Collection

Published Jul 18, 2022

A Collection is an interface allowing items to be grouped within a single container object. It is found in the java.util package and is implemented by List, Set, and Queue.

There are many varied Collection types in Java, and the special Collections class is provided to manipulate them.

Syntax

import java.util;

Collection<DataType> c = new CollectionClass<DataType>();

Where DataType is the data type to be stored in the collection and CollectionClass is some class implementing the Collection interface.

Methods

The Collection interface utilizes the following methods:

Method	Syntax	Description
`.add()`	`.add(item)`	Adds `item` to the collection if it wasn’t a member already.
`.addAll()`	`.addAll(collection)`	Adds the elements in `collection` to the collection.
`.clear()`	`.clear()`	Removes all the items in the collection.
`.contains()`	`.contains(item)`	Returns `true` if `item` is a member of the collection.
`.containsAll()`	`.contains(collection)`	Returns true if all the items in `collection` are contained in the collection.
`.isEmpty()`	`.isEmpty()`	Returns `true` if the collection contains no elements.
`.remove()`	`.remove(item)`	Removes `item` from the collection.
`.removeAll()`	`.removeAll(collection)`	Removes all items in `collection` from the collection.
`.size()`	`.size()`	Returns the number of elements in the collection.
`.toArray()`	`.toArray()`	Returns an array containing all the elements in the collection.
	`.toArray(array)`	Returns an array whose data type is the same as `array`.

Example

This is an example of the Collection interface being implemented by an ArrayList:

import java.util.*;

public class Main {
 public static void main(String[] args) {
   Collection<String> food = new ArrayList<String>();
   food.add("Cabbage");
   food.add("Pizza");
   food.add("Sausage");
   food.add("Potatoes");
   food.add("Salad");
   System.out.println(food.contains("Cabbage"));
   food.remove("Cabbage");
   System.out.println(food);
 }
}