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.
Convert String to Map Java
Convert String to Map Java | As we all know the string is the collection of a sequence of characters and a Map is an interface under the collection framework which stores the elements in the key, value pair form.
The map operations can be done with a key or if you want to retrieve the value then you can use the respective key to do so. Also, the map only stores unique key values, therefore, no duplicate values are allowed on the map. Now this blog, teaches you how to convert the string to a map. The string has a particular type of elements but in the map, we need to store the elements in the form of key and value pairs. Analyzing this problem might feel difficult but this blog helps you to solve it in an easy way. In this blog, we use two methods to do so. Observe the below examples you might get more clarity.
Example:-
1. String = “Apple:1, Banana:2, Mango:3”
Map =
2. String array = < “Apple”, “Pomegranate”, “Strawberries”, “Watermelons”, “Green Grapes” >
Integer array = < 1, 2, 3, 4, 5 >
Map =
In this section, we will implement both the above methods the first method is converting a single string to the map the next one is taking two string array for key and value and then converting it to a map.
Apart from these two examples, we will also see how to convert JSON string to map using Jackson API. Example:-
Json String =
Map: .
Java Program To Convert String To Map
import java.util.HashMap; import java.util.Map; public class Main < public static void main(String[] args) < String data = "Apple:1, Banana:2, Mango:3"; Mapmap = new HashMap(); String fruits[] = data.split(","); for (String fruit : fruits) < String string1[] = fruit.split(":"); String string2 = string1[0].trim(); String string3 = string1[1].trim(); map.put(string2, string3); >System.out.println("String: " + data); System.out.println("Map: " + map); > >String: Apple:1, Banana:2, Mango:3
Map:
In the above program to convert string to map Java, the string contains the fruit name and value which are separated by a colon (:), and each fruit is separated by a comma. Therefore first we have split based on the comma and then we have fetched the name and value. Both data had been placed on the map as key & value.
Convert String To Map Java
Now we will see an example where we have a string array and an integer array. Using these two arrays we want to create a map. In the map, we will make integer value as key and string element as value.
import java.util.HashMap; import java.util.Map; public class Main < public static void main(String[] args) < String fruits[] = < "Apple", "Pomegranate", "Strawberries", "Watermelons", "Green Grapes" >; Integer number[] = < 1, 2, 3, 4, 5 >; Map fruitMap = new HashMap(); for (int i = 0; i < fruits.length && i < number.length; i++) < fruitMap.put(number[i], fruits[i]); >System.out.println("Map: " + fruitMap); > >Convert JSON String to Map Java
To convert JSON string to map we are going to use Jackson API. For this, we will need the following dependencies:- Jackson-core, Jackson-databind & Jackson-annotations.
We have the following JSON which needs to be converted into the map.
Create Map in Java
- Creating Map Using HashMap in Java
- Creating Map Using Map.ofEntries in Java
- Creating Map Along With Initialization in Java
- Creating Map Using the Map.of() Method
This tutorial introduces how to create Map in Java and lists some example codes to understand the topic.
Map is an interface in Java and belongs to java.util package. It is used to store data in key-value pairs. It provides several implementation classes such as HashMap , LinkedHashMap , TreeMap , etc.
We can create Map by using these classes and then hold a reference to Map. Let’s understand by some examples.
Creating Map Using HashMap in Java
Let’s create a Map that holds the integer key and String values. See, we used the HashMap class and inserted elements by using the put() method. See the example below.
package javaexample; import java.util.HashMap; import java.util.Map; public class SimpleTesting public static void main(String[] args) MapInteger, String> hm = new HashMapInteger, String>(); hm.put(1, "Red"); hm.put(2, "Green"); hm.put(3, "Blue"); hm.put(4, "White"); System.out.println(hm); > > Creating Map Using Map.ofEntries in Java
It is a static method of Map interface and was added into Java 9. We can use it to create an immutable map containing keys and values extracted from the given entries. See the example below.
import java.util.Map; public class SimpleTesting public static void main(String[] args) MapInteger,String> map = Map.ofEntries( Map.entry(1, "Red"), Map.entry(2, "Green"), Map.entry(3, "Blue") ); System.out.println(map); > >