Section 3.5 Arrays

As was said at the outset of this section, we are going to use Java ArrayLists because they are easier to use and more closely match the way that Python lists behave. However, if you look at Java code on the internet or even in your core Java books you are going to see examples of something called an array. In fact, you have already seen one example of an array declared in the ‘Hello World’ program. Listing 3.5.1 is a rewritten version of Listing 3.4.4 that uses primitive arrays rather than array lists.

Listing 3.5.1.

import java.util.Scanner; // import the Scanner class to read input from the user
import java.io.File; // import the File class to read from files
import java.io.IOException; // import the IOException class to handle file input/output exceptions
/**
 * Program to read numbers from a file and produce a histogram using arrays in Java.
 */
public class HistoArray {
    public static void main(String[] args) {
        Scanner data = null;
        Integer[] count = {0,0,0,0,0,0,0,0,0,0}; // create an array of 10 integers initialized to 0
        Integer idx;
        try {
            data = new Scanner(new File("test.dat"));
        }
        catch ( IOException e) {
            System.out.println("Unable to open data file");
            e.printStackTrace();
            System.exit(0);
        }
        while(data.hasNextInt()) {
            idx = data.nextInt();
            count[idx] = count[idx] + 1; // increment the count for the number read from the file, using array indexing
        }
        idx = 0;
        for(Integer i : count) {
            System.out.println(idx + " occurred " + i + " times.");
            idx++;
        }
    }
}

The main difference between Listing 3.5.1 and Listing 3.4.4 is that we declare count to be an Array of integers. We also can initialize short arrays directly using the syntax shown here: Integer[] count = {0,0,0,0,0,0,0,0,0,0}Then notice that we can use the square bracket notation count[idx] to index into an array.

Checkpoint 3.5.2.

Construct a short Java program that creates an array of three integers, changes the first value, and prints it. Drag the blocks into the correct order on the right.

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

---
        Integer[] nums = {1, 2, 3};

---
        Integer[] nums = (1, 2, 3);
 #paired
---
        nums[0] = 5;

---
        nums(0) = 5;
 #paired
---
        System.out.println(nums[0]);

---
    }
 }

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