Basics of Java Programming - Part 2

Arrays

Arrays in Java, like Python lists, Javascript arrays, and C arrays, store ordered values of the same data type. They are built-in. However, there are some differences.

Java arrays cannot be resized once created, and you must declare the type of the variables that the array will store in advance. There is a class you can use to work with arrays in Java: the Arrays class. It is more flexible, and a frequent alternative to arrays, but first let’s see how to use arrays.

Example:

class ArrayExample {
  public static void main(String[] args) {

    // Declare and initialize an array of integers
    int[] myCalorieIntake = {2000, 4500, 3000, 1500, 1000};

    System.out.println(myCalorieIntake[2]);

    // Arrays have an instance variable that stores
    // the length of the array
    System.out.println(myCalorieIntake.length);

    // Array of strings
    String[] myFruits = {"Bananas", "Mangoes", "Apples", "Oranges"}; 
    System.out.println("Today, I would love to eat " + myStrings[3]);

    // Arrays can be modified
    myStrings[2] = "Pineapple";
  }
}

Note: Although you can determine the length of a string using the method someString.length(), you determine the length of an array using the instance variable someArray.length.

new keyword

new is a special keyword in Java that can be used to create arrays. To use it, you first have to declare the array.

Example:

class NewArrayExample {
  public static void main(String[] args) {
    // Declare the array
    int[] myCalories;
    myCalorieIntake = new myCalories[7];

    myCalorieIntake[0] = 1400;
    myCalorieIntake[1] = 1460;
    myCalorieIntake[2] = 2000;
    myCalorieIntake[3] = 1390;
    myCalorieIntake[4] = 2900;
    myCalorieIntake[5] = 780
    myCalorieIntake[6] = 2300

    System.out.println("I have a list of my calorie intake for " + myCalories.length " days.");
  }

}

The array in main (String[] args)

The main method is used to run Java programs. You have to have a main method if you want to execute any Java program.

The syntax of the main method is usually: public static void main (String[] args).

You can now see that main takes a single parameter, args, of type String.

If you execute a program from the command line (terminal on Unix or Mac), you can pass parameters to the program. For example, the command

cat calories.txt

can be typed into the command line to display the information contained in calories.txt. The first word, cat, is the name of the program to execute. The second word, calories.txt, is passed as a string as the first item in the array of strings, args.

A similar mechanism is used in C, but since C arrays do not internally know their lengths, the syntax declaration of main is a bit different. However, this can be determined using argc , which is the count of the number of arguments passed to main. In Python, sys.argv can be imported to fetch command-line arguments.

The args array can be used to access the arguments that were passed to the program. For example, the following code will print the value of the second argument:

public static void main(String[] args) {
  System.out.println(args[1]);
}

java.util.Arrays

You can work with arrays using the Arrays class. This is sometimes a better alternative since they are more flexible. The Arrays class in the java.util package provides some methods that let you do things like copy, compare, sort, and search arrays.

Unlike some other classes, like Math or String that are available by default and hence don't need to be imported, you must import the Arrays class, using an import statement at the top of any file in which you would like to use it.

You can find out more about the Arrays class here: Arrays (Java Platform SE 8 ) (oracle.com)

import java.util.Arrays;

class ArraysExample {
  public static void main(String[] args) {
    int[] myCalorieIntake = {42, 1, 17, 27, 16};
    Arrays.sort(myCalorieIntake);
    System.out.println(Arrays.toString(myCalorieIntake));

  }
}

With this information, you now have what it takes to use and manipulate arrays effectively! 🚀 Feel free to reach out if you have any questions and comment below! Happy coding! 🎉