One ： Collection Overview

What is the difference between a set and an array ？
An array is java A data structure in , The length of the data is fixed , Storage is linear . And can store basic data types and objects , Basic data objects can be boxed and stored according to basic types . Our array is of reference data type .
A set is java Another kind of data in i structure , Compared to arrays , Collections are more flexible . In terms of implementation , Collections are implemented in a variety of ways , It has a wide range of applications , The array adopts the method of continuous space allocation storage .
in addition , Collections take the form of classes and interfaces , have java Three characteristics of object-oriented , The comparison array more clearly reflects the object-oriented logical thinking .
java The set in is divided into single column set and double column set ,Collection Is the top-level interface of a single column set ,Map Is the top-level interface of a two column set . In terms of basic storage , The data stored in a single column only contains the data itself , Double columns contain keys and values , That is, double columns not only store the data itself , Also store the corresponding index . This article mainly introduces single column set Collection, And some sub interfaces below it .

Generally, we will learn these two basic List,Set, But actually , There is another kind. . This article will Collection All three interface implementation classes are described .

Two ：Collection Set parsing

our JDK API That's right Collection General .

Collection hierarchy Root interface in .Collection Represents a set of objects , These objects are also called collection The elements of . some collection Duplicate elements are allowed , Others are not allowed to . some collection Is ordered , Others are disordered .JDK No direct access to this interface is provided Realization ： It provides a more specific sub interface （ Such as Set and List） Realization . This interface is usually used to pass collection, And operate them where they need the most universality collection.
package (bag) Or more sets (multiset)（ Unordered that may contain duplicate elements collection） This interface should be implemented directly .
All in common Collection Implementation class （ It is usually realized indirectly through one of its sub interfaces Collection） Two... Should be provided “ standard ” Construction method ： One is void（ No parameter ） Construction method , Used to create an empty collection; The other is with Collection The construction method of type single parameter , Used to create a new element with the same parameters collection. actually , The latter allows users to copy any collection, To generate an equivalent of the desired implementation type collection. Although this agreement cannot be enforced （ Because an interface cannot contain constructor ）, however Java All common in the platform library Collection All implementations follow it .
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The specific implementation interface class is mentioned here ,Set,List.

in addition , The document also professionally explains the following points that need to be paid attention to

Collections Framework Many methods in the interface are based on equals Method defined . for example ,contains(Object o) Method specification declaration ：“ If and only if collection Contains at least one that satisfies (o==null ? e==null :o.equals(e)) The elements of e when , return true.” No This specification should be understood as implying that the call has non null parameters o Of Collection.contains Method will result in arbitrary e Element call o.equals(e) Method . Various implementations can be optimized at will , Just avoid calling equals that will do , for example , By first comparing the hash codes of the two elements .（Object.hashCode() The specification guarantees that two objects with unequal hash codes will not be equal ）. More commonly , Various Collections Framework The implementation of the interface can use the bottom layer at will Object The specified behavior of the method , Regardless of whether the implementation program thinks it's appropriate .

It is embodied in its implementation interface class , Explain in detail later in the article

Collection Itself provides some of its own methods

Extract common methods

1.  Add functionality     boolean add(E e)          Add an element     boolean addAll(Collection c)           Add a batch of elements 2.  Delete function    boolean remove(Object o)         Delete an element 3.  Judgment function    boolean contains(Object o)         Determines whether the collection contains the specified element    boolean isEmpty()          Determines if the set is empty ( There are no elements in the collection )4.  Acquisition function    int size()         Get the length of the collection 5.  Conversion function    Object[] toArray()         Convert a set to an array