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Java Map

Jakob Jenkov
Last update: 2018-11-26

The Java Map interface, java.util.Map, represents a mapping between a key and a value. More specifically, a Java Map 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 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.

Here are a few examples of how to create a Map instance: 

Map mapA = new HashMap();
Map mapB = new TreeMap();

Inserting Elements Into a Java Map

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

Map mapA = new HashMap();

mapA.put("key1", "element 1");
mapA.put("key2", "element 2");
mapA.put("key3", "element 3");

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: 

mapA.put("key", 123);

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 Are Not Allowed

Please note, that the key cannot be null ! The Map uses the hashCode() and equals() methods of the key, to store the key + value pair internally, so if the key is null, the Map cannot place the key + value pair correctly internally.

Null Values Are Allowed

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

mapA.put("D", null);

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

Object value = mapA.get("D");

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(): 

Map mapA = new HashMap();
mapA.put("key1", "value1");
mapA.put("key2", "value2");

Map mapB = new HashMap();
mapB.putAll(mapA);

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: 

String element1 = (String) mapA.get("key1");

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.

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: 

Map map = new HashMap();

map.put("A", "1");
map.put("B", "2");
map.put("C", "3");

Object value = map.getOrDefault("E", "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 Map getOrDefault() 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: 

boolean hasKey = mapA.containsKey("123");

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: 

boolean hasValue = mapA.containsValue("value 1");

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: 

Iterator iterator = mapA.keySet().iterator();
while(iterator.hasNext(){
  Object key   = iterator.next();
  Object value = mapA.get(key);
}

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 Map get() method.

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: 

for(Object key : mapA.keySet()) {
    Object value = mapA.get(key);
}

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

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: 

Map<String, String> map = new HashMap<>();

map.put("one", "first");
map.put("two", "second");
map.put("three", "third");

Stream<String> stream = map.keySet().stream();
stream.forEach((value) -> {
    System.out.println(value);
});

Iterating the Values of a Java Map

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

Iterator iterator = map.values().iterator();

while(iterator.hasNext()) {
    Object nextValue  iterator.next();
}

Since a Set is unordered, you do not have any guarantees about the order in which the values are iterated.

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: 

for(Object value : mapA.values()){
    System.out.println(value);
}

This example will print out all the values store in the mapA Map 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: 

Map<String, String> map = new HashMap<>();

map.put("one", "first");
map.put("two", "second");
map.put("three", "third");

Stream<String> stream = map.values().stream();
stream.forEach((value) -> {
    System.out.println(value);
});

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: 

Set entries = map.entrySet();
Iterator iterator = entries.iterator();
while(iterator.hasNext()) {
    Map.Entry entry = (Map.Entry) iterator.next();
    Object key = entry.getKey();
    Object value = entry.getValue();
}

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: 

for(Object entryObj : map.entrySet()){
    Map.Entry entry = (Map.Entry) entryObj;
    Object key = entry.getKey();
    Object value = entry.getValue();
}

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 : 

mapA.remove("key1");

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: 

mapA.clear();

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 Map replace() method: 

Map map = new HashMap();

map.replace("key", "new value");   //nothing is replaced, no mapping to "key"

map.put("key", "value");           //now the map contains an entry on "key"

map.replace("key", "newer value"); //now the entry is replaced

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 Map size - hence the method name size() . Here is an example of reading the number of entries in a Map using the size() method: 

int entryCount = mapA.size();

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.

Generic Java Maps

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: 

Map<String, MyObject> map = new HashMap<String, MyObject>();

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: 

for(MyObject anObject : map.values()){
   //do someting with anObject...
}

for(String key : map.keySet()){
   MyObject value = map.get(key);
   //do something to value
}

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

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 Map compute() 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 Map compute() method: 

map.compute("123", (key, value) -> value == null ? null : value.toString().toUpperCase());

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 Map computeIfAbsent() 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 Map computeIfAbsent() method: 

map.computeIfAbsent("123", (key) -> "abc");

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 Map computeIfPresent() 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: 

map.computeIfPresent("123", (key, value) -> value == null ? null : value.toString().toUpperCase());

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 Map merge() 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 Map merge() method: 

map.merge("123", "XYZ", (oldValue, newValue) -> newValue + "-abc");

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.

Jakob Jenkov




Copyright  Jenkov Aps
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