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Java Set, List, Map what is the difference between the three, many novices are not very clear about this, in order to help you solve this problem, the following small series will be explained in detail for everyone, there are people who need this can learn, I hope you can harvest.

JAVA collections are mainly divided into three types: Set, List and Map.

Collection interface: Collection is the most basic collection interface, declaring common methods applicable to JAVA collections (only Set and List). Set and List both inherit Conllection, Map

Methods of the Collection interface:

boolean add(Object o): Adds a reference to an object to the collection

void clear(): deletes all objects in the collection, i.e. no longer holds references to them

boolean isEmpty(): Determines if the collection is empty

boolean contains(Object o): Determines whether a collection holds a reference to a particular object

Iterartor iterator(): Returns an Iterator object that can be used to iterate over elements in a collection

boolean remove(Object o): Remove a reference to an object from the collection

int size(): Returns the number of elements in the collection

Object[] toArray(): Returns an array containing all the elements in the collection

Both Iterator() and toArray() methods are used for all elements of a collection, the former returning an Iterator object and the latter an array containing all elements of the collection.

The Iterator interface declares the following methods:

hasNext(): Determine whether the elements in the collection have been traversed, if not, return true

next(): Returns the next element

remove(): removes the last element returned by the next() method from the collection.

Set: Set is the simplest kind of set. The objects in the collection are not sorted in a particular way and there are no duplicate objects. The Set interface mainly implements two implementation classes:

HashSet: The HashSet class accesses objects in the collection according to the hash algorithm, and the access speed is relatively fast.

TreeSet: The TreeSet class implements the SortedSet interface, enabling sorting of objects in a collection.

Usage of Set: What is stored is a reference to the object, no duplicate object

Set set=new HashSet();

String s1=new String("hello");

String s2=s1;

String s3=new String("world");

set.add(s1);

set.add(s2);

set.add(s3);

System.out.println(set.size());//Print the number of objects in the collection is 2.

How does the add() method of Set determine whether an object is already stored in the set?

boolean isExists=false;

Iterator iterator=set.iterator();

while(it.hasNext()) {

String oldStr=it.next();

if(newStr.equals(oldStr)){

isExists=true;

}

}

List: A List is characterized by its elements being stored linearly, allowing duplicate objects to be stored in the collection.

The main implementation classes of List interface include:

ArrayList() : Represents an array of variable length. Random access to elements is possible, and insertion and deletion of elements into ArrayList() is slow.

LinkedList(): Uses a linked list data structure in the implementation. Fast insertion and deletion, slow access.

Random access to a List is to randomly retrieve only elements located at a particular location. The get(int index) method of List puts back the object in the collection at the index position specified by the parameter index, with subscripts starting with "0." There are two basic ways to retrieve all objects in a collection:

1: for loop and get() method:

for(int i=0; i

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