash code, and synchronization) are unreliable and should be avoided.
- They are serialized as specified on the Serialized Form page.
This interface is a member of the Java Collections Framework .