Computer Science 15-100, Spring 2009
Class Notes:  Getting Started

1. Hello World
2. Basic Console Output
   1. print vs println
   2. Printing Numbers (not Strings)
   3. Printing Numbers and Strings
3. Comments
   1. End-of-Line Comments
   2. Block Comments
4. Errors
   1. Syntax Errors (Compile-Time Errors)
   2. Runtime Errors ("Crash")
   3. Logical Errors (Compiles and Runs, but is Wrong!)
5. Variables and Expressions
6. Basic Console Input

Getting Started

1. Hello World
   
   class HelloWorld {
     public static void main(String[] args) {
       System.out.println("Hello World!");
     }
   }
    
2. Basic Console Output
    
   1. print vs println

      class MyCode {
        public static void main(String[] args) {
          System.out.print("This is");
          System.out.print("a test");
          System.out.println("that will run, albeit");
          System.out.println();
          System.out.println("with some problems...");
        }
      }

   2. Printing Numbers (not Strings)

      class MyCode {
        public static void main(String[] args) {
          System.out.println(1);
          System.out.println(1 + 2);
        }
      }

   3. Printing Numbers and Strings

      class MyCode {
        public static void main(String[] args) {
          System.out.println("1 + 2 = " + (1 + 2));
          System.out.println("1 + 2 = " + 1 + 2);  // surprised?
        }
      }

3. Comments
    
   1. End-Of-Line Comments

      class MyCode {
        public static void main(String[] args) {
          System.out.println("Howdy!"); // this is an end-of-line comment
                                        // (but not a very good one!)
      
          // The next line is commented out, so it is ignored!
          // System.out.println("This will be ignored");
      
          System.out.println("And howdy on line #2");
        }
      }

   2. Block Comments

      class MyCode {
        public static void main(String[] args) {
          System.out.println("Howdy!");
          /* The preceding comments end at the end of the line, but this
             is a block comment, started with a slash-star, which will continue
             across new lines until a star-slash (on the next line):
          */
          System.out.println("And howdy on line #2");  /* another block comment! */
        }
      }

4. Errors
    
   1. Syntax Errors (Compile-Time Errors)

      class MyCode {
        public static void main(String[] args) {
          System.out.println("Uh oh!); // ERROR!  missing close-quote
        }
      }
      
      Compiler output:
         MyCode.java:3: unclosed string literal
         System.out.println("Uh oh!); // ERROR! missing close-quote

   2. Runtime Errors ("Crash")

      class MyCode {
        public static void main(String[] args) {
          System.out.println(1/0); // ERROR!  Division by zero!
        }
      }
      
      Runtime (Console) output:
         Exception in thread "main" java.lang.ArithmeticException: / by zero
         at MyCode.main(Foo.java:3)

   3. Logical Errors (Compiles and Runs, but is Wrong!)

      class MyCode {
        public static void main(String[] args) {
          System.out.println("2+2=5"); // ERROR!  Untrue!!!
        }
      }
      
      Runtime (Console) output:
         2+2=5

5. Variables and Expressions
   
   class MyCode {
     public static void main(String[] args) {
       int x = 5;
       int y = 3;
       int sum = x + y;
       System.out.print(x);
       System.out.print("+");
       System.out.print(y);
       System.out.print("=");
       System.out.println(sum);
     }
   }
   
   Once again, but more concisely:
   
   class MyCode {
     public static void main(String[] args) {
       int x = 5;
       int y = 3;
       int sum = x + y;
       System.out.println(x + "+" + y + "=" + sum);
     }
   }
    
6. Basic Console Input
   
   import java.util.Scanner;
   class MyCode {
     public static void main(String[] args) {
       Scanner scanner = new Scanner(System.in);
       int x = scanner.nextInt();
       int y = scanner.nextInt();
       int sum = x + y;
       System.out.println(x + "+" + y + "=" + sum);
     }
   }
   
   Once again, but with nicer prompts and more operators:
   
   import java.util.Scanner;
   class MyCode {
     public static void main(String[] args) {
       Scanner scanner = new Scanner(System.in);
       System.out.print("Enter an integer: ");
       int x = scanner.nextInt();
       System.out.print("Enter another integer: ");
       int y = scanner.nextInt();
   
       int sum = x + y;
       System.out.println(x + "+" + y + "=" + sum);
   
       int difference = x - y;
       System.out.println(x + "-" + y + "=" + difference);
   
       int product = x * y;
       System.out.println(x + "*" + y + "=" + product);
   
       int quotient = x / y;
       System.out.println(x + "/" + y + "=" + quotient);
   
       int remainder = x % y;
       System.out.println(x + "%" + y + "=" + remainder);
     }
   }
    

carpe diem   -   carpe diem   -   carpe diem   -   carpe diem   -   carpe diem   -   carpe diem   -   carpe diem   -   carpe diem   -   carpe diem