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 –.

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.

ClassDescription
HashMapHashMap is the implementation of Map, but it doesn't maintain any order.
LinkedHashMapLinkedHashMap is the implementation of Map. It inherits HashMap class. It maintains insertion order.
TreeMapTreeMap is the implementation of Map and SortedMap. It maintains ascending order.

Useful methods of Map interface

MethodDescription
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 values()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

MethodDescription
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)

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Java Map Example: Generic (New Style)

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Java Map Example: comparingByKey()

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Java Map Example: comparingByKey() in Descending Order

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Java Map Example: comparingByValue()

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Java Map Example: comparingByValue() in Descending Order

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Next TopicJava HashMap class

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 Maps 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 Setand Listinterfaces, 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 — the Set of keys contained in the Map.
  • values — The Collection of values contained in the Map. This Collection is not a Set, because multiple keys can map to the same value.
  • entrySet — the Set of key-value pairs contained in the Map. The Map interface provides a small nested interface called Map.Entry, the type of the elements in this Set.

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.

Java get value from map

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 Sets 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.

What are key values

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.