Using map in java

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.

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Overview

In this article, we’ll cover how a map works in Java and explore what’s new in Java 9.

What is a Map?

A map is a data structure that’s designed for fast lookups. Data is stored in key-value pairs with every key being unique. Each key maps to a value hence the name. These pairs are called map entries.

In the JDK, java.util.Map is an interface that includes method signatures for insertion, removal, and retrieval.

Collections Framework

The Map interface is implemented by a number of classes in the Collections Framework. Each class offers different functionality and thread safety. The most common implementation is the HashMap so we’ll be using this for most of our examples.

Map is the only collection that doesn’t extend or implement the Collection interface. It doesn’t fit the same contract since it needs to work with pairs instead of single values.

Create a Map

The keys and values of a map can be any reference type. We can’t use primitive types because of a restriction around the way generics were designed.

A HashMap allows one null key and multiple null values. It doesn’t preserve the order of the elements and doesn’t guarantee the order will remain the same over time.

Let’s create a HashMap with Integer keys and String values:

Since all maps implement the Map interface, the following methods will work for any of the map implementations illustrated above.

Add to a Map

The put() method allows us to insert entries into our map. It requires two parameters: a key and its value.

Now, let’s populate our map with id’s and names:

map.put(1, "Petyr Baelish"); map.put(2, "Sansa Stark"); map.put(3, "Jon Snow"); map.put(4, "Jamie Lannister");

Here’s what our map looks like now:

Sometimes we may want to add multiple entries in bulk or combine two maps. There’s a method that handles this called putAll(). It copies the entry references from another map in order to populate ours.

Duplicate Keys

We mentioned before that duplicate keys aren’t allowed in maps.

Let’s see what happens when we try to insert a key that already exists:

map.put(4, "Daenerys Targaryen");

The method doesn’t complain, but notice how the new value overwrote the previous one:

The put method returns the previous value if we care to use it. If there was no previous value, it returns null.

To check whether a key already exists, we use the containsKey() boolean method:

Similarly, the containsValue() method checks for existence of a value:

boolean check = map.containsValue("Brienne of Tarth"); // false

Retrieve from a Map

The get() method accepts a key and returns the value associated with that key or null if there is no value.

String value = map.get(4); // Daenerys Targaryen

Remove from a Map

The remove() method accepts a key so it can find the entry to remove. It returns the value associated with the removed entry or null if there is no value.

map.remove(3); // removes and returns "Jon Snow" map.remove(3); // removes nothing and returns null

If we want to empty the map, we can call clear(). This is a void method so it does not return anything.

Map Size

The size() method returns number of entries in our map.

The isEmpty() method returns a boolean indicating if the map is empty or not.

boolean isEmpty = map.isEmpty();

Collection Views

The Map interface provides Collection view methods which allow us to view our map in terms of a collection type. These views provide the only mechanism for us to iterate over a map.

• keySet() — returns a Set of keys from the map
• values() — returns a Collection of values from the map
• entrySet() — returns a Set of Map.Entry objects which represent the key-value pairs in the map

It’s important to remember that views are backed by our map. This means any changes we make to the view update the underlying map and vice versa.

The collection views support removal of entries but not insertion.

keySet()

The keySet() method returns a Set view of the keys contained in our map:

Set keys = map.keySet(); // [1, 2, 4]

values()

The values() method returns a Collection of the values contained in our map:

Collection values = map.values(); // [Petyr Baelish, Sansa Stark, Daenerys Targaryen]

Why does it return a Collection instead of a Set? Because the values of a map aren’t guaranteed to be unique so a Set wouldn’t work.

entrySet()

The entrySet() method is used to get a Set view of the entries in our map. The Set will contain Map.Entry objects. A Map.Entry object is simply a key-value pair. We can call getKey() or getValue() on this object.

The most common usage of the entrySet is for looping which we’ll cover in the next section.

Iterating over a Map

There are many ways of iterating over a map. Let’s go over some common approaches.

Keep in mind that loops will throw a NullPointerException if we try to iterate over a map that is null.

Using foreach and Map.Entry

This is the most common method and is preferable in most cases. We get access to both keys and values in the loop.

for (Map.Entry entry : map.entrySet()) < System.out.println( entry.getKey() + " ->" + entry.getValue() ); >

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