ArrayList

Earlier on we discussed array objects. We created several array objects but something that we might have or not have discussed is the static size of the array object. This means that when we create an array of some size, n, the total number of elements that this n sized array can hold is n elements. The size of the array is not resizable - can not be changed after initialization.

Java provides a solution to this problem, using an ArrayList. An ArrayList created with an initial size n changes as the number of elements it holds grow.

To use an ArrayList we have to import it from java.util as import java.util.ArrayList and just like any of the classes that we have used so far, we create an instance of it.

To create an instance of an ArrayList, we need the type of elements the ArrayList and the initial size. We then create the instance like this: ArrayList<ObjectType> arrayList = new ArrayList<ObjectType>(size);

ArrayList<Integer> numbers = new ArrayList<Integer>(5);
ArrayList<String> names = new ArrayList<>();

An ArrayList stores Objects - meaning we can't pass a primitive type to it. We would Integer in place of int, Double for double, etc. Note that in new ArrayList<>(), the type and size was not passed. It would be inferred.

An ArrayList is a data structure (stores and manages data). We can represent it logically or abstractly (ADT), just the specifications.

ADT:

Stores data of non-primitive data
adds an element to the list using the add method
removes an element from the list using the remove method
returns a boolean whether the list has a certain element using the contains method
returns the element at a specified index using the get(index) method
returns the current size of the list using the size method
removes all the elements in the list using the clear method

Let's create a class, ArrayListAverage that uses an ArrayList instead of an array to find the average of the elements in the array list.

import java.util.ArrayList;

public class ArrayListAverage {
    public static void main(String[] args) {
        ArrayList<Integer> nums = new ArrayList<>(5);

        nums.add(3);
        nums.add(2);
        nums.add(1);
        nums.add(5);
        nums.add(4);

        int numSum = nums.get(0) + nums.get(1) + nums.get(2) + nums.get(3) + nums.get(4);

        double numAvg = (double)numSum / nums.size();

        System.out.println("Sum: " + numSum);
        System.out.println("Average: " + numAvg);

        System.out.println("\n\nadd new element and find the sum and average");

        nums.add(6);
        numSum += nums.get(5);

        numAvg = (double)numSum / nums.size();

        System.out.println("New Sum: " + numSum);
        System.out.println("New  Average: " + numAvg);
    }
}

Try your hands on the contains, remove and clear methods.

HashMap

ArrayList is an index based data structure just like the array and others. There is a HashMap which is an implementation of a Map. A map stores data using a key-value pair approach. This is similar to object in javascript and dictionary in python.

Just like we created an instance of the ArrayList class, where we passed the type of data to hold, we pass the type of the key and the value.

import java.util.HashMap;

...

HashMap<String, String> hmStringKey = new HashMap<>();
HashMap<Integer, String> hmIntegerKey = new HashMap<>();

ADT

Stores data of non-primitive data
adds an element to the map using the put(key, value) method
returns a boolean whether the map has certain key or value using the containsKey(key) or containsValue(value) method respectively
return a boolean whether the map is equal to another map using the equals method
returns an element by key using the get(key) method
returns a boolean whether the map is empty using the isEmpty method
removes all the elements in the map using the clear method
removes an element from the map using the remove method
returns the current size of the map using the size method
etc

import java.util.HashMap;
...

HashMap<String, String> profile = new HashMap<>();

profile.put("fullName", "John Doe");
profile.put("dob", "1st October, 1976");
profile.put("job", "Project Manager at DooDev LLC");

System.out.println(profile);

Sets

We will use the ADT approach to make describing Set and its operations easier.

Stores data of non-primitive data
Has no duplicate - all its elements are unique
adds an element to the set using the add method
returns a boolean whether the set has a certain element using the contains method
return a boolean whether the set is equal to another set using the equals method
returns a boolean whether the set is empty using the isEmpty method
removes all the elements in the set using the clear method
removes an element from the set using the remove method
returns the current size of the set using the size method

An implementation of a set is the HashSet. We create an instance of a HashSet as we did for the ArrayList.

There is an iterator method on HashSet, ArrayList, etc

Iterators

An Iterator is an object that enables cycling through a collection, obtaining or removing an element. Any collection class provides an iterator method. An example of the ArrayList.

The Iterator ADT

hasNext method returns a boolean whether there is a next element
next method returns the next item
remove method removes the last next object (that is returned by the next method)

import java.util.ArrayList;
import java.util.Iterator;

...

ArrayList<Integer> list = new ArrayList<>();
list.add(23);
list.add(34);
list.add(7);
list.add(-18);
list.add(8);
list.add(3);

// call the iterator
Iterator iterator = arrayList.iterator();

while(iterator.hasNext()) {
    System.out.println(iterator.next());
}

Conclusion

An array as we know has a limitation and we can bypass this limitation of our writing some hacks. Java has the ArrayList data structure. Its size is dynamic and can easily be used.

Projects

Write a program that takes a sentence as an argument and returns a Map of the following:
- number of characters in the sentence with spaces
- number of characters in the sentence without spaces
- number of words in the sentence
Write a program that takes a sentence as an argument and returns a Map of the number of each unique character (white spaces and non-alphabetic characters like numbers and ampersand are excluded). So hello there will return a Map with the following keys and value pairs:
- h:2
- e:3
- l:2
- ...

Source

Sololearn
DS Malik

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