Instances & Testing

Your class should now look something like this, although your default values may be different:

public class Ingredient{
    public String name;
    public double amount;
    public String units;

    public Ingredient(){
        this.name = "flour";
        this.amount = 2.0;
        this.units = "cup";
    }
}

As classes grow large you will want to test methods as you add them. In CC 410 we will learn about test frameworks and will write formal tests in parallel with project development.

For now we will follow the convention of a main method in each class which can be used for testing. The basic flow would be:

1. Create class
2. Write the constructor, it may be blank
3. Create main(args), instantiate an object and test (print out) the initialized value to test for correctness.
4. Compile and run the class as a program
5. Repeat until done:
   1. Create new method
   2. Update main(), usually involves instantiating and object and using it to call the new method
   3. Run and check output

Instantiation

To instantiate, or create, an object in Java, we use the keyword new and call the class constructor.

Ingredient ingr1 = new Ingredient();

This will create a new Ingredient object, and then store it in a variable of type Ingredient named ingr1. While this may seem a bit confusing at first, it is very similar to how we’ve already been working with variables of types like int and double.

Accessing Attributes

Once we’ve created a new object, we can access the instance attributes as defined in the class from which it is created.

For example, to access the name attribute in the object stored in ingr1, we could put the following code in main()¹

Ingredient ingr1 = new Ingredient();
String n = ingr1.name; // n is assigned the current value of
                       // ingr1's name attribute
System.out.println(n == ingr1.name); // prints true they are equal

Java uses what is called dot notation to access attributes and methods within instances of a class. So, we start with an object created from that class and stored in a variable, and then use a period or dot . directly after the variable name followed by the attribute or method we’d like to access. Therefore, we can easily access all of the attributes in an Ingredient object using this notation:

Ingredient ingr1 = new Ingredient();
System.out.println(ingr1.name);
System.out.println(ingr1.amount);
System.out.println(ingr1.units);

We can then treat each of these attributes just like any normal variable, allowing us to use or change the value stored in it:

Ingredient ingr1 = new Ingredient();
System.out.println(ingr1.name);
System.out.println(ingr1.amount);
System.out.println(ingr1.units);

n = ingr1.name;
String ingr1.name = "cardamom";
System.out.println (n == ingr1.name); // False they are not equal we changed ingr1.name

How to Test

When testing it is important to avoid “feature creep” in the class. We want to avoid adding add attributes or methods that are not called for by the UML class diagram. In software development you will drive up test and maintenance cost².

In this class it is always acceptable to add a private static void main() method, even if it is not on the UML, to facilitate testing. We will put all our test code for instance classes in main().

public class Ingredient{
    public String name;
    public double amount;
    public String units;

    public Ingredient(){
        this.name = "flour";
        this.amount = 2.0;
        this.units = "cup";
    }
    private static void main(String[] args){
        Ingredient ingr1 = new Ingredient();
        System.out.println(ingr1.name);
        System.out.println(ingr1.amount);
        System.out.println(ingr1.units);    
    }
}

This is how one might test a constructor method, we check to see all object variables have the correct values.

Things to keep in mind for this course:

• YOU need to know what the answer (print out) should be
• Never use input (keyboard or file) in your test code
  • Hard code the values you want to test with
• Consider deleting main() from instance classes before submitting a Project
• Consider deleting this code after testing each method
  • You can continue to append to the end but it makes for a long program

private static void test(){  put you test code here}
public static void main(String[] args){
    test(); // delete this line when you are done testing
            // and ready to start writing main
}