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This article mainly introduces what the Java collection is, has a certain reference value, interested friends can refer to, I hope you can learn a lot after reading this article, the following let the editor take you to understand it.

Java collection 1. What is a collection?

Concept: a collection is a container for storing objects, and defines common methods for manipulating multiple objects, which can realize the function of arrays.

The difference between and array:

(1) the length of the array is fixed and the length of the set is not fixed.

(2) arrays can store basic types and reference types, and collections can only store reference types.

Location: java.util.*

2.Collection system set

3.Collection interface

The Collection interface defines many operations for collections through top-level design, which can be roughly divided into several methods, such as adding, deleting, traversing, judging, and so on.

Increase

Boolean add (E e) ensures that this collection contains the specified elements (optional). Boolean addAll (Collection c) removes all elements (optional actions) of all elements contained in this collection (optional actions). Default boolean removeIf (Predicate c) retains only the elements contained in the specified collection (optional operation).

Ergodic

Iterator iterator () returns an iterator for the elements in this collection. Object [] toArray () returns an array containing all the elements in this collection. T [] toArray (T [] a) returns an array containing all the elements in this collection; the runtime type of the returned array is the runtime type of the specified array.

Judge

Boolean contains (Object o) returns true if the collection contains the specified element. Boolean containsAll (Collection c) returns true if this collection contains all the elements of the specified collection. Boolean isEmpty () returns true if the collection does not contain any elements.

Other

Int size () returns the number of elements in this collection. Boolean equals (Object o) compares the specified object to this collection for equality. Int hashCode () returns the hash code value of this collection. The use of 4.Collection

Two commonly used methods of traversal: forEach and Iterator

Public class Demo1 {public static void main (String [] args) {Collection collection = new ArrayList (); collection.add ("apple"); collection.add ("banana"); collection.add ("watermelon"); / / 1.foreach for (Object object: collection) {System.out.println (object);} / * * 2. Using Iterator, it includes three methods: * hasnext (): determine whether there is a next element * next (): get the next element * remove (): delete the last element returned by the iterator * / Iterator it = collection.iterator (); while (it.hasNext ()) {String object = (String) it.next () / / collection.remove (object); cannot use the deletion method of the traversed collection. Concurrent modification exception it.remove (); System.out.println (object);} System.out.println (collection.size ());}} 5.List subinterface (1) basic information will be reported.

Features: orderly, subscript, elements can be repeated.

List adds a lot of methods to the parent interface Collection.

Increase

Void add (int index, E element) inserts the specified element at the specified location in the list (optional operation). Boolean addAll (Collection list) reverses the order of the elements in the specified list. Static void shuffle (List list) Random substitution specifies that the list uses a random default source. Static void swap (List list, int I, int j) exchanges elements at the specified location in the specified list. Static void copy (List

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