Initializing Arryas

Below is a program from the text book that creates an array of String objects in two different ways and illustrates how to change the values stored in an array. Program the code and be able to explain how it works. 
class ArrayTest {

    String[] firstNames = { "Dennis", "Grace", "Bjarne", "James" };
    String[] lastNames = new String[firstNames.length];

    void printNames() {
        int i = 0;
        System.out.println(firstNames[i]
            + " " + lastNames[i]);
        i++;
        System.out.println(firstNames[i]
            + " " + lastNames[i]);
        i++;
        System.out.println(firstNames[i]
            + " " + lastNames[i]);
        i++;
        System.out.println(firstNames[i]
            + " " + lastNames[i]);
    }

    public static void main (String arguments[]) {
        ArrayTest a = new ArrayTest();
        a.printNames();
        System.out.println("-----");
        a.lastNames[0] = "Ritchie";
        a.lastNames[1] = "Hopper";
        a.lastNames[2] = "Stroustrup";
        a.lastNames[3] = "Gosling";
        a.printNames();
    }
}