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This article will explain in detail how to master the use of Map and Set in the Java collection framework, the content of the article is of high quality, so the editor shares it for you to do a reference. I hope you will have a certain understanding of the relevant knowledge after reading this article.

1. Search

1.1 scene introduction

When learning programming, our common search methods are:

Direct traversal: the time complexity is O (N), and the efficiency of elements will be very slow if there are many elements.

Binary search: the time complexity is O (logN). The sequence must be ordered before searching.

However, the above sort is more suitable for static type search, that is, the interval will not be inserted or deleted.

And real-world search such as:

Check the test results according to the name.

Address book (query contact information according to name)

You may need to do some insert and delete operations during the search, that is, dynamic lookup. Therefore, the above sorting method is not very appropriate.

1.2 Model

Generally speaking, the data to be searched is called Key, and the corresponding keyword is called Value, and the two can be combined into key-value pairs of Key-Value.

Therefore, there are two models for dynamic lookup:

Pure Key model: the storage mode of Set, such as:

There is an English dictionary to quickly find out whether a word is in the dictionary.

Quickly find out if a name is in the address book

Key-Value model: the storage mode of Map, such as:

Count the number of times each word appears in the file, and the result is that each word has a corresponding number of times:

Liangshan hero's jianghu nickname, each hero has his own jianghu nickname:

2. Map

2.1 introduction to Map

A brief introduction:

Map is an interface class that does not inherit from Collection. What is stored in this class is the key-value pair of the structure, and K must be unique and cannot be repeated.

Note:

The Map interface is located in the java.util package and needs to be introduced before using it.

Map is an interface class, so objects cannot be instantiated directly. If you want to instantiate an object, you can only instantiate its implementation class TreeMap or HashMap

The Key of the key-value pair stored in Map is unique, and Value can be repeated.

Document information:

2.2 introduction to Map.Entry

A brief introduction:

Map.Entry is an inner class implemented within Map to store the mapping relationship between key and value pairs. This inner class mainly provides the acquisition, Value setting and Key comparison method.

Common methods:

Method description K getKey () returns keyV getValue () in entry returns valueV setValue in entry (V value) replaces the value in the key-value pair with the specified value

Note:

Map.Entry does not provide a method to set Key

Document information:

2.3 description of common methods for Map

Note:

When storing data in Map, a series of operations are performed according to key, and then stored in Map

If the key is the same, then the value of the newly inserted key will overwrite the value of the original key

When inserting key-value pairs in Map, both key and value can be null

All the key in Map can be separated and stored in Set for access (because key cannot be repeated)

All the value in Map can be separated and stored in any subset of Collection (note that value may be duplicated)

The key of key-value pairs in Map cannot be modified directly, but value can be modified. If you want to modify the key, you can only delete the key and then reinsert it (or overwrite it directly)

2.4 introduction to HashMap

A brief introduction:

HashMap is a hash table that stores key-value pair (key-value) mappings

HashMap implements the Map interface, stores data according to the HashCode value of the key, has a fast access speed, allows a record key to be null at most, and does not support thread synchronization.

HashMap is unordered, that is, the order of insertion is not recorded

HashMap inherits from AbstractMap and implements Map, Cloneable and java.io.Serializable interfaces.

Note:

The HashMap class is located in the java.util package and needs to be introduced before using it.

Document information:

2.5 introduction to TreeMap

A brief introduction:

TreeMap is a collection of Map that can compare the size of elements and sorts the size of the incoming key. You can use the natural order of elements, or you can use a custom comparator in the collection to sort.

Different from the hash mapping of HashMap, the underlying layer of TreeMap implements the tree structure, that is, the structure of red-black tree.

Note:

The TreeMap class is located in the java.util package and needs to be introduced before using it.

Document information:

2.6 the difference between HashMap and TreeMap

2.7 Map uses sample code

Note: Map is an interface class, so objects cannot be instantiated directly. Take HashMap as an example.

Example 1: create a Map instance

Map map=new HashMap ()

Example 2: insert key and its corresponding value is value

Map.put ("Huicheng", "Master"); map.put ("Xiao", "Assassin"); map.put ("Meow squad leader", "thief swordsman"); System.out.println (map); / / the result is: {Huicheng = mage, Xiao = assassin, Meow squad leader = robber swordsman}

Example 3: return the corresponding value of key

String str1=map.get ("dawn"); System.out.println (str1); / / the result is: assassin

Example 4: return the value corresponding to key. If key does not exist, return the default value defaultValue.

String str2=map.getOrDefault ("Mei Hui", "adventurer"); System.out.println (str2); / / the result is: adventurer

Example 5: delete the mapping relationship corresponding to key

String str3=map.remove ("Meow monitor"); System.out.println (str3); System.out.println (map); / / the result is: sword thief and {Huicheng = mage, dawn = assassin}

Example 6: in addition to printing map directly above, traversal printing can also be done through the method of Set entrySet ().

Set set=map.entrySet (); for (Map.Entry str: set) {System.out.println ("Key=" + str.getKey () + "Value=" + str.getValue ());} / * * result: Key= Huicheng Value= Master Key= Xiao Value= assassin Key= Meow squad leader Value= thief swordsman * /

Example 7: determine whether to include key

System.out.println (map.containsKey ("Huicheng")); / / result: true3. Set

3.1 introduction to Set

A brief introduction:

Set is an interface inherited from Collection. It is a collection that does not allow duplicate elements and is unordered. It mainly has two implementation classes: HashSet and TreeSet.

Note:

The Set interface is located in the java.util package and needs to be introduced before using it.

Document information:

3.1 description of common methods of Set

Note:

Only Key is inherited in Set and Key is required to be unique

The bottom layer of Set is implemented using Map, which uses a default object of Key and Object as a key-value pair insertion into Map.

The most important function of Set is to deduplicate the elements in the collection.

The common classes that implement the Set interface are TreeSet and HashSet, and a LinkedHashSet,LinkedHashSet maintains a two-way linked list on the basis of HashSet to record the insertion order of elements.

The Key in Set cannot be modified. If you modify it, delete the original one first.

Null can be inserted into Set

3.3 introduction to TreeSet

A brief introduction:

TreeSet implements the Set-like interface, which is based on Map, and its underlying structure is red-black tree.

Note:

The TreeSet class is located in the java.util package and needs to be introduced before using it.

Document information:

3.4 introduction to HashSet

A brief introduction:

HashSet implements the Set interface, and the underlying layer is implemented by HashMap, which is a hash table structure.

When a new element is added, the key,value equivalent to HashMap defaults to a fixed Object

Note:

The HashSet class is located in the java.util package and needs to be introduced before using it.

Document information:

3.5 the difference between TreeSet and HashSet

3.6 Set uses sample code

Note: Set is an interface class, so objects cannot be instantiated directly. Take HashSet as an example.

Example 1: create a Set instance

Set set=new HashSet ()

Example 2: add elements (duplicate elements cannot be added successfully)

Set.add (1); set.add (2); set.add (3); set.add (4); set.add (5); System.out.println (set); / / the result is: [1,2,3,4,5]

Example 3: determine whether 1 is in the collection

System.out.println (set.contains (1)); / / result: true

Example 4: delete elements in the collection

System.out.println (set.remove (3)); / / result: true

Example 5: returns the number of collections in set

System.out.println (set.size ()); / / result: 4

Example 6: check whether set is empty

System.out.println (set.isEmpty ()); / / result: false

Example 7: return the iterator for traversal

Iterator it=set.iterator (); while (it.hasNext ()) {System.out.println (it.next ());} / / result: 1 2 4 5

The hashNext () method determines whether the current element has the next element, and next () gets the current element and points to the next element.

Example 8: clear the collection

Set.clear (); System.out.println (set); / / the result is: [] 4. Programming exercises

4.1 find the first duplicate data

Title:

Print the first duplicate data from some data

Code:

Public static void findNum (int [] array) {Set set=new HashSet (); for (int iTuno)

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