Interface Map
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.
The 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 equals and 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 NullPointerException or 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 clone() , equals() , hashCode() and toString() methods. Implementations may optionally handle the self-referential scenario, however most current implementations do not do so.
Unmodifiable Maps
- 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 NullPointerException .
- 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 IllegalArgumentException .
- The iteration order of mappings is unspecified and is subject to change.
- They are value-based. Programmers should treat instances that are equal as interchangeable and should not use them for synchronization, or unpredictable behavior may occur. For example, in a future release, synchronization may fail. Callers should make no assumptions about the identity of the returned instances. Factories are free to create new instances or reuse existing ones.
- They are serialized as specified on the Serialized Form page.
This interface is a member of the Java Collections Framework.
How to Compare Two Maps in Java
Learn different ways to compare two hashmaps in Java by keys, values and key-value pairs. Also, learn to compare Maps while allowing or restricting duplicate values.
1. Compare Maps for Same Keys and Values
By default, HashMap.equals() method compares two hashmaps by key-value pairs. It means both HashMap instances must have exactly the same key-value pairs and both must be of the same size. The order of key-value pairs can be different and does not play in role in comparison.
Map map1 = Map.of("A", 1, "B", 2); Map map2 = Map.of("A", 1, "B", 2); Map map3 = Map.of("C", 1, "D", 2); Assertions.assertTrue( map1.equals(map2) ); Assertions.assertFalse( map1.equals(map3) );1.2. Comparing Maps with Array Type Values
It is worth noting the Map keys and values are compared using their equals() method so the key and value objects must properly implement the equals() method to give a consistent result. For example, if Map value is an array then the comparison will not work as array.equals() compares object identity and not the contents of the array.
Map map4 = Map.of("A", new Integer[], "B", new Integer[]); Map map5 = Map.of("A", new Integer[], "B", new Integer[]); Assertions.assertFalse(map4.equals(map5));In such a case, we can create a custom method to compare the array contents using the Arrays.equals() method.
private static boolean checkEqualMapsWithArrayTypeValues( Map firstMap, Map secondMap) < if (firstMap.size() != secondMap.size()) return false; return firstMap.entrySet().stream() .allMatch(e ->Arrays.equals(e.getValue(), secondMap.get(e.getKey()))); >Now the map comparison will check for array content and gives the correct result.
Map map4 = Map.of("A", new Integer[], "B", new Integer[]); Map map5 = Map.of("A", new Integer[], "B", new Integer[]); Assertions.assertFalse( checkEqualMapsWithArrayTypeValues(map4, map5) );We can compare two Maps to have the same keys or not. Or we can find the missing keys in the second Map, if needed.
2.1. Both Maps Have Same Keys
If we want to compare hashmaps by keys i.e. two hashmaps will be equal if they have the exactly the same set of keys, we can use HashMap.keySet() function. It returns all the map keys in HashSet.
Then we can compare the HashSet for both maps using Set.equals() method. It returns true if the two sets have the same size, and every element of the specified set is contained in another set.
Map map1 = Map.of("A", 1, "B", 2); Map map2 = Map.of("A", 1, "B", 2); Assertions.assertTrue( map1.keySet().equals(map2.keySet()) ); Map map3 = Map.of("A", 1, "B", 2, "C", 3, "D", 4); Assertions.assertFalse( map1.keySet().equals(map6.keySet()) );2.2. Difference in Map Keys
We may be interested in finding out what extra keys the first hashmap has than the second hashmap. To get this difference, do a union of keys from both hashmaps, and then remove all keys present in the first hashmap.
Java program to find out the difference between two hashmaps.
HashSet unionKeys = new HashSet<>(map1.keySet()); unionKeys.addAll(map3.keySet()); unionKeys.removeAll(map1.keySet()); Assertions.assertEquals(Set.of("C", "D"), unionKeys );If we want to compare hashmaps by values i.e. two hashmaps will be equal if they have exactly the same set of values. Please note that HashMap allows duplicate values, so decide if you want to compare hashmaps with duplicate or without duplicate values.
3.1. Duplicate Values Are NOT Allowed
Add all values from HashMap.values() to an ArrayList for both maps. Now compare both array lists for equality.
new ArrayList<>( map1.values() ).equals(new ArrayList<>( map2.values() )); //true new ArrayList<>( map1.values() ).equals(new ArrayList<>( map3.values() )); //false3.2. Duplicate Values Are Allowed
If you want to remove duplicate values before comparing the hashmaps, add all values into a HashSet that automatically ignores duplicate values.
new HashSet<>( map1.values() ).equals(new HashSet<>( map2.values() )); //true new HashSet<>( map1.values() ).equals(new HashSet<>( map3.values() )); //true4. Map Difference with Guava
If we are interested in finding the difference between two Maps then Guava provides an excellent API Maps.difference().
com.google.guava guava 31.1-jre The Map.difference() returns an instance of MapDifference class. By inspecting the MapDifference, we can find the difference in Maps in multiple ways.
MapDifference diff = Maps.difference(map1, map2);Let us see the API in action using a simple example. Both map1 and map2 have some entries in common, and each map has a few distinct entries.
Map map1 = Map.of("A", 1, "B", 2, "E", 5, "F", 6); Map map2 = Map.of("A", 1, "B", 2, "C", 3, "D", 4); MapDifference diff = Maps.difference(map1, map2); Assertions.assertFalse(diff.areEqual()); Assertions.assertEquals(Map.of("A", 1, "B", 2), diff.entriesInCommon()); Assertions.assertEquals(Map.of("E", 5, "F", 6), diff.entriesOnlyOnLeft()); Assertions.assertEquals(Map.of("C", 3, "D", 4), diff.entriesOnlyOnRight());In this short tutorial, we learned to compare two Maps for the equality of their entries, key and values. We also learned to find the Map differences using the plain Java APIs as well as Guava’s MapDifference API.
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