gather

                                         gather

   Earlier we introduced the concept of arrays. Both collections and arrays can be mistaken as containers, but the length of the array is fixed, and the length of the collection can be changed, but the stored elements must be elements of reference data types. It is also a few differences from arrays

We have introduced the Arraylist collection earlier, so I won't go into details here. Today we'll take a look at the collection interface

    collection interface

The secondary interface is a top-level interface that defines many functions and methods that subclasses can call　

public static void main(String[] args) {
        // TODO Auto-generated method stub
        Collection<String>coll=new ArrayList<String>();
        //add方法
        coll.add("1号");
        coll.add("2号");
        //集合转属组
        Object[] arr=coll.toArray();
        for(int i=0;i<arr.length;i++){
            System.out.println(arr[i]);
        }
        coll.remove( "No.2"); // collection element removal method 
        System.out.println(coll.size());
         boolean b=coll.contains("No.2"); // Determine whether there is any in the collection This element 
        
        coll.clear(); // Clear the contents of your container 
    }

This is the method of the collection's collection to array and adding and removing elements. When turning the array, it can only be an array of Object type.

iterator

Iterator interface: its main function is to get the elements in the collection

 

public static void main(String[] args) {
        Collection<String>coll=new HashSet<String>();
        coll.add( "Little girl" );
        coll.add( "You have to study hard" );
        coll.add( "You are so old" );
        coll.add( "Like a baby" );
         // 1. Get the iterator object
         // Iterator<String>it=coll.iterator();
 //         while(it.hasNext()){
 //             String str=it.next();
 //             System.out.println(str);
 //         }
         // System.out.println(it.hasNext());
         // System.out.println(it.next() ); 
        for (Iterator<String>it= coll.iterator();it.hasNext();){
            System.out.println(it.next());
        }
    }

 

The hasnext method will return a boolean type data to determine whether there are elements in the collection, and then traverse the collection. Two methods are introduced here, for and while