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. In the previous tutorial we learned about Java Stream Filter.In this guide, we will see how to use Stream filter method to filter a Map by keys and Values. Java 8 – Filter Map by Keys. 10 Examples of HashMap in Java - Programming Tutorial. TreeMap has a constructor which accepts Map and can create a Map sorted on the natural order of key or any custom sorting order defined by Comparator. And individual entries seprated by comma (,) My only problem is I don't know how to get all key values from Map? Reply Delete.
The remainder of this page discusses the
Map
interface in detail. But first, here are some more examples of collecting to Map
s 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
Set
and List
interfaces, 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: Aperture 3.6 mac free download.keySet
— theSet
of keys contained in theMap
.values
— TheCollection
of values contained in theMap
. ThisCollection
is not aSet
, because multiple keys can map to the same value.entrySet
— theSet
of key-value pairs contained in theMap
. TheMap
interface provides a small nested interface calledMap.Entry
, the type of the elements in thisSet
.
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.Along similar lines, suppose you want to know whether two
Map
objects contain mappings for all of the same keys.![Values Values](/uploads/1/3/3/8/133849041/540455909.png)
Suppose you have a
Map
that represents a collection of attribute-value pairs, and two Set
s 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
https://equiever786.weebly.com/screen-shot-for-mac.html. has in common with another.Suppose you want to remove from one
Map
all of the keys that have mappings in another. Mac office 2008 product key generator free.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
Office 365 product key generator 2016 online. 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
Free steam game cd-key generator online. 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.Java Generate All Key Values For A Maple
Hallmark ornaments value guide download. 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 the
dictionary file
we used.It was derived from the Public Domain ENABLE benchmark reference word list.- Related Questions & Answers
- Selected Reading
Object Oriented ProgrammingJava8Java Programming
The Map class’s object contains key and value pairs. You can convert it into two list objects one which contains key values and the one which contains map values separately.
Personal Key Values
To convert a map to list −
Java Get Key From Map
![Java Generate All Key Values For A Map Java Generate All Key Values For A Map](/uploads/1/3/3/8/133849041/303697609.png)
- Create a Map object.
- Using the put() method insert elements to it as key, value pairs
- Create an ArrayList of integer type to hold the keys of the map. In its constructor call the method keySet() of the Map class.
- Create an ArrayList of String type to hold the values of the map. In its constructor call the method values() of the Map class.
- Print the contents of both lists.