In the last lesson, we learned that objects have state and behavior. We have seen how to give objects state through instance fields. Now, we’re going to learn how to create object behavior using methods. Remember our example of a Savings Account.

A diagram of a software object with fields and behavior

The state tells us what a savings account should know:

  • The balance of money available

The behavior tells us what tasks a savings account should be able to perform:

  • Depositing - increasing the amount available
  • Withdrawing - decreasing the amount available
  • Checking the balance - displaying the amount available.

Methods are repeatable, modular blocks of code used to accomplish specific tasks. We have the ability to define our own methods that will take input, do something with it, and return the kind of output we want.

Looking at the example above, recreating a savings account is no easy task. How can one program tackle such a large problem? This is where methods with their ability to accomplish smaller, specific tasks come in handy. Through method decomposition, we can use methods to break down a large problem into smaller, more manageable problems.

Methods are also reusable. Imagine we wrote a sandwich-making program that used 20 lines of code to make a single sandwich. Our program would become very long very quickly if we were making multiple sandwiches. By creating a makeSandwich() method, we can make a sandwich anytime simply by calling it.

makeSandwich() adds ingredients together to make a sandwich

In this lesson, we’ll learn how to create and call our very own methods inside of our programs.

Keep Reading: AP Computer Science A Students

If we were to share this sandwich-making program with another person, they wouldn’t have to understand how makeSandwich() worked. If we wrote our program well, all they would need to know is that if they called makeSandwich(), they would receive a sandwich. This concept is known as procedural abstraction: knowing what a method does, but not how it accomplishes it.



We have made a SavingsAccount class without using any methods beyond main() and the constructor, SavingsAccount().

Run the code to see some account behavior happen.

Look at the main method! It’s so long! There is so much repeated code! Can you imagine how messy it would look if you needed to make 10 deposits?

Throughout this lesson, we will learn how to make methods that would make checking the balance, depositing, and withdrawing all behavior that would take only one line of code.

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