CSCI A201/A597 Lecture Notes 2 Spring 2000

We start from the first program:

public class Hello {
  public static void main(String[] args) {
    System.out.println("Hello, and welcome to A201!"); 
  } 
}

System.out.println("Hello, and " + "welcome to A201!");

System.out.println("Hello, and " + "we" + "lcome to A201!");

System.out.println("Hello, " + "and welcome" + " to " + "A201!");

We identify a pattern that we need to be using for now without completely understanding it:

public class _______ {
  public static void main(String[] args) {
    _______________;
    _______________; 
    ...
    _______________; 
  } 
}

I will not skip classes again!
I will not skip classes again!
I will not skip classes again!
I will not skip classes again!
I will not skip classes again!
I will not skip classes again!

public class Penitence {
  public static void main(String[] args) {
    System.out.println("I will not skip classes again!"); 
    System.out.println("I will not skip classes again!"); 
    System.out.println("I will not skip classes again!"); 
    System.out.println("I will not skip classes again!"); 
    System.out.println("I will not skip classes again!"); 
    System.out.println("I will not skip classes again!"); 
  } 
}

Individual work:

1. Write a program that produces the following output:

       4
      4
     4
    4   4
   44444444
        4
        4

(The program should produce a big 4).

2. Also write a program that produces the following output:

Please       your 
       bring 
  own        floppies!

End of individual work.

Now we start looking at the structure and components of a Java program. Java programs are composed of classes. The action happens in methods. The methods are composed of statements, which are made up of expressions. We start in the simplest possible way.

We know what an integer number is and we can give examples of such a number. 5, or 3, or 19 are such numbers. For all practical purposes we refer to integer numbers in Java by the keyword int: 5 is an int, 3 is an int, and so is 19. All these are constants, integer constants.

From math we know that there are a few operations defined on the set of ints:

• addition (+)
• subtraction (-)
• comparison (<)

If we write

5 + 3

Of course, it's important where (the context, that is, in which) you can write this expression. We'll look at that in a second. Meanwhile, by the same token, if we write

12 - 103

Expressions like these:

5 < 3

5 < 13

true and false are booleans.

boolean is a type just like int.

How many boolean values do you think there are? How many ints?

When it evaluates an expression Java gives us the resulting value, and if we don't store it anywhere it simply gets lost. We can store results of expressions in named locations. The names are called identifiers, and resemble notation in mathematics. The locations where we store the result of an expression must be of the same type as the result that needs to be stored.

So, for example, we can store the result of

1 + 2

result = 1 + 2

If we write several statements one after another we need to separate them by semicolons (;) as we shall promptly see in the example below.

Names of locations can be used on the right hand side of an assignment statement, in which case they will be replaced (when the expression is evaluated) by their current value.

Here are some examples:

a = 2 + 4;
b = a + 3;

Individual work:

What's the value of i at the end of the following sequence of statements:

i = -3;
i = i + 1;
i = i + 1;
i = i + 1; 

End of individual work section.

The type of the named locations that we plan to use must be declared ahead of time, before they are used. So in the examples above we need to declare the type of result, a, b, i (to be int) before they are first used.

Declarations have the following format: type followed by name of variable (or named location) followed by semi-colon.

Methods are composed of sequences of declarations and executable statements. main is a method. To start with we will be working with main for a while. We will later write many other methods.

Let's write a program that computes and prints the sum of the first 10 integers, from 1 to 10.

Here's a solution:

public class First {
  public static void main(String[] args) {
    int result; 
    result = 1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 + 10; 
    System.out.println("Sum of first 10 integers is: " + result);
  } 
}

while loops

Java has a language construct that looks like this:

while (condition) {
  statements
}