Lecture Notes 08: Methods, encapsulation, and scope

Objectives

By the end of this module, for simple HelloWorld-like programs, you will be able to:

• Understand why methods exist and know how to write one yourself
• Understand return statements
• Understand method arguments

Why are functions useful in computation?

• Sharing complex libraries, such as SSL, deep learning, etc.
• Allowing groups of people to work on the same codebase in parallel, to make large systems
• Other examples?

Writing and using Methods that use Input and/or return Output

In the previous module with Strings, we practiced using methods that resolve (or return) something and that might need input parameters to run.
In this segment we'll practice writing and invoking such methods.

First, two definitions:

• Parameters are those specified in the Method Declaration as being required as input for the method to function
.
• Arguments are the actual explicit values or variables whose values we want to pass to the method by putting the inside the parenthesis when doing the method Invocation.

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  public class IOMethods1
  {
    public static void main(String args[])
    {
      int num1 = 5;
      int num2 = 3;
      // Invoke the myOperation method and save result in the num3 variable
      int num3 = myOperation(num1, num2);  // myOperation(num1, num2) resolves into the resulting value
      System.out.println ("myOperation on " + num1 + " and " + num2 +  " is: " + num3);
    }

    public static int myOperation(int localNum1, int localNum2)
    {
      int localNum3 = (localNum1 * localNum2) - (localNum1 - localNum2);
      // this causes the method invocation to resolve into localNum3
      return localNum3;
    }
  }
  

Note the following:

• Since the method does not know how it will be invoked, it has default names for the input parameters. For example, it could be called like this:
  • int z = myOperation (8,2);
  
  And it has to place the arguments 8 and 2 somewhere!
• The parameters that the method myOperation takes are two ints that will be locally called localNum1 and localNum2 (but that you could rename to whatever you want as long as you use them properly inside the method).
• When we invoke the method, the argument values are copied from the invocation arguments to the method's input parameters.
  That is, from num1 to localNum1, and from num2 to localNum2.
• The local method variables are only "alive" during the execution of the method. Once the method returns, those variables are no longer in existence.

localNum1 = localNum1 + 99;
localNum2 = localNum2 + 99;

then, inside the main, add the following statement:

System.out.println ("num1 after modifying localNum1 is: " + num1 );
System.out.println ("num2 after modifying localNum2 is: " + num2 );

What do you print? why?

Activity 3: Write a method that takes no parameters but returns a value. Then invoke that method from main and print the return value.

Activity 4: Run the code below where the name of at least one variable is the same between the two methods. What happens?

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  public class IOMethods1
  {
    public static void main(String args[])
    {
      int num1 = 5;
      int num2 = 3;
      // Invoke the myOperation method and save result in the num3 variable
      int num3 = myOperation(num1, num2);  // myOperation(num1, num2) resolves into the resulting value
      System.out.println ("myOperation on " + num1 + " and " + num2 +  " is: " + num3);
    }

    public static int myOperation(int num1, int localNum2)
    {
      num1 = num1 + 1;
      int num2 = (num1 * localNum2) - (num1 - localNum2);
      return num2;
    }
  }
  

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  public class IOMethods1
  {
     // ignore the static keyword for now (we'll learn more about static next class)
     static int num1 = 11;
     static int num4 = 3;

    public static void main(String args[])
    {
      int num2 = 3;
      // Invoke the myOperation method and save result in the num3 variable
      int num3 = myOperation(num1, num2);  // myOperation(num1, num2) resolves into the resulting value
      System.out.println ("myOperation on " + num1 + " and " + num2 +  " is: " + num3);
      num4++;
    }

    public static int myOperation(int num1, int localNum2)
    {
      num1 = num1 + 1;
      int num2 = (num1 * localNum2) - (num1 - localNum2);
      int num4 = 8;
      return num2;
    }
  }