1. Personal Key Values
  2. Java Map Value To Key
  3. Java Get Value From Map

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.

  • What this is. This file is included in the DevDaily.com 'Java Source Code Warehouse' project.The intent of this project is to help you 'Learn Java by Example' TM.Other links. The search page; Other source code files at this package level.
  • Map with different types for values. Ask Question. 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. Compare this to the typesafe heterogeneous container as defined in Effective Java. There the type is part of the key (actually.
  • Inserting Elements Into a Java Map
  • Get Elements From a Java Map
  • Iterating the Keys of a Java Map
  • Iterating the Values of a Java Map
  • Iterating the Entries of a Java Map
  • Functional Operations in Java Map

The Java Map interface, java.util.Map, represents a mapping between a key and a value. More specifically, a JavaMap can store pairs of keys and values. Each key is linked to a specific value. Once stored in a Map, you can later look up the value using just the key.

The Java Map interface is not a subtype of the Collection interface. Therefore it behaves a bit different from the rest of the collection types.

Java Map Tutorial Video

If you prefer video, I have a video version of this Java Map tutorial available here: Java Map Tutorial

Java Map Implementations

Since Map is an interface you need to instantiate a concrete implementation of the Map interface in order to use it. The Java Collections API contains the following Map implementations:

  • java.util.HashMap
  • java.util.Hashtable
  • java.util.EnumMap
  • java.util.IdentityHashMap
  • java.util.LinkedHashMap
  • java.util.Properties
  • java.util.TreeMap
  • java.util.WeakHashMap

In my experience, the most commonly used Map implementations are HashMap and TreeMap.

Each of these Map implementations behaves a little differently with respect to the order of the elements when iterating the Map, and the time (big O notation) it takes to insert and access elements in the maps.

HashMap maps a key and a value. It does not guarantee any order of the elements stored internally in the map.

TreeMap also maps a key and a value. Furthermore it guarantees the order in which keys or values are iterated - which is the sort order of the keys or values. Check out the Java Map JavaDoc for more details.

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The HashMap implementation is typically the fastest of the two Map implementations, so whenever you don't need to sort the elements in the Map you can just use a HashMap. Otherwise use a TreeMap.

Create a Map

To create a Java Map you must create an instance of one the classes that implement the Java Map interface. Here are a few examples of how to create a Map instance:

Java

Generic Java Map

By default you can put any Object into a Map, but from Java 5, Java Generics makes it possible to limit the types of object you can use for both keys and values in a Map. Here is an example:

This Map can now only accept String objects for keys, and MyObject instances for values. You can then access and iterate keys and values without casting them. Here is how it looks:

If you know the type of the objects being stored in the Map it is considered good practice to always specify a generic type when declaring and creating a Java Map. The generic type helps you avoid inserting the wrong objects, and makes it easier for people reading your code to understand what kind of objects the Map contains. Throughout the rest of this tutorial I will be using generic types on all Map examples whenever it makes sense.

For more information about Java Generics, see the Java Generics Tutorial.

Inserting Elements Into a Java Map

To add elements to a Map you call its put() method. Here are a few examples:

The three put() calls maps a string value to a string key. You can then obtain the value using that key, as we will see in the next section.

Only Objects Can Be Inserted

Only Java objects can be used as keys and values in a Java Map. In case you pass primitive values (e.g. int, double etc.) to a Map as key or value, the primitive values will be auto-boxed before being passed as parameters. Here is an example of auto-boxing primitive parameters passed to the put() method:

The value passed to the put() method in the above example is a primitive int. Java auto-boxes it inside an Integer instance though, because the put() method requires an Oject instance as both key and value. Auto-boxing would also happen if you passed a primitive as key to the put() method.

Subsequent Inserts With Same Key

A given key can only occur in a Java Map one time. That means, that only a single key + value pair for each key can exist in the Map at the same time. In other words, for the key 'key1' only one value can be stored in the same Map instance. Of course you can store values for the same key in different Map instances.

If you call put() more than once with the same key, the latest value passed to put() for that key will overwrite what is already stored in the Map for that key. In other words, the latest value replaces the existing value for the given key.

Null Keys

Quite surprisingly you can use the value null as a key in a Java Map. Here is an example of using a null key in a Java Map:

To obtain the value stored by the null key you call the get() method with null as parameter value. Here is an example of getting the value for the null key of a Java Map:

Null Values

The value of a key + value pair stored in a Map is allowed to be null - so this is valid:

Just keep in mind that you will get a null out when you call get() later with that key - so this will return null:

The value variable will have the value null after this code has been executed, if a null value was inserted for this key earlier (like in the previous example).

Inserting All Elements From Another Map

The Java Map interface has a method called putAll() which can copy all key + value pairs (entries) from another Map instance into itself. In set theory, this is also referred to as the union of the two Map instances.

Here is an example of copying all entries from one Java Map into another via putAll():

After running this code the Map referenced by variable mapB will contain both of the key + value entries inserted into mapA at the beginning of the code example.

The copying of entries only goes one way. Calling mapB.putAll(mapA) will only copy entries from mapA into mapB, not from mapB into mapA. To copy entries the other way, you would have to execute the code mapA.putAll(mapB).

Get Elements From a Java Map

To get a specific element stored in a Java Map you call its get() method, passing along the key for that element as parameter. Here is an example of getting a value stored in a Java Map:

Notice that the get() method returns a Java Object, so we have to cast it to a String (because we know the value is a String). Later in this Java Map tutorial you will see how to use Java Generics to type the Map so it knows what specific key and value types it contains. This makes type casting unnecessary, and makes it harder to insert the wrong values into the Map by accident.

If we had specified a generic type for the key and value of the Map, then it would not have been necessary to cast the object returned by get() method. Here is how that would look:

Get or Default Value

The Java Map interface has a getOrDefault() method which can return a default value supplied by you - in case no value is stored in the Map by the given key. Here is an example of getting a value from a Java Map with a backup default value:

This example creates a Map and stores three values in it using the keys A, B and C. Then the example calls the MapgetOrDefault() method, passing the String E as key, along with a default value - the String default value. Since the Map does not contain any object stored by the key E the given default value will be returned - which is the String default value passed as the last parameter to the getOrDefault() method.

Checking if Map Contains Key

You can check if a Java Map contains a specific key using the containsKey() method. Here is how that looks:

After running this code, the hasKey variable will have the value true if a key + value pair was inserted earlier with the String key 123, and false if no such key + value pair was inserted.

Checking if Map Contains Value

The Java Map interface also has a method that enables you to check if the Map contains a certain value. The method is called containsValue() . Here is how calling the containsValue() looks:

After executing this code the hasValue variable will contain the value true if a key + value pair was inserted ealier with the String value 'value 1', and false if not.

Iterating the Keys of a Java Map

There are several ways to iterate the keys stored in a Java Map. The most used methods for iterating the keys are:

  • Via the key Iterator
  • Via the for-each loop
  • Via a Stream

All methods will be covered in the following sections.

Using a Key Iterator

You can iterate all the keys of a Java Map via its keySet() method. Here is how iterating the keys of a Java Map looks:

As you can see, the key Iterator returns every key stored in a Java Map, one by one (one for each call to next()). Once you have the key, you can obtain the element stored for that key using the Mapget() method.

In the example above, the Iterator next() method returns an Object - and so does the get() method. With generic types specified for the Map these methods would have returned the type of the key and value objects respectively. Here is how that would look:

Notice how a generic type is now also specified for the Iterator obtained from map.keySet().iterator().

Using a Key For-Each Loop

From Java 5 you can also use the for-each loop to iterate the keys stored in a Java Map. Here is how that looks:

The effect of the above code is pretty similar to the code shown in the previous section.

If you have specified a generic type for the Java Map, then you can use that type inside the for-each loop. Here is how that looks:

Using a Key Stream

From Java 8 you can use a Java Stream to iterate the keys of a Java Map. The Stream interface is part of the Java Stream API which was added in Java 8. You first obtain the key Set from the Map and from that you can get a Stream. Here is an example of iterating the keys of a Java Map via a Stream:

Iterating the Values of a Java Map

It is also possible to just iterate the values stored in a Java Map. You obtain a Collection of the values stored in a Map via the values() method. You can iterate the values in the Collection in following ways:

  • Using an Iterator
  • Using the for-each Loop
  • Using a value Stream

All of these options are covered in the following sections.

Using a Value Iterator

The first way to iterate all values stored in a Java Map is to obtain a value Iterator instance from the value Set, and iterate that. Here is how iterating the values stored in a Java Map using a value Iterator:

Since a Set is unordered, you do not have any guarantees about the order in which the values in the value set are iterated. However, if you are using a TreeSet you can control this order still.

Using a Value For-Each Loop

The second method of iterating the values stores in a Java Map is via the Java for-each loop. Here is how iterating the values of a Java Map using the for-each loop looks in code:

This example will print out all the values store in the mapAMap variable.

Using a Value Stream

The third way to iterate the values stored in a Java Map is by using a value Stream, by using the Java Stream API. You first obtain the value Set from the Map, and from the value Set you can obtain the Stream. Here is an example of iterating the values of a Java Map via a value Stream:

Iterating the Entries of a Java Map

It is also possible to iterate all entries of a Java Map. By entries I mean key + value pairs. An entry contains both the key and value for that entry. Earlier we have only iterated either the keys, or the values, but by iterating the entries we iterate both at the same time.

Like with keys and values, there are two ways to iterate the entries of a Map:

  • Using an Entry Iterator
  • Using the for-each loop

Both of these options will be explained in the following sections.

Using an Entry Iterator

The first way to iterate the entries of a Java Map is via an entry Iterator obtained from the entry Set. Here is an example of iterating the entries of Java Map:

Notice how the key and value can be obtained from each Map.Entry instance.

Keep in mind that the above code can be made a bit nicer using a Map typed with Java Generics as shown later in this tutorial.

Using an Entry For-Each Loop

The second way to iterate the entries of a Java Map is to use a for-each loop. Here is an example of iterating the entries of a Java Map using a for-each loop:

Notice, that this example too can be made a bit prettier using a generic Map. Generic Map instance are explained a bit later in this Java Map tutorial.

Removing Entries From a Java Map

You remove Entries by calling the remove(Object key) method. You thus remove the (key, value) pair matching the key. Here is an example of removing the entry for a given key in a Java Map :

After executing this instruction, the Map referenced by mapA will no longer contain an entry (key + value pair) for the key key1.

Removing All Entries

You can remove all entries in a Java Map using the clear() method. Here is how that looks:

Replacing an Entry in a Java Map

It is possible to replace an element in a Java Map using the replace() method. The replace() method will only insert the new value if there is already an existing value mapped to the key. If no existing value is mapped to the given key, no value is inserted. This is different from how put() works, which always insert the value no matter what.

Here is an example of replacing one value with another using the Java Mapreplace() method:

After running this code the Map instance will contain the String value newer value for the String key key .

Reading Number of Entries in Map

You can read the number of entries in a Java Map using the size() method. The number of entries in a Java Map is also referred to as the Mapsize - hence the method name size() . Here is an example of reading the number of entries in a Map using the size() method:

Checking if a Java Map is Empty

The Java Map interface has a special method for checking if a Map is empty. This method is called isEmpty() and it returns either true or false. The isEmpty() method will return true if the Map instance contains 1 or more entries. If the Map contains 0 entries, isEmpty() will return false.

Functional Operations in Java Map

The Java Map interface had a few functional operations added from Java 8. These functional operations make it possible to interact with a Map in a more functional style. For instance, you pass a Java Lambda Expression as parameter to these functional style methods. You can read more about functional programming in my tutorial about Java Functional Programming .

The functional operation methods are:

  • compute()
  • computeIfAbsent()
  • computeIfPresent()
  • merge()

Each of these functional methods will be described in more detail in the following sections.

compute()

The Mapcompute() method takes a key object and a lambda expression as parameters. The lambda expression must implement the java.util.function.BiFunction interface. Here is an example of calling the Java Mapcompute() method:

The compute() method will call the lambda expression internally, passing the key object and whatever value is stored in the Map for that key object, as parameters to the lambda expression.

Whatever value the lambda expression returns is stored instead of the currently stored value for that key. If the lambda expression returns null, the entry is removed. There will not be a key ->null mapping stored in the Map.

In the example above you can see that the lambda expression checks if the value mapped to the given key is null or not, before calling toString().toUpperCase() on it.

It the lambda expression throws an exception, the entry is also removed.

computeIfAbsent()

The MapcomputeIfAbsent() method works similarly to the compute() method, but the lambda expression is only called if no entry exists already for the given key.

The value returned by the lambda expression is inserted into the Map. If null is returned, no entry is inserted.

If an exception is thrown by the lambda expression, no entry is inserted either.

Here is an example of calling the MapcomputeIfAbsent() method:

This example actually just returns a constant value - the string 123 . However, the lambda expression could have calculated the value in any way it needed to - e.g. extract the value from another object, or concatenate it from other values etc.

computeIfPresent()

The MapcomputeIfPresent() method works oppositely of computeIfAbsent(). It only calls the lambda expression passed as parameter to it, if an entry already exists in the Map for that key. Here is an example of calling the computeIfPresent() method:

The value returned by the lambda expression will be inserted into the Map instance. If the lambda expression returns null, the entry for the given key is removed.

If the lambda expression throws an exception, the exception is rethrown, and the current entry for the given key is left unchanged.

merge()

The Mapmerge() method takes a key, a value, and a lambda expression implementing the BiFunction interface as parameters.

If the Map does not have an entry for the key, or if the value for the key is null, the value passed as parameter to the merge() method is inserted for the given key.

If, however, an existing value is already mapped to the given key, the lambda expression passed as parameter is called instead. The lambda expression thus gets a chance to merge the existing value with a new value. The value returned by the lambda expression is then inserted into the Map for the given key. If the lambda expression returns null, the entry for the given key is removed.

Here is an example of calling the Mapmerge() method:

This example will insert the value XYZ into the Map if no value is mapped to the key (123), or if null is mapped to the key. If a non-null value is already mapped to the key, the lambda expression is called. The lambda expression returns the new value (XYZ) + the value -abc, meaning XYZ-abc.

If an exception is thrown by the lambda expression, the exception is rethrown, and the current mapping for the given key is kept unchanged.

More Details in the Java Map JavaDoc

There is more you can do with a Map, but you will have to check out the JavaDoc for more details. This text focused on the two most common operations: Adding / removing elements, and iterating the keys and values.

Right 1

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.

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.

Personal Key Values

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.

Java Map Value To Key

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 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.

Java Get Value From Map

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.

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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.

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.