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2.3. Calling Methods Without Parameters

Methods are a set of instructions that define behaviors for all objects of a class. For example, in the Turtle class, methods like forward() and turnRight() give Turtle objects the ability to move forward and turn 90 degrees right.

To use an object’s method, you must use the object name and the dot (.) operator followed by the method name, for example, yertle.forward(); calls yertle’s forward method to move a turtle object forward 100 pixels. These are called object methods or non-static methods. An object method must be called on an object of the class that the method is defined in. Object methods work with the attributes of the object, such as the direction the turtle is heading or its position.

Every method call is followed by parentheses. The parentheses () after method names are there in case you need to give the method parameters (data) to do its job, which we will see in the next lesson. You must always include the parentheses after the method name.

Note

object.method(); is used to call an object’s method.

exercise Check Your Understanding: Mixed-up Code

The following code uses a turtle to draw the digital number 7, but the lines are mixed up. Drag the code blocks to the right and put them in the correct order to first draw the line going up (towards the top of the page) and then turn and draw a line to the left to make a 7. Remember that the turtle is facing the top of the page when it is first created. Click on the “Check Me” button to check your solution.

        public class DrawL
{
---
    public static void main(String[] args)
    {
---
        World habitat = new World(300,300);
---
        Turtle yertle = new Turtle(habitat);
---
        yertle.forward();
---
        yertle.turnLeft();
        yertle.forward();
---
        habitat.show(true);
---
    } // end main
---
} // end class
        

coding exercise Coding Exercise:

After you put the mixed up code in order above, type in the same code below to make the turtle draw a 7. (If the code below does not work for you, you can also use the Turtle code at this repl.it link (refresh page after forking and if it gets stuck) or download the files here to use in your own IDE.)

coding exercise Coding Exercise:

Can you make yertle draw the digital number 8, as 2 squares on top of each other?

2.3.1. Procedural Abstraction

Procedural abstraction allows a programmer to use a method and not worry about the details of how it exactly works. For example, we know that if we hit the brakes, the car will stop, and we can still use the brakes even if we don’t really know how they work.

You will learn to write your own methods in Unit 5. In this unit, you should be able to use methods already written for you and figure out what they do. When we use methods for a class in a library, we can look up the method signature (or method header), which is the method name followed by a parameter list, in its documentation. For example, here is a Student class with a method signature public void print() which has an empty parameter list with no parameters. Methods are defined after the instance variables (attributes) and constructors in a class.

A Student class showing instance variables, constructors, and methods

Figure 1: A Student class showing instance variables, constructors, and methods

exercise Check Your Understanding

The Java visualization below shows how a song can be divided up into methods. Click on the next button below the code to step through the code. Execution in Java always begins in the main method in the current class. Then, the flow of control skips from method to method as they are called. The Song’s print method calls the chorus() and animal() methods to help it print out the whole song.

When you call the chorus() method, it skips to the chorus code, executes and prints out the chorus, and then returns back to the method that called it.

Activity: CodeLens 2.3.1.2 (songviz1)

Methods inside the same class can call each other using just methodName(), but to call non-static methods in another class or from a main method, you must first create an object of that class and then call its methods using object.methodName().

Calling Methods

Figure 2: Calling non-static methods from main() or from other methods inside the same class.

exercise Check your understanding

Try this visualization to see this code in action.

Note

method(); is used to call a method within the same class, but object.method(); is necessary if you are calling the method from the main method or from a different class.

Before you call a method from main or from outside of the current class, you must make sure that you have created and initialized an object. Remember that if you just declare an object reference without setting it to refer to a new object the value will be null meaning that it doesn’t reference an object. If you call a method on a variable whose value is null, you will get a NullPointerException error, where a pointer is another name for a reference.

2.3.2. groupwork Programming Challenge : Draw a Letter

Working in pairs, use the area below (or the repl.it link) to use a turtle to draw a simple block-style letter or number that uses just straight lines (no curves or diagonals). It could be one of your initials or a number from today’s date.

It may help to act out the code pretending you are the turtle. Remember that which way you turn depends on which direction you are facing, and the turtle begins facing north (towards the top of the page).

Here are some simple turtle methods that you can use:

  • forward()

  • turnLeft()

  • turnRight()

  • backward()

  • penUp()

  • penDown()

You may notice that it is challenging to have your turtle draw with these simple methods. In the next lesson, we will use more complex Turtle methods where you can indicate how many steps to take or what angle to turn that will make drawing a lot easier!

Create a drawing of a simple letter or number that uses just straight lines (no curves or diagonals). It could be an initial in your name or a number from today’s date.

2.3.3. Summary

  • Methods are a set of instructions that define the behaviors for all objects of the class.

  • Use dot notation to execute an object’s method. This is the object’s name followed by the dot (.) operator followed by the method name and parentheses: object.method();

  • A method signature is the method name followed by the parameter list which gives the type and name for each parameter. Note that methods do not have to take any parameters, but you still need the parentheses after the method name.

  • Procedural abstraction allows a programmer to use a method by knowing in general what it does without knowing what lines of code execute. This is how we can drive a car without knowing how the brakes work.

  • A method or constructor call interrupts the sequential execution of statements, causing the program to first execute the statements in the method or constructor before continuing. Once the last statement in the method or constructor has executed or a return statement is executed, the flow of control is returned to the point immediately following the method or constructor call.

  • A NullPointerException will happen if you try to call an object method on an object variable whose value is null. This usually means that you forgot to create the object using the new operator followed by the class name and parentheses.

  • An object method or non-static method is one that must be called on an object of a class. It usually works with the object’s attributes.

  • A static method or class method method is one that doesn’t need to be called on an object of a class.

2.3.4. AP Practice

You have attempted of activities on this page