Intro to Arrays

When you create an array, you will need to do three steps:

1. Declare the reference, or array variable.
2. Allocate the slots for the array with new.
3. Create the elements in the array with new (unless it is an array of primitives).

1. The first step is to declare the array variable.

   TestScore    tests[] = null;    // declare the array variable and set it to null

   The square brackets indicate that the variable tests is an array. It will reference a list of objects, all of which will be of class TestScore.

   

   Notice that at this point we do not actually have a list. That is the next step.

2. Now we allocate the slots with new.

   TestScore    tests[] = null;   // declare the array variable and set it to null
   
   tests = new TestScore[5];      // allocate the slots in the array

   The number in square brackets indicates how big the array should be. In this case we want the array to be size 5.

   Notice that we still do not have any TestScores. That is the third step in creating an array.

   

3. Now we allocate the elements in the array with new.

   TestScore    tests[] = null;   // declare the array variable and set it to null
   
   tests    = new TestScore[5];        // allocate the slots in the array
   tests[0] = new TestScore(19, 20);   // create element 0

   The number in the square brackets is the index into the array. In this case we are setting the first element in the array (index 0) to the TestScore that we created with new and calling the constructor that takes two arguments. What do you think the 2 arguments represent?

   

   Notice that we just write the values tied to a TestScore object inside the new box. Also notice that we have only filled one element in the array.

   It is important to start thinking about the details of the types involved.

Let's create two more TestScore elements in our array

TestScore    tests[] = null;    // declare the array variable and set it to null

tests = new TestScore[5];           // allocate the slots in the array

tests[0] = new TestScore(19, 20);   // create element 0
tests[1] = new TestScore(23, 25);   // create element 1
tests[2] = new TestScore(47, 50);   // create element 2

Here's a more complete code example; try drawing it yourself:

TestScore   tests[] = new TestScore[5]; // declare and allocate the slots in the array
int         used = 0;
int         i = 0;
String      scoreLabel;

tests[0] = new TestScore(19, 20);
tests[1] = new TestScore(20, 20);
tests[2] = new TestScore(23, 25);

used = 3;

System.out.println("The size of our array is " + tests.length +
    ". We are using " + used + " elements.");


for (i = 0; i < used; i++)
{
    scoreLabel = "Score " + (i + 1);
    System.out.println(scoreLabel + " is " + tests[i]);
    System.out.println("    Letter grade: " + tests[i].getLetterGrade());
}