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This article mainly explains "how to use the Java set method". The content of the article is simple and clear, and it is easy to learn and understand. Please follow the editor's train of thought to study and learn "how to use the Java set method".

Overview:

Object-oriented language embodies things in the form of objects. In order to facilitate the operation of multiple objects, it is necessary to store objects. On the other hand, there are some drawbacks to using Array to store objects, and the Java collection is like a container in which references to multiple objects can be placed dynamically.

Java collection classes can be used to store a varying number of pairs of objects, as well as associative arrays with mapped relationships.

Use:

Storage objects can be considered: ① array ② collection

Characteristics of array storage objects: Student [] stu=new Student [20]; stu [0] = new Student ();

The downside of using an array: once created, its length is immutable.

The number of objects stored in the real array is unknown.

Collection:

Collection interface:

List: an ordered, repeatable collection of elements-a "dynamic" array

① ArrayList (the main implementation class) (the underlying is implemented in an array)

② LinkedList (recommended for frequent inserts and deletions) (low-level linked list)

③ Vector (ancient implementation classes, thread-safe)

Set: unordered, non-repeatable collections of elements (the common methods in Set are defined under Collection-similar to high school "collections"

Set: stored elements are unordered and unrepeatable!

Disorder: disorder! = randomness. True disorder means that the location where the element is stored at the underlying level is out of order.

Non-repeatability: when the same element is added to the Set, the latter cannot be added.

Description: the equals () and hashCode () methods must be overridden for the class where the elements are added to the Set. And then ensure the non-repeatability of the elements in Set!

How are the elements in Set stored? The hash algorithm is used.

When you add an object to Set, you first call the hashCode () method of the class in which the object resides to calculate the hash value of the object, which determines where the object is stored in Set. If there is no object storage before this location, the object is stored directly to this location. If there is already an object store in this location, compare whether the two objects are the same through the equals () method. If they are the same, the latter object can no longer be added.

What if I return to false? All stored (not recommended)

> the hashCode () method is required to be consistent with the equals () method.

① HashSet (main implementation class)

② LinkedHashSet

A linked list is used to maintain an order that is added to the collection. As a result, when we iterate through the LinkedHashSet collection elements, we iterate in the order in which they are added.

The insertion performance of LinkedHashSet is slightly lower than that of HashSet, but has good performance when iterating through all the elements in Set.

LinkedHashSet does not allow duplicate collection elements.

③ TreeSet

The element added to the TreeSet must be of the same class

TreeSet does not allow duplicate collection elements.

You can traverse in the specified order of the elements added to the collection. By default, such as String, wrapper classes, etc. are traversed in order from small to large.

When the Person class does not implement the Comparable interface, the ClassCaseException is reported when a Person object is added to the TreeSet.

When you add objects of a custom class to TrreSet, there are two sorting methods:

① natural sorting: an abstract method that requires a custom class to implement the java.lang.Comparable interface and override its compareTo (Object obj), in which it indicates which property of the custom class is installed for sorting.

② custom sorting: comparetTo () is consistent with hashCode () and equal s ()! See the following steps for sorting

① creates a class object that implements the Comparator interface

> ① adds objects of class Customer to TrreSet. In this compare () method, it is indicated that it is sorted by which attribute of Customer.

> ② passes this object as a formal parameter to the constructor of TreeSet.

> ③ adds objects of the classes involved in the compare method in the Comparator interface to the TrreSet.

When you add an element to TreeSet, first compare it according to the compareTo () method. Once 0 is returned, although only two objects have the same value of this attribute, the program will think that the two objects are the same, so the latter object cannot be added.

> comparetTo () is consistent with hashCode () and equal s ()!

Map interface: a set with mapping relationship "key-value pairs"-similar to the "function" of high school yyogf (x) (x1primey1) (x2jiny2)

① HashMap ② LinkedHashMap ③ TreeMap ④ Hashtable (subclass: Properties)

Collection:

Size (): returns the number of elements (objects) in the collection

Add (Object obj): add an element (object) to the collection, any type is fine.

AddAll (Collection coll): adds all elements contained in the parameter coll to the current collection.

IsEmpty (): determine whether the collection is empty

Clear (): clear collection elements

Contains (Objetc obj): determines whether the collection contains the specified obj element. If included, returns true. Otherwise, return false.

The basis of judgment: judge according to the equals () method of the class in which the element is located.

Be clear: if the element saved in the collection is an object of a custom class. Requirement: the custom class overrides the equals () method.

System.out.println (object); you can view the elements of the collection

ContainsAll (Collection coll): determines whether the current collection contains all the elements in the coll.

RetainAll (Collction coll): find the common elements of the current collection and coll, and return them to the current collection

Remove (Object obj): removes the obj element from the collection. If the deletion is successful, return true. Otherwise, return false.

RemoveAll (Collection coll): removes the elements contained in the coll from the current collection.

Equals (Object obj): determines whether all elements in two sets are exactly the same

HashCode (): hash value (not understood yet)

RoArray (): converts the collection to an array. (receive with Object).

Collection coll=Arrays. AsList (1, 2, 3): an array is converted into a collection.

Iterator (): returns an object of the Iterator interface implementation class, which in turn implements traversal of the collection.

/ / method 1: do not use Iterator iterator=new Iterator (); System.out.println (iterator.next ()); / / output one, and print several times. / / method 2: do not use for (int iTuno void add (int index, Object ele): add elements at the specified index location

Boolean addAll (int index, Collection eles): adds a collection of elements at the specified index location

Object get (int index): gets the element at the specified index location

Object remove (int index): deletes the element at the specified index location

Object set (int index, Object ele): sets the element value at the specified index location

> int indexOf (Object obj): returns the location where obj first appears in the collection. If not. Return-1

Int lastIndexOf (Object obj): returns the location where obj last appeared in the collection. If not. Return-1

List subList (int fromIndex, int toIndex): returns a child list that closes left and opens right from fromeIndex to the end of toIndex

Common methods of List: ① increase (add (Object ele)) ③ change (set (int index, Object ele)) ⑤ length (sizi ())

② delete (remove) ④ search (get (int index)) ⑥ insert (addAll (int index, Collection eles))

Conllection interface

Map interface

The main implementation class of HashMap:Map, key is stored in Set and cannot be repeated.

Value is stored in Collection, and a Key-value pair can be repeated. It's an Entry. All Entry are stored in Set and cannot be repeated.

When you add an element to a HashMap, the equals () method of the class in which the key belongs is called to determine whether the two key are the same. If it is the same, you can only add the element that is added later.

LinkedHashMap: use linked lists to maintain the order added to the Map, so when traversing the Map, it traverses in the order in which it was added.

TreeMap: sorts by the specified attribute of the Key added to the element in the Map. Requirement: key must be an object of the same class!

Customized sorting for key: ① natural sorting vs ②

Hashtable: an ancient implementation class, thread-safe, not recommended

Properties: commonly used to work with properties files. Keys and values are of type String

Map and Collection exist side by side. Used to save data with mapped relationships: Key-Value

Key and value in Map can be data of any reference type

The key in Map is stored in Set, and repetition is not allowed, that is, the class corresponding to the same Map object must override the hashCode () and equals () methods.

The String class is commonly used as the "key" of Map.

There is an one-way one-to-one relationship between key and value, that is, a unique and definite value can always be found through a specified key.

Add, delete

Object put (Object key,Object value): add an element to the Map

Object remove (Object key): delete this key-value according to the specified Key

Void putAll (Map t): add all the elements in a new object

Void clear (): empty

Element query

Object get (Object key): gets the value of the specified key. If there is no such Key, then null.

Boolean containsKey (Object key): determines whether this map contains pairs of the specified key.

Boolean containsValue (Object value): the pair that determines whether it contains value.

Int size (): returns the length of the collection

Boolean isEmpty (): determines whether the element is empty

Boolean equals (Object obj): whether to equals another

Meta view operation: traversing Map

Set keySet ()

Collection values ()

Set entrySet ()

① traverses the key set

Set set=new map.keySet (); for (Object obj:set) {System.out.println (obj);}

② traverses the value set

Collection values=map.value (); Iterator i=values.iterator (); while (i.hasNext ()) {System.out.println (i.next ());}

③ traverses the key-value pair.

Set set=new map.keySet (); for (Object obj:set) {System.out.println (obj+ "-->" + map.get (obj));}

Or:

Set set=map.entruySet (); for (Object obj:set) {Map.Entry entry = (Map.Entry) obj; System.out.println (entry.getKey () + "-->" + entry.getValue ());} Collections:

Tool class for operating Collection and Map: Collections

Interview questions: distinguish between Collection and Collections

Sort operation:

① reverse (List): reverses the order of elements in List

② shuffle (List): randomly sorts the elements of the List collection

③ sort (List): sorts the specified List collection elements in ascending order according to their natural order

④ sort (List,Comparator): sorts List collection elements according to the order in which the specified Comparator is generated

⑤ swap (List,int, int): swaps elements at I and j in the specified list collection

Find, replace:

① Object max (Collection): returns the largest element in a given collection based on the natural order of the elements

② Object max (Collection,Comparator): returns the largest element in a given collection in the order specified by Comparator

③ Object min (Collection)

④ Object min (Collection,Comparator)

⑤ int frequency (Collection,Object): returns the number of occurrences of a specified element in a specified collection

⑥ void copy (List dest,List src): copy the contents of src to dest

⑦ boolean replaceAll (List list,Object oldVal,Object newVal): replaces all old values of the List object with new values

Thank you for your reading, the above is the content of "how to use the Java collection method", after the study of this article, I believe you have a deeper understanding of how to use the Java collection method, and the specific use needs to be verified in practice. Here is, the editor will push for you more related knowledge points of the article, welcome to follow!

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