Collection Common Functions

Collection is the parent interface of all single-column collections, so some common methods of single-column collections (List and Set) are defined in Collection, which can be used to manipulate all single-column collections. The methods are as follows:

• Public Boolean add (E): Adds a given object to the current collection.
• public void clear(): Clears all elements in the collection.
• Public Boolean remove (E): Delete a given object from the current collection.
• Public Boolean contains (E): Determines whether a given object is included in the current collection.
• public boolean isEmpty(): Determines whether the current collection is empty.
• public int size(): Returns the number of elements in the collection.
• public Object[] toArray(): Stores elements in a collection into an array.

import java.util.ArrayList;
import java.util.Collection;

public class Collection_basic {
    public static void main(String[] args) {
        Collection<String> coll = new ArrayList<>();
        System.out.println(coll); // Printed as [], indicating that soString() has been overwritten.

        /* 1. add Method
            public boolean add(E e): Add objects to geometry
            The return value is a Boolean value, which returns true successfully, but it is generally error-free and generally not accepted.
        */
        boolean b1 = coll.add("Zhang San"); // The return value of add is that Boolean value true is add to CD-ROM migrant workers
        System.out.println("b1:" + b1);
        System.out.println(coll); // [Zhang three]

        coll.add("Li Si");
        coll.add("Wang Wu");
        coll.add("Zhao Liu");
        System.out.println(coll);

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

        /* 2. remove Method
            public boolean remove(E e): Delete elements from collections
            The return value is boolean, which exists in the collection, deletes successfully, and returns true
                             The collection does not exist, deletion fails, and false is returned.
         */
        boolean b2 = coll.remove("Zhao Liu");
        System.out.println("b2:" + b2);
        boolean b3 = coll.remove("I do not exist.");
        System.out.println("b3" + b3);

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

        /* 3. contains
            public boolean contains(E e): Determine whether an element is included in a collection
         */
        boolean b4 = coll.contains("Li Si");
        System.out.println("b4:" + b4);

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

        /* 4. isEmpty
            public boolean isEmpty(); Set returns true for empty, not false for empty
         */
        boolean b5 = coll.isEmpty();
        System.out.println("b5:" + b5);

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

        /* 5. size
            public int size(); Number of elements returned
         */
        int s = coll.size();
        System.out.println("size:" + s);

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

        /* 6. toArray
            public Object[] toArray(); Store elements in a collection in an array
         */
        Object[] arr = coll.toArray();
        System.out.println(arr);
        for (int i = 0; i < arr.length; i++) {
            System.out.println(arr[i]);
        }

        /* 7. clear
            public void clear(); Empty all elements in the collection, but do not delete the collection, and become an empty collection
         */
        coll.clear();
        System.out.println(coll);
        System.out.println(coll.isEmpty());

    }
}