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Collections and Iterators

2022-08-07 03:33:00 【l'amourХэ рутилин】

Collection

When programming, you can use an array to store multiple objects, but the length of the array cannot be changed. Once the array length is specified when the array is initialized, the length of the array is immutable.If you need to hold a varying amount of data, arrays are a bit powerless.Moreover, arrays cannot store data with mapping relationships. In order to store an indeterminate amount of data and data with mapping relationships, Java provides a collection class.Collection classes are mainly responsible for storing and holding other data, so collection classes are also called container classes.

The collection class is different from the array. The elements of the array can be either the value of the basic type or the object, and the collection can only store the object.

Java collection types are divided into Collection and Map, which are the root interfaces of Java collections, and these two interfaces contain some sub-interfaces or implementation classes.

Collection interface basic structure

Basic structure of Map interface

Collection common functions

Code Examples:

public static void main(String[] args) {ArrayList objects = new ArrayList<>();objects.add("123456");objects.add(652);objects.add(true);objects.remove("123456");boolean contains = objects.contains("123456");System.out.println(contains);System.out.println(objects);for (Object next : objects) {System.out.println(next);}
Iterator
In program development, it is often necessary to traverse all elements in a collection.In response to this demand, JDK provides an interface Iteration is a way to traverse Collection elements.Before taking the element, first determine whether there is an element in the collection, and if so, take the element out.Continue to judge, if there is, continue to obtain elements until all elements in the collection are taken out.The technical term for this retrieval method is called Iterative.
boolean hasNext(): Determine if there is a next elementE next(): returns the next elementvoid remove(): delete the next element
Get iterator
Iterator iter = collection.iterator();
Note:
Iterable: Iterable, indicating an iterable ability (provides a method to obtain an iterator)
Iterator: The iterator implements the iteration function (real iterator)
Using iterators
1. Get the iterator: iterator
2. Determine if there is a next element: hasNext
3. If it exists, get the next element: next
4. Repeat steps 2 and 3 until there is no next element
Code example:
 public static void main(String[] args) {ArrayList arrayList = new ArrayList();arrayList.add(56341);arrayList.add(87465);arrayList.add(94864);Iterator iterator = arrayList.iterator();while (iterator.hasNext()){System.out.println(iterator.next());}}
Note:
1. When the collection element is acquired, if there is no element in the collection, and the next method of the iterator is continued to be used, a java.util.NoSuchElementException will be thrown. There is no collection element exception.
2. During collection element acquisition, if you add or remove elements in the collection, it will not be able to continue to iterate, and ConcurrentModificationException will be thrown.
Enhanced for loop [foreach]
Enhanced for loop (also called for each loop) is an advanced for loop after JDK1.5, which is specially used to traverse arrays and collections.Its internal principle is actually an Iterator iterator, so in the process of traversal, you cannot add or delete elements in the collection.
Syntax:
for(variable type variable: array/collection){The traversed data is processed, and the data is stored in this variable}
Tips: array name/collection name.for
Code example:
public static void main(String[] args) {ArrayList arrayList = new ArrayList();arrayList.add(56341);arrayList.add(87465);arrayList.add(94864);for (Object o : arrayList) {System.out.println(o);}}
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