I genius official free Tutorial 34: Generic collection of Java collection framework

Generic Collection

A generic collection does not refer to a collection, but rather to the addition of generics on the basis of the collection.
In a generic collection, once the generic parameter "Class A" is passed in, only objects of Class A or class A subclasses can be added to the collection, and no other object can be added.
When you get an object from a generic collection, the object's type is Class A, not an object of type objects.

Generic List

For example: The two list sets mentioned above can be added to a generic

package collection.list.arraylist;import java.util.arraylist;import java.util.list;/** *   Demonstrating the use of generic collections  *  @author   Genius Federation  -  Yukun  */public class genericlistdemo  {public static void main (String[] args)  {//Create object Father f = new  father (); Son s = new son ();/*******************  does not use a generic collection  ********************/// The reference to the parent list refers to the subclass ArrayList object, the polymorphic application list list = new arraylist ();//Add the two objects created above to the collection// Any object can be joined List.add (f); List.add (s);//To use a member specific to the object, the type conversion father f1 =  (Father)  list.get (0) must be enforced; F1.work (); son s1 =  (Son)  list.get (1); S1.play ();/*******************  used a generic collection  ************* /system.out.println ("-----------------------------");/* *  create generic collection objects, you can use ArrayList, You can also use linkedlist *  Note:  * 1, generics can only use reference types, cannot use basic data types  *  such as:list<int> is the wrong  *  2, List<father> and aRraylist<father> There are inheritance relationships  *  but arraylist<father> and arraylist<son> are two different types  *  There is also no inheritance relationship  *  such as:list<father> fatherlist = new arraylist<son> ();  *  ArrayList<Father> fatherList = new ArrayList<Son> (); *  are all wrong.  */List<Father> fatherList = new ArrayList<Father> (); List<son> sonlist = new arraylist<son> ();//After using generics, the Fatherlist collection can only store father types// No type other than father can be added to the collection fatherlist.add (f);//After using generics, the Sonlist collection can only store the son type Sonlist.add (s);//You do not need to force type conversions when fetching elements father  f2 = fatherlist.get (0); F2.work (); Son s2 = sonlist.get (0); S2.play ();}} Class father{public void work () {System.out.println ("work");}} Class son extends father{public void play () {System.out.println ("Play");}} Running results: Work and play-----------------------------work and play

Generic Set

Package Collection.set.hashset;import java.util.hashset;import java.util.set;/** * Demo Generic Set Collection * * @author Genius Federation-Yukun */PUBL IC class Genericsetdemo {public static void main (string[] args) {//Create HashSet object set<string> set = new Hashset<strin G> ();//generic specifies that only string types can be added, so it is not possible to add other types of element Set.add ("Zhang San"), Set.add ("John Doe"), Set.add ("Zhang San"), or//using a generic string type, So the generic collection set here is the string type for (string str:set) {System.out.println (str);}}} Running Result: Li Shizhang

Generic Map

Package Map.hashmap;import Java.util.hashmap;import java.util.map;/** * Demo add generic after map collection * * When using multiple generics, the middle of the placeholder is separated by a comma in English format * For example: Map<k,v> * Use is also in the order of the location of the incoming * For example:map<string,integer> * All K in its Map class will be replaced with a String * All V will be replaced with an Integer * When you want to store the value in the collection is also to correspond to the data type * * @author Genius Federation-Yukun */public class Genericmapdemo {public static void main (string[] args) {//Create generics MAPM ap<string,integer> map = new hashmap<string,integer> ();/* Because generics are used when creating the object map, only the corresponding data type value is added when the value is added */map.put ( "Zhang San", Map.put ("John Doe"); Map.put ("Harry", 23);/* * Gets the value corresponding to the value of the key * use generics without forcing type conversion */int age = Map.get ("Zhang San"); System.out.println ("Zhang San's age is:" + Ages);}} Operation Result: Age of Zhang San is: 18

Generic Collection Summary

Syntax Format: Collection class name a< class name B>
Function: Only data of type B can be stored in collection a
Declaring and creating an object:list<string> List = new arraylist<string> ()
Note: 1, Collection < parent class > and Collection < subclasses > are incompatible two types
2, Collection < parent class > object, can store child class object;
3. The element type in a generic collection can only be a generic specified type or its subclasses
4. The type of the element to be removed from the generic collection is the type of the generic type

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I genius official free Tutorial 34: Generic collection of Java collection framework