Detailed explanation of the use of collection classes in Java language

The Java language is one of the most widely used programming languages ​​in the world, and collection classes are a very important part of the Java language. In the Java language, a collection class is a container for a set of data types that can store and operate multiple objects. These objects can be basic types, custom objects, or other collection objects. Java collection classes are widely favored by programmers for their efficiency, flexibility and ease of use. This article will introduce in detail how to use Java collection classes.

1. Classification of Java collection classes

The Java collection framework contains two types of collection classes: one is an object container generally provided through a packaging class, such as ArrayList passing objects , a container that wraps Java's basic data types, provides features that ordinary arrays do not have; the other is a Map-based data structure, such as HashMap, TreeMap, etc., which is used to map key objects to value objects. Java collection classes can be divided into four categories: List, Map, Set and Queue. Among them, List is an ordered collection, Map is a storage relationship collection of key/value pairs, Set is an unordered collection of elements that does not allow duplication, and Queue is a collection of elements managed according to the FIFO (first in, first out) principle. Below we will introduce the characteristics and usage of these four types of collections respectively.

2. List collection

List collection is an ordered collection (that is, it can be repeated). It can store a set of elements and sort them in a certain order. There are three commonly used implementations of the List interface: ArrayList, LinkedList and Vector.

1. ArrayList

ArrayList is a dynamic array that can dynamically increase and decrease the size of the array. Its internal implementation uses an array structure, so query operations are fast, but insertion and deletion operations are slow. The usage of ArrayList is similar to that of array, but it has the flexibility of dynamically increasing and shrinking.

2. LinkedList

LinkedList is a doubly linked list, its elements can be inserted and deleted at any time, and can better support concurrent operations. LinkedList's query operations are slower, but insertion and deletion operations are faster. LinkedList can be used as a queue or stack.

3. Vector

Vector is a thread-safe version of ArrayList. Its execution efficiency is slower than ArrayList, so it is less used in actual development. Vector has many similar methods to ArrayList, but it is thread-safe.

3. Map collection

Map collection is a storage relationship collection of key/value pairs. It has very fast query speed and the ability to add and delete elements. A Map collection can guarantee that its keys are unique, but values ​​can be repeated. There are three commonly used implementations of the Map interface: HashMap, TreeMap and LinkedHashMap.

1. HashMap

HashMap is an implementation based on a hash table, which can perform insertion and query operations in constant time, but the order of the elements is uncertain. The keys or values ​​of HashMap can be null objects.

2. TreeMap

TreeMap is based on the implementation of red-black trees and can sort elements. The time complexity of its insertion and query operations is O(log N). TreeMap requires its keys to be comparable, so it must implement the Comparable or Comparator interface.

3. LinkedHashMap

LinkedHashMap is a variant of HashMap that is able to maintain the insertion order of elements while also knowing how to use soft references or weak references in the element access order. Reference to the deleted element. The execution efficiency of LinkedHashMap is slightly lower than HashMap.

4. Set collection

Set collection is an unordered collection that does not allow duplicate elements. It can be used to process objects with a single attribute. There are two commonly used implementations of the Set interface: HashSet and TreeSet.

1. HashSet

HashSet is implemented based on HashMap. It can provide fast element insertion and query operations, but does not allow duplicate elements. It is undetermined whether a HashSet has order, so the order of the elements is not required.

2. TreeSet

TreeSet is implemented based on red-black trees and can perform ordered insertion and query operations on elements. TreeSet requires its elements to implement the Comparable or Comparator interface to achieve sorting purposes.

5. Queue Collection

Queue collection is a collection of elements managed according to the FIFO (first in, first out) principle, including two interfaces: Queue and Deque. Deque is a double-ended queue expanded on the basis of Queue. It can insert or delete elements at the beginning and end of the queue. Both Queue and Deque have two commonly used implementations: LinkedList and ArrayDeque.

1. LinkedList

LinkedList can be used as a List collection, or as a Queue and Deque. For Queue and Deque, LinkedList is an implementation based on a linked list.

2. ArrayDeque

ArrayDeque is an array-based implementation that can add or remove elements from the tail and get elements from the head.

6. Use cases of collection classes

As a Java programmer, you must skillfully use Java collection classes to improve development efficiency and program performance. Below we take ArrayList as an example to show how to use Java collection classes.

import java.util.ArrayList;

public class ArrayListTest {
    public static void main(String[] args) {
        // 创建ArrayList对象
        ArrayList<Integer> list = new ArrayList<Integer>();
        // 添加元素
        list.add(1);
        list.add(2);
        list.add(3);
        list.add(1);
        System.out.println(list);
        // 删除元素
        list.remove(0);
        System.out.println(list);
        // 插入元素
        list.add(0, 4);
        System.out.println(list);
        // 获取元素
        int element = list.get(2);
        System.out.println("Element at index 2: " + element);
        // 修改元素
        list.set(2, 5);
        System.out.println(list);
        // 清空列表
        list.clear();
        System.out.println(list);
    }
}

The above code demonstrates how to create an ArrayList object, add elements to it, delete elements, insert elements, get elements, modify elements and clear the list. In actual development, programmers can also use Java collection classes to implement more complex business logic.

7. Summary

This article provides a detailed introduction to the use of Java collection classes. The Java collection class is one of the knowledge points that Java programmers must master proficiently. In actual development, it can provide us with a more efficient, flexible, and easier-to-use data storage and operation method. When choosing which collection class to use, you need to make a choice based on the actual situation to ensure the performance and maintainability of the program. We hope this article can help everyone master the use of Java language collection classes.

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