7.4 Developing Algorithms Using ArrayLists

Common Arraylist Methods:

size(): Returns the size of the arraylist as an Integer
add(object): Adds an object to the end of your ArrayList
void add(index, object): Addes an object to an index of your choice. Shifts the index of everything to the right by one and increases size by 1
get(index): Retrieves the object at the index specified
set(index, obj): Like void add, but instead of adding, it replaces the object that’s already in that index
remove(index): Removes the object at specified index

//size() & add(object)

ArrayList<Double> numbers = new ArrayList<>();
    numbers.add(1.0);
    numbers.add(2.0);
    numbers.add(3.0);

int size = numbers.size();

System.out.println(size);

//void add(index, object)
//get(index)
ArrayList<Double> numbers = new ArrayList<>();
    numbers.add(1.0);
    numbers.add(2.0);
    numbers.add(3.0);

System.out.println(numbers.get(2));

    numbers.add(2,4.0);

System.out.println(numbers.get(2));
System.out.println(numbers.get(3));

0
0
0

// set(index, obj)


ArrayList<Double> numbers = new ArrayList<>();
    numbers.add(1.0);
    numbers.add(2.0);
    numbers.add(3.0);

System.out.println(numbers.get(2));

    numbers.set(2,4.0);

System.out.println(numbers.get(2));

3.0
4.0

// remove(index)


ArrayList<Double> numbers = new ArrayList<>();
    numbers.add(1.0);
    numbers.add(2.0);
    numbers.add(3.0);
System.out.println(numbers.get(2));
    numbers.remove(2);

System.out.println(numbers.get(0));
System.out.println(numbers.get(1));
System.out.println(numbers.get(2));

//anybody know why we get an IndexOutofBoundsException eror?

3.0
1.0
2.0

---------------------------------------------------------------------------

java.lang.IndexOutOfBoundsException: Index 2 out of bounds for length 2

	at java.base/jdk.internal.util.Preconditions.outOfBounds(Preconditions.java:64)

	at java.base/jdk.internal.util.Preconditions.outOfBoundsCheckIndex(Preconditions.java:70)

	at java.base/jdk.internal.util.Preconditions.checkIndex(Preconditions.java:266)

	at java.base/java.util.Objects.checkIndex(Objects.java:361)

	at java.base/java.util.ArrayList.get(ArrayList.java:427)

	at .(#38:1)

Here’s an example of a program using Arrays that finds the maximum value:

public class Main {
    public static void main(String[] args) {
        double[] values = {1, 2, 3, 4, 5};

        double maxValue = findMax(values);
        System.out.println("The maximum value is: " + maxValue);
    }

    private static double findMax(double[] values) {
        double max = values[0];
        for (int index = 1; index < values.length; index++) {
            if (values[index] > max) {
                max = values[index];
            }
        }
        return max;
    }
}
Main.main(null);

The maximum value is: 5.0

Now, how can we modify this to use an ArrayList?


public class Main {
    public static void main(String[] args) {
        ArrayList<Double> values = new ArrayList<>();
        values.add(1.2);
        values.add(3.4);
        values.add(2.6);
        values.add(4.9);
        values.add(0.8);

        double maxValue = findMax(values);
        System.out.println("The maximum value is: " + maxValue);
    }

    private static double findMax(ArrayList<Double> values) {
        double max = values.get(0);

        for (int index = 1; index < values.size(); index++) {
            if (values.get(index) > max) {
                max = values.get(index);
            }
        }
        return max; 
    }
}
Main.main(null);

The maximum value is: 4.9

Homework:

(Paragraph Answer)

What is the difference between the two examples above. Which one is better and why?

(Code Answer)

Make your own algorithm using ArrayLists that finds the sum of the elements in the ArrayList

Homework Paragraph - Dinesh Sahai

Written Response

Example 1, with the double[] array, is definitely the better choice for a fixed-size dataset. Arrays are quicker to access and more efficient when you already know the data size, so it makes the code simpler and faster. Example 2 uses an ArrayList, which is useful for adding or removing values, but that flexibility isn’t needed here. The extra overhead makes it less efficient. So, if your data is static, Example 1 is the way to go.

//Homework Code Answer - Dinesh Sahai

import java.util.ArrayList;

ArrayList<Double> values = new ArrayList<>();
values.add(1.2);
values.add(3.4);
values.add(2.6);
values.add(4.9);
values.add(0.8);

double sum = 0;

// Loop through the ArrayList and add each element to the sum
for (double value : values) {
    sum += value;
}

System.out.println("The sum of the elements is: " + sum);

The sum of the elements is: 12.9