Java Generate All Key Values For A Map
- I am trying to implements a map (that can work as a cache) that has strings as keys and values of different types. In my real case once set the key/values in the map will not change. I used these.
- The java.util.HashMap.keySet method in Java is used to create a set out of the key elements contained in the hash map. It basically returns a set view of the keys or we can create a new set and store the key elements in them.
- 5 ways to Iterate Map using keySet in Java We know that keySet method returns a Set view of the keys contained in the map and values method returns a Set view of the values contained in the map. So we can use keySet to print all keys present in the Map and values to print all values. There are several ways to do that –.
- Java Generate All Key Values For A Map Of The World
- Java Generate All Key Values For A Map Of America
A map contains values on the basis of key, i.e. key and value pair. Each key and value pair is known as an entry. A Map contains unique keys.
A Map is useful if you have to search, update or delete elements on the basis of a key.
This example shows how to count the occurrences of key or value in a map. Creating sample map of data by initialize a map of months and then using java 8 and guava to count the number of times a key or value occurs. A comparable example demonstrates how to count keys or values in a groovy map.
Java Map Hierarchy
There are two interfaces for implementing Map in java: Map and SortedMap, and three classes: HashMap, LinkedHashMap, and TreeMap. The hierarchy of Java Map is given below:
A Map doesn't allow duplicate keys, but you can have duplicate values. HashMap and LinkedHashMap allow null keys and values, but TreeMap doesn't allow any null key or value.
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A Map can't be traversed, so you need to convert it into Set using keySet() or entrySet() method.
Class | Description |
---|---|
HashMap | HashMap is the implementation of Map, but it doesn't maintain any order. |
LinkedHashMap | LinkedHashMap is the implementation of Map. It inherits HashMap class. It maintains insertion order. |
TreeMap | TreeMap is the implementation of Map and SortedMap. It maintains ascending order. |
Useful methods of Map interface
Method | Description |
---|---|
V put(Object key, Object value) | It is used to insert an entry in the map. |
void putAll(Map map) | It is used to insert the specified map in the map. |
V putIfAbsent(K key, V value) | It inserts the specified value with the specified key in the map only if it is not already specified. |
V remove(Object key) | It is used to delete an entry for the specified key. |
boolean remove(Object key, Object value) | It removes the specified values with the associated specified keys from the map. |
Set keySet() | It returns the Set view containing all the keys. |
Set<Map.Entry<K,V>> entrySet() | It returns the Set view containing all the keys and values. |
void clear() | It is used to reset the map. |
V compute(K key, BiFunction<? super K,? super V,? extends V> remappingFunction) | It is used to compute a mapping for the specified key and its current mapped value (or null if there is no current mapping). |
V computeIfAbsent(K key, Function<? super K,? extends V> mappingFunction) | It is used to compute its value using the given mapping function, if the specified key is not already associated with a value (or is mapped to null), and enters it into this map unless null. |
V computeIfPresent(K key, BiFunction<? super K,? super V,? extends V> remappingFunction) | It is used to compute a new mapping given the key and its current mapped value if the value for the specified key is present and non-null. |
boolean containsValue(Object value) | This method returns true if some value equal to the value exists within the map, else return false. |
boolean containsKey(Object key) | This method returns true if some key equal to the key exists within the map, else return false. |
boolean equals(Object o) | It is used to compare the specified Object with the Map. |
void forEach(BiConsumer<? super K,? super V> action) | It performs the given action for each entry in the map until all entries have been processed or the action throws an exception. |
V get(Object key) | This method returns the object that contains the value associated with the key. |
V getOrDefault(Object key, V defaultValue) | It returns the value to which the specified key is mapped, or defaultValue if the map contains no mapping for the key. |
int hashCode() | It returns the hash code value for the Map |
boolean isEmpty() | This method returns true if the map is empty; returns false if it contains at least one key. |
V merge(K key, V value, BiFunction<? super V,? super V,? extends V> remappingFunction) | If the specified key is not already associated with a value or is associated with null, associates it with the given non-null value. |
V replace(K key, V value) | It replaces the specified value for a specified key. |
boolean replace(K key, V oldValue, V newValue) | It replaces the old value with the new value for a specified key. |
void replaceAll(BiFunction<? super K,? super V,? extends V> function) | It replaces each entry's value with the result of invoking the given function on that entry until all entries have been processed or the function throws an exception. |
Collection | It returns a collection view of the values contained in the map. |
int size() | This method returns the number of entries in the map. |
Map.Entry Interface
Java Generate All Key Values For A Map Of The World
Entry is the subinterface of Map. So we will be accessed it by Map.Entry name. It returns a collection-view of the map, whose elements are of this class. It provides methods to get key and value.
Methods of Map.Entry interface
Method | Description |
---|---|
K getKey() | It is used to obtain a key. |
V getValue() | It is used to obtain value. |
int hashCode() | It is used to obtain hashCode. |
V setValue(V value) | It is used to replace the value corresponding to this entry with the specified value. |
boolean equals(Object o) | It is used to compare the specified object with the other existing objects. |
static <K extends Comparable<? super K>,V>Comparator<Map.Entry<K,V>> comparingByKey() | It returns a comparator that compare the objects in natural order on key. |
static <K,V> Comparator<Map.Entry<K,V>> comparingByKey(Comparator<? super K> cmp) | It returns a comparator that compare the objects by key using the given Comparator. |
static <K,V extends Comparable<? super V>>Comparator<Map.Entry<K,V>> comparingByValue() | It returns a comparator that compare the objects in natural order on value. |
static <K,V> Comparator<Map.Entry<K,V>> comparingByValue(Comparator<? super V> cmp) | It returns a comparator that compare the objects by value using the given Comparator. |
Java Map Example: Non-Generic (Old Style)
Output:
Java Map Example: Generic (New Style)
Output:
Java Map Example: comparingByKey()
Output:
Java Map Example: comparingByKey() in Descending Order
Output:
Java Map Example: comparingByValue()
Output:
Java Map Example: comparingByValue() in Descending Order
Output:
A Map
is an object that maps keys to values. A map cannot contain duplicate keys: Each key can map to at most one value. It models the mathematical function abstraction. The Map
interface includes methods for basic operations (such as put
, get
, remove
, containsKey
, containsValue
, size
, and empty
),bulk operations (such as putAll
and clear
), andcollection views (such as keySet
, entrySet
, and values
).
The Java platform contains three general-purpose Map
implementations: HashMap
, TreeMap
, and LinkedHashMap
. Their behavior and performance are precisely analogous to HashSet
, TreeSet
, and LinkedHashSet
, as described in The Set Interface section.
The remainder of this page discusses the Map
interface in detail. But first, here are some more examples of collecting to Map
s using JDK 8 aggregate operations. Modeling real-world objects is a common task in object-oriented programming, so it is reasonable to think that some programs might, for example, group employees by department:
Or compute the sum of all salaries by department:
Or perhaps group students by passing or failing grades:
You could also group people by city:
Or even cascade two collectors to classify people by state and city:
Again, these are but a few examples of how to use the new JDK 8 APIs. For in-depthcoverage of lambda expressions and aggregate operations see the lesson entitledAggregate Operations.
Map Interface Basic Operations
The basic operations of Map
(put
, get
, containsKey
, containsValue
, size
, and isEmpty
) behave exactly like their counterparts in Hashtable
. The following program
generates a frequency table of the words found in its argument list. The frequency table maps each word to the number of times it occurs in the argument list.
The only tricky thing about this program is the second argument of the put
statement. That argument is a conditional expression that has the effect of setting the frequency to one if the word has never been seen before or one more than its current value if the word has already been seen. Try running this program with the command:
The program yields the following output.
Suppose you'd prefer to see the frequency table in alphabetical order. All you have to do is change the implementation type of the Map
from HashMap
to TreeMap
. Making this four-character change causes the program to generate the following output from the same command line.
Similarly, you could make the program print the frequency table in the order the words first appear on the command line simply by changing the implementation type of the map to LinkedHashMap
. Doing so results in the following output.
This flexibility provides a potent illustration of the power of an interface-based framework.
Like the Set
and List
interfaces, Map
strengthens the requirements on the equals
and hashCode
methods so that two Map
objects can be compared for logical equality without regard to their implementation types. Two Map
instances are equal if they represent the same key-value mappings.
By convention, all general-purpose Map
implementations provide constructors that take a Map
object and initialize the new Map
to contain all the key-value mappings in the specified Map
. This standard Map
conversion constructor is entirely analogous to the standard Collection
constructor: It allows the caller to create a Map
of a desired implementation type that initially contains all of the mappings in another Map
, regardless of the other Map
's implementation type. For example, suppose you have a Map
, named m
. The following one-liner creates a new HashMap
initially containing all of the same key-value mappings as m
.
Map Interface Bulk Operations
The clear
operation does exactly what you would think it could do: It removes all the mappings from the Map
. The putAll
operation is the Map
analogue of the Collection
interface's addAll
operation. In addition to its obvious use of dumping one Map
into another, it has a second, more subtle use. Suppose a Map
is used to represent a collection of attribute-value pairs; the putAll
operation, in combination with the Map
conversion constructor, provides a neat way to implement attribute map creation with default values. The following is a static factory method that demonstrates this technique.
Java Generate All Key Values For A Map Of America
Collection Views
The Collection
view methods allow a Map
to be viewed as a Collection
in these three ways:
keySet
— theSet
of keys contained in theMap
.values
— TheCollection
of values contained in theMap
. ThisCollection
is not aSet
, because multiple keys can map to the same value.entrySet
— theSet
of key-value pairs contained in theMap
. TheMap
interface provides a small nested interface calledMap.Entry
, the type of the elements in thisSet
.
The Collection
views provide the only means to iterate over a Map
. This example illustrates the standard idiom for iterating over the keys in a Map
with a for-each
construct:
and with an iterator
:
The idiom for iterating over values is analogous. Following is the idiom for iterating over key-value pairs.
At first, many people worry that these idioms may be slow because the Map
has to create a new Collection
instance each time a Collection
view operation is called. Rest easy: There's no reason that a Map
cannot always return the same object each time it is asked for a given Collection
view. This is precisely what all the Map
implementations in java.util
do.
With all three Collection
views, calling an Iterator
's remove
operation removes the associated entry from the backing Map
, assuming that the backing Map
supports element removal to begin with. This is illustrated by the preceding filtering idiom.
With the entrySet
view, it is also possible to change the value associated with a key by calling a Map.Entry
's setValue
method during iteration (again, assuming the Map
supports value modification to begin with). Note that these are the only safe ways to modify a Map
during iteration; the behavior is unspecified if the underlying Map
is modified in any other way while the iteration is in progress.
The Collection
views support element removal in all its many forms — remove
, removeAll
, retainAll
, and clear
operations, as well as the Iterator.remove
operation. (Yet again, this assumes that the backing Map
supports element removal.)
The Collection
views do not support element addition under any circumstances. It would make no sense for the keySet
and values
views, and it's unnecessary for the entrySet
view, because the backing Map
's put
and putAll
methods provide the same functionality.
Fancy Uses of Collection Views: Map Algebra
When applied to the Collection
views, bulk operations (containsAll
, removeAll
, and retainAll
) are surprisingly potent tools. For starters, suppose you want to know whether one Map
is a submap of another — that is, whether the first Map
contains all the key-value mappings in the second. The following idiom does the trick.
Along similar lines, suppose you want to know whether two Map
objects contain mappings for all of the same keys.
Suppose you have a Map
that represents a collection of attribute-value pairs, and two Set
s representing required attributes and permissible attributes. (The permissible attributes include the required attributes.) The following snippet determines whether the attribute map conforms to these constraints and prints a detailed error message if it doesn't.
Suppose you want to know all the keys common to two Map
objects.
A similar idiom gets you the common values.
All the idioms presented thus far have been nondestructive; that is, they don't modify the backing Map
. Here are a few that do. Suppose you want to remove all of the key-value pairs that one Map
has in common with another.
Suppose you want to remove from one Map
all of the keys that have mappings in another.
What happens when you start mixing keys and values in the same bulk operation? Suppose you have a Map
, managers
, that maps each employee in a company to the employee's manager. We'll be deliberately vague about the types of the key and the value objects. It doesn't matter, as long as they're the same. Now suppose you want to know who all the 'individual contributors' (or nonmanagers) are. The following snippet tells you exactly what you want to know.
Suppose you want to fire all the employees who report directly to some manager, Simon.
Note that this idiom makes use of Collections.singleton
, a static factory method that returns an immutable Set
with the single, specified element.
Once you've done this, you may have a bunch of employees whose managers no longer work for the company (if any of Simon's direct-reports were themselves managers). The following code will tell you which employees have managers who no longer works for the company.
This example is a bit tricky. First, it makes a temporary copy of the Map
, and it removes from the temporary copy all entries whose (manager) value is a key in the original Map
. Remember that the original Map
has an entry for each employee. Thus, the remaining entries in the temporary Map
comprise all the entries from the original Map
whose (manager) values are no longer employees. The keys in the temporary copy, then, represent precisely the employees that we're looking for.
There are many more idioms like the ones contained in this section, but it would be impractical and tedious to list them all. Once you get the hang of it, it's not that difficult to come up with the right one when you need it.
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Multimaps
A multimap is like a Map
but it can map each key to multiple values. The Java Collections Framework doesn't include an interface for multimaps because they aren't used all that commonly. It's a fairly simple matter to use a Map
whose values are List
instances as a multimap. This technique is demonstrated in the next code example, which reads a word list containing one word per line (all lowercase) and prints out all the anagram groups that meet a size criterion. An anagram group is a bunch of words, all of which contain exactly the same letters but in a different order. The program takes two arguments on the command line: (1) the name of the dictionary file and (2) the minimum size of anagram group to print out. Anagram groups containing fewer words than the specified minimum are not printed.
There is a standard trick for finding anagram groups: For each word in the dictionary, alphabetize the letters in the word (that is, reorder the word's letters into alphabetical order) and put an entry into a multimap, mapping the alphabetized word to the original word. For example, the word bad causes an entry mapping abd into bad to be put into the multimap. A moment's reflection will show that all the words to which any given key maps form an anagram group. It's a simple matter to iterate over the keys in the multimap, printing out each anagram group that meets the size constraint.
The following program
is a straightforward implementation of this technique.
Running this program on a 173,000-word dictionary file with a minimum anagram group size of eight produces the following output.
Many of these words seem a bit bogus, but that's not the program's fault; they're in the dictionary file. Here's thedictionary file
we used.It was derived from the Public Domain ENABLE benchmark reference word list.