Single row collection system

Common functions of top-level interface Collection

Overview

​ Collection is the top-level interface of single-column collections, and the top-level interface of single-column collections defines the functions common to all single-column collections. JDK does not provide any direct implementation of this interface. He provides more specific sub-interfaces (such as Set and List) to achieve

Create Collection object

Collection interface, you cannot create objects directly, you need an implementation class ArrayList to create objects

​ 1) Polymorphism: the way of polymorphism-the parent class reference points to the child class object Collection c = new ArrayList();

​ 2) Specific implementation class creation-this class reference points to this class object ArrayList a = new ArrayList();

Collection collection commonly used methods:

​ 1) boolean add (Object e): add elements

​ 2) boolean remove (Object o): remove the specified element from the collection

​ 3) void clear(): Clear the elements in the collection

​ 4) boolean contains(Object o): Determine whether the specified element exists in the collection

​ 5) boolean isEmpty(): Determine whether the collection is empty (the collection exists, there are no elements), if the collection is empty, then return true, if the collection is not empty false

​ 6) int size(): Returns the number of elements in the collection, returns the length of the collection

​ 7) Object[] toArray(): returns an array of objects of the collection

package com.ujiuye.collection;
import java.util.ArrayList;
import java.util.Collection;

public class Demo01_Collection常用方法 {
    
    
    public static void main(String[] args) {
    
    
        // 1. 创建出一个集合
        Collection c = new ArrayList();
        // 2. boolean add(Object e): 添加元素
        c.add("a");
        c.add("hello");
        c.add("b");
        System.out.println(c);// [a, hello, b]

        // 4. boolean contains(Object o): 判断集合中是否存在指定的元素
        System.out.println(c.contains("hello"));// true
        System.out.println(c.contains("he"));// false

        // 5.boolean isEmpty(): 判断集合是否为空(集合存在,没有元素), 如果集合为空, 那么返回true, 如果集合不为空 false
        System.out.println(c.isEmpty());// false

        // 3. boolean remove (Object o): 从集合中移除指定的元素
        c.remove("a");
        System.out.println(c);// [hello, b]

        //6. int size(): 返回集合中元素的数量，返回集合的长度。
        System.out.println(c.size());// 2

        // 4. void clear(): 清空集合中的元素
        c.clear();
        System.out.println(c);// []

        // boolean isEmpty(): 判断集合是否为空(集合存在,没有元素), 如果集合为空, 那么返回true, 如果集合不为空 false
        System.out.println(c.isEmpty());// true

        //6. int size(): 返回集合中元素的数量，返回集合的长度。
        System.out.println(c.size());// 0

        System.out.println("-------------------");

        // 7. Object[] toArray(): 返回集合的对象的数组
        Object[] objArr = c.toArray();
        // 通过遍历数组相当于在遍历集合中元素内容
        for(int index = 0; index < objArr.length; index++){
    
    
            Object obj = objArr[index];
            String s = (String)obj;
            System.out.println(s);
        }
    }
}