Outline of Class 8: Variables and Subroutines

Held: Thursday, February 12, 1998

Administrivia

• Our Convocation speaker, Prof. Doris Schattschneider, will be having lunch with students at Grinnell House on Thursday , February 26. Please send a note to Prof. Herman if you would like to join her for lunch.
• Because of intent and time restrictions, we won't be covering anything close to all of Java. In general, I'll work in a demand-driven mode: if there's something you want to know how to do, ask me and I'll come up with an answer. This is part of your problem-solving ability: identifying the things you know (or at least learned about) and those that you don't know.
• We decided to end this class early, as everyone seemed somewhat exhausted.

Variables

• As you may have noted in the programs and algorithms you've seen and written, it is useful to name values and objects.
• The names we use in computer programs are called variables.
• In Java, we must assign types to these variables. A type describes the roles the variable can play and/or the values it can take on.
  • It wouldn't make sense to add two people.
  • It wouldn't make sense to have a number write something on the board.
• In Java, you must declare all the variables you will use in a program. A declaration requires a type and a name.
• You may assign values to variables using the assignment operator (the equals sign).

Subroutines

• As you develop more complex algorithms / programs, you will find it useful and necessary to write subroutines that describe how to do one component of your algorithm.
  • In finding the cheapest route, we needed to list all routes
  • In finding the cheapest route, we needed to find the minimum in a list
  • We might use our Centrigade to Fahrenheit conversion as part of a more general computation.
• In Java, such subroutines are called methods and are usually attached to objects.
• Most methods are used to compute a value. When describing methods, we must indicate the type of the value. For example, a minimum method might compute a number.
• You use the command return to return a value from a method.
• There are four or so key parts to the definition of a method:
  • The name of the method.
  • The type of the return value.
  • The parameters to the method (the information that serves as input to the method).
  • The body of the method. These are the instructions for doing whatever the method does.
• The structure of a method definition is as follows ...

  public type name(parameters)
  {
    body
  } // name
  

• Each parameter has a type and a name.
• For example, a method that computes the square of a number might resemble

  public double square(double param)
  {
    return param * param;
  } // square
  

• You may recall that methods are performed by objects. In order to use the methods you define, you will need to create an object.
• In order to create an object, you must indicate what type it has and use new.
• For example,

  public class MyProgram
  {
    public static void main(String[] args)
    {
      ...
      MyProgram compute = new MyProgram();
      ...
      out.println(5 + " squared is " + compute.square(5));
    } // main
  } // MyProgram
  

See squares, revisited and temperatures in computing exercise 5 for an example and some exercises.