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.
Java create map with default values in java
But is strikes me that what you might be trying to implement is a «multimap»; i.e. a mapping from a key to a collection of values. Solution 3: As noted in comments: Guava had the idea of a «computing map» which will execute a function to provide a value if it’s not present.
Create Map with provided set of keys and default values for them
The easiest answer came to me seconds ago:
final Map labelsToReturn = keys.stream().collect(toMap(x -> x, x -> "")); solves the problem perfectly.
Just use stream and toMap with key / value mapping to initialize the map.
I advise you to use Stream API Collectors.toMap() method. There is a way:
private static void build(List keys) < MapdefaultValues = keys.stream().collect( Collectors.toMap(key -> key, key -> "default value") ); // further computations > Map map = new HashMap<>(); String emptyString = ""; for(String key:keys) < Object yourcomputation = emptyString; //asign your computation to new value base on key map.put(key,yourcomputation); >Inititialize Map keys from list with a default value
Yes, you can create a Stream of that List ‘s elements and collect them to a Map :
Map map = keys.stream () .collect (Collectors.toMap (Function.identity (), k -> "default")); HashMap in Java, Java HashMap allows null key and null values. · HashMap is not an ordered collection. · HashMap is almost similar to Hashtable except that it’s
Java Map implementation that returns a default value instead of null
Thanks to default methods, Java 8 now has this built in with Map::getOrDefault :
Map map = . map.put(1, "1"); System.out.println(map.getOrDefault(1, "2")); // "1" System.out.println(map.getOrDefault(2, "2")); // "2" @Jon’s answer is a good way to do what you are asking directly.
But is strikes me that what you might be trying to implement is a «multimap»; i.e. a mapping from a key to a collection of values. If that is the case, then you should also look at the multimap classes in guava or apache commons collections.
- the com.google.common.collect.Multimap interface and its implementations, or
- the org.apache.commons.collections.MultiMap interface and its implementations.
- the org.apache.commons.collections4.MultiMap interface and its implementations (a new version of the previous MultiMap; introduced in v4).
Guava’s computing map concept was superseded with LoadingCache . Also java 8 introduced to Map interface nice computeIfAbsent default method which doesn’t break map contract and features lazy evaluation .
Guava had the idea of a «computing map» which will execute a function to provide a value if it’s not present. It was implemented in MapMaker.makeComputingMap ; you could now use CacheBuilder — see CacheBuilder.build for more details.
It may be overkill for what you’re after — you might be better off just writing a Map implementation which has a Map (using composition rather than extending any particular implementation) and then just return the default if it’s not present. Every method other than get could probably just delegate to the other map:
public class DefaultingMap implements Map < private final Mapmap; private final V defaultValue; public DefaultingMap(Map map, V defaultValue) < this.map = map; this.defaultValue = defaultValue; >@Override public V get(Object key) < V ret = map.get(key); if (ret == null) < ret = defaultValue; >return ret; > @Override public int size() < return map.size(); >// etc > HashMap initialization in java, Becoming
Default Values in a Hashmap
The easiest way by far is to write
Integer r = Letters.get(. ); if (r==null) r=0; rather than mess about with arranging for your map to return a default value.
int r = Letters.contains(. ) ? Letters.get(. ) : 0; You can override get(Object key) when you create your HashMap . Try this during Letters creation in your code, hope it helps.
HashMap Letters = new HashMap() < @Override public Integer get(Object key) < if(containsKey(key))< return super.get(key); >return 0; > >; You can use an array instead of a Map. If you are just using ASCII, the range of characters maps to a range of indices. This code is also quicker to run, if performance matters in your case.
int[] Letters = new int['Z' - 'A' + 1]; . // Inside the for loop Letters[LettersWord[j] - 'A'] += (int) Math.pow(10, LettersWord.length - 1 - j); HashMap to return default value for non-found keys?, In Java 8, use Map.getOrDefault. It takes the key, and the value to return if no matching key is found.