Computer Science 15-100, Spring 2009
Class Notes:  Data and Expressions

1. int values
   1. Declare a variable and use it
   2. Use a variable without declaring it
   3. Declaring and assigning on different lines
   4. Assigning and re-assigning
   5. Using before assigning a value
   6. Using two variables
   7. Declaring two variables on one line
   8. Using two variables in an expression
   9. Operator Precedence  (just as you'd expect)
   10. Overflow
   11. Integer division
   12. The Modulus or Remainder operator (%)
   13. Increment and Decrement Operators (as statements)
   14. Assignment Operators
   15. Some int Math methods (abs, min, max)
2. boolean values
   1. Boolean literals (true and false)
   2. Equality operators
   3. Relational operators
   4. Boolean operators (and, or, not)
   5. Short-circuit evaluation
3. double values
   1. Print a floating-point value
   2. Use "double" variables
   3. Integer vs Floating-point division
   4. Approximate values of floating-point numbers
   5. "almost equal" comparison of doubles
   6. Some double Math methods (abs, min, max, pow, sqrt)
4. String values
   1. String literals
   2. Getting the length of a String
   3. null versus empty Strings
   4. String concatenation
   5. Escape Sequences
5. char values
   1. char literals
   2. char comparisons
6. Type conversion

Data and Expressions

1. int values
    
   1. Declare a variable and use it

      class MyCode {
        public static void main(String[] args) {
          int x = 3;
          System.out.println(x);
        }
      }

      Same, but with a nicer UI

      class MyCode {
        public static void main(String[] args) {
          int x = 3;
          System.out.println("x = " + x);
        }
      }

   2. Use a variable without declaring it

      class MyCode {
        public static void main(String[] args) {
          System.out.println("x = " + x);  // ERROR!  No such variable as x
        }
      }

   3. Declaring and assigning on different lines

      class MyCode {
        public static void main(String[] args) {
          int x;  // declare the variable
          x = 5;  // initialize it (assign it its first value)
          System.out.println("x = " + x);
        }
      }

   4. Assigning and re-assigning

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

   5. Using before assigning a value

      class MyCode {
        public static void main(String[] args) {
          int x;  // declared, but not assigned a value
          System.out.println("x = " + x);  // ERROR!  x is not initialized
        }
      }

   6. Using two variables

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

   7. Declaring two variables on one line

      class MyCode {
        public static void main(String[] args) {
          int x = 1, y = 2;
          System.out.println("x = " + x);
          System.out.println("y = " + y);
        }
      }

   8. Using two variables in an expression

      class MyCode {
        public static void main(String[] args) {
          int x = 1, y = 2;
          System.out.println("x = " + x);
          System.out.println("y = " + y);
          System.out.println("x + y = " + (x + y));
        }
      }

      Same, but with a nicer UI

      class MyCode {
        public static void main(String[] args) {
          int x = 1, y = 2;
          System.out.println("x = " + x);
          System.out.println("y = " + y);
          System.out.println("x + y = "
                             + x + " + " + y
                             + " = " + (x + y));
        }
      }

   9. Operator Precedence  (just as you'd expect)

      class MyCode {
        public static void main(String[] args) {
          System.out.println(2+3*4); // prints 14, not 20
        }
      }

   10. Overflow

       class MyCode {
         public static void main(String[] args) {
           int x = 1*1000*1000*1000;  // 1 billion
           int y = 2*1000*1000*1000;  // 2 billion
           System.out.println(x + y); // Prints -1294967296
         }
       }

   11. Integer division

       class MyCode {
         public static void main(String[] args) {
            System.out.println("20/3 = " + (20/3));
            System.out.println(" 6/3 = " + ( 6/3));
            System.out.println(" 5/3 = " + ( 5/3));
            System.out.println(" 2/3 = " + ( 2/3));
         }
       }

   12. The Modulus or Remainder operator (%)

       class MyCode {
         public static void main(String[] args) {
            System.out.println(" 7%3 = " + (7%3));
            System.out.println(" 6%3 = " + (6%3));
            System.out.println(" 5%3 = " + (5%3));
            System.out.println(" 3%3 = " + (3%3));
            System.out.println(" 2%3 = " + (2%3));
            System.out.println(" 0%3 = " + (0%3));
            System.out.println(" 3%0 = " + (3%0));
         }
       }

   13. Increment and Decrement Operators (as statements)

       class MyCode {
         public static void main(String[] args) {
           int x = 5;
           System.out.println(x); // 5
           x++;
           System.out.println(x); // 6
           ++x;
           System.out.println(x); // 7
           x--;
           System.out.println(x); // 6
           --x;
           System.out.println(x); // 5
         }
       }

   14. Assignment Operators

       class MyCode {
         public static void main(String[] args) {
           int x = 5;
           System.out.println(x); // 5
           x += 2;
           System.out.println(x); // 7
           x *= 2;
           System.out.println(x); // 14
           x %= 9;
           System.out.println(x); // 5
           x /= 2;
           System.out.println(x); // 2
           x -= 5;
           System.out.println(x); // -3
         }
       }

   15. Some int Math methods (abs, min, max)

       class MyCode {
         public static void main(String[] args) {
           System.out.println(Math.abs(4));   // 4
           System.out.println(Math.abs(-4));  // 4
       
           System.out.println(Math.min(3, 2)); // 2
           System.out.println(Math.min(2, 3)); // 2
       
           System.out.println(Math.max(3, 2)); // 3
           System.out.println(Math.max(2, 3)); // 3
       
           // We can use these in expressions, too:
       
           int x = Math.min(5, -13);
           int y = Math.max(x, x/2);
           int z = Math.abs(x) + Math.abs(y);
           System.out.println(x);
           System.out.println(y);
           System.out.println(z);
         }
       }

2. boolean values
    
   1. Boolean literals ( true , false )

      class MyCode {
        public static void main(String[] args) {
          boolean b = true;
          System.out.println(b);     // prints true
          System.out.println(false); // prints false
        }
      }

   2. Equality operators ( == , != )

      class MyCode {
        public static void main(String[] args) {
          System.out.println(5 == 4);
          System.out.println(5 == 5);
          System.out.println(5 != 4);
          System.out.println(5 != 5);
        }
      }

   3. Relational operators ( < , <= , >= , > )

      class MyCode {
        public static void main(String[] args) {
          System.out.println(5 < 4);
          System.out.println(5 > 4);
          System.out.println(5 < 5);
          System.out.println(5 <= 5);
          System.out.println(5 >= 5);
          System.out.println(5 > 5);
        }
      }

   4. Boolean operators ( && , || , ! )

      class MyCode {
        public static void main(String[] args) {
          System.out.println("&& (AND)");
          System.out.println(true  && true );
          System.out.println(true  && false);
          System.out.println(false && true );
          System.out.println(false && false);
      
          System.out.println("|| (OR)");
          System.out.println(true  || true );
          System.out.println(true  || false);
          System.out.println(false || true );
          System.out.println(false || false);
      
          System.out.println("! (NOT)");
          System.out.println( !true );
          System.out.println( !false );
        }
      }

   5. Short-circuit evaluation

      class MyCode {
        public static void main(String[] args) {
           int x = 0, y = 0;
           System.out.println((y != 0) && ((x/y) != 0)); // Works!
           System.out.println(((x/y) != 0) && (y != 0)); // Crashes!
        }
      }

      Once again, using the "or" operator ( || )

      class MyCode {
        public static void main(String[] args) {
           int x = 0, y = 0;
           System.out.println((y == 0) || ((x/y) == 0)); // Works!
           System.out.println(((x/y) == 0) || (y == 0)); // Crashes!
        }
      }

3. double values
    
   1. Print a floating-point value

      class MyCode {
        public static void main(String[] args) {
          System.out.println(1.2);
        }
      }

   2. Use "double" variables

      class MyCode {
        public static void main(String[] args) {
          double x = 1.2;
          System.out.println(2*x);
        }
      }

   3. Integer vs Floating-point division

      class MyCode {
        public static void main(String[] args) {
          int i = 10;
          double d = 10.0;
          System.out.println( 10 / 3 );
          System.out.println(  i / 3 );
          System.out.println(  d / 3 );
          System.out.println( 10 / 3.0 );
          System.out.println(  i / 3.0 );
        }
      }

   4. Approximate values of floating-point numbers

      class MyCode {
        public static void main(String[] args) {
          double d1 = (29.0 / 7.0) * 7.0;
          double d2 = 29.0;
          System.out.println(d1 == d2); // false!
          System.out.println(d2 - d1);  // -3.552713678800501E-15  (tiny!)
        }
      }

   5. "almost equal" comparison of doubles

      class MyCode {
        public static void main(String[] args) {
          double d1 = (29.0 / 7.0) * 7.0;
          double d2 = 29.0;
          System.out.println(d1 == d2); // still false (of course)
      
          // now compare if their difference is very small...
          double epsilon = 0.000001;
          System.out.println(Math.abs(d2 - d1) < epsilon); // true!
        }
      }

   6. Some double Math methods (abs, min, max, pow, sqrt)

      class MyCode {
        public static void main(String[] args) {
          System.out.println(Math.abs(4.2));   // 4.2
          System.out.println(Math.abs(-4.2));  // 4.2
      
          System.out.println(Math.min(3.5, 2.5)); // 2.5
          System.out.println(Math.min(2.5, 3.5)); // 2.5
      
          System.out.println(Math.max(3.5, 2.5)); // 3.5
          System.out.println(Math.max(2.5, 3.5)); // 3.5
      
          // We can use these in expressions, too:
      
          double x = Math.min(5, -13);
          double y = Math.max(x, x/2);
          double z = Math.abs(x) + Math.abs(y);
          System.out.println(x);
          System.out.println(y);
          System.out.println(z);
        }
      }

      Some Math methods (like pow and sqrt) do not have int versions:

      class MyCode {
        public static void main(String[] args) {
          System.out.println(Math.pow(2, 3));   // 8.0 (and not int 8), which is 2^3
          System.out.println(Math.sqrt(9));     // 3.0 which is the square root of 9
          System.out.println(Math.pow(9, 0.5)); // ditto (why?)
        }
      }

4. String values
    
   1. String literals

      class MyCode {
        public static void main(String[] args) {
          String s = "A"; // use double-quotes, not single-quotes
          System.out.println(s);
      
          s = "";         // Empty strings (of zero length) are allowed!
          System.out.println(s);
      
          s = 'b';        // ERROR! Must use double-quotes!
          System.out.println(s);
       }
      }

   2. Getting the length of a String

      class MyCode {
        public static void main(String[] args) {
          String s = "a";
          System.out.println(s.length());
      
          s = "ab";
          System.out.println(s.length());
      
          s = "";
          System.out.println(s.length());
          System.out.println(s.length() == 0);
       }
      }

   3. null versus empty Strings

      class MyCode {
        public static void main(String[] args) {
          String s = "";  // empty string
          System.out.println(s == null);
          System.out.println(s.length());
      
          s = null;       // null string
          System.out.println(s == null);
          System.out.println(s.length()); // Runtime Error!
        }
      }

   4. String concatenation

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

   5. Escape Sequences

      class MyCode {
        public static void main(String[] args) {
          System.out.println("Double-quote: \"");
          System.out.println("Backslash: \\");
          System.out.println("Newline (in brackets): [\n]");
          System.out.println("Tab (in brackets): [\t]");
      
          System.out.println("These items are tab-delimited, 3-per-line:");
          System.out.println("abc\tdef\tg\nhi\tj\\\tk\n---");
        }
      }

5. char values
    
   1. char literals

      class MyCode {
        public static void main(String[] args) {
          char c = 'A'; // use single-quotes, not double-quotes
          System.out.println(c);
      
          c = '\\';     // Escape sequences work here, too!
          System.out.println(c);
      
          c = "b";      // ERROR! Must use single-quotes!
          System.out.println(c);
      
          c = 'ab';     // ERROR! Only one character allowed!
          System.out.println(c);
      
          c = '';       // ERROR! No empty char literals allowed!
          System.out.println(c);
      
          c = null;     // ERROR! No null char allowed!
          System.out.println(c);
        }
      }

   2. char comparisons

      class MyCode {
        public static void main(String[] args) {
          System.out.println('D' >= 'A');
          System.out.println('D' <= 'Z');
          System.out.println('D' >= 'a');
          System.out.println('D' <= 'z');
      
          System.out.println("Testing for uppercase:");
          char c = 'D';
          System.out.println((c >= 'A') && (c <= 'Z'));
          c = 'd'; 
          System.out.println((c >= 'A') && (c <= 'Z'));
        }
      }

6. Type conversion
   
   Quick-reference table (see details below):
    

   	to String	to int	to double	to char	to boolean
   from String	n/a	// use "parse" method int i =   Integer.parseInt(s);	// use "parse" method double d =   Double.parseDouble(s);	// use "charAt" method char c = s.charAt(0);	// use "parse" method boolean b =   Boolean.parseBoolean(s);
   from int	// String concatenation String s = "" + i;	n/a	// Widening, so assign double d = i;	// Narrowing, so cast char c = (char) i;	n/a
   from double	// String concatenation String s = "" + d;	// Narrowing, so cast int i = (int) d;	n/a	n/a	n/a
   from char	// String concatenation String s = "" + c;	// Widening, so assign int i = c;	// Widening, so assign double d = c;	n/a	n/a
   from boolean	// String concatenation String s = "" + b;	n/a	n/a	n/a	n/a

   
   Same table, with more details:
    

   	to String	to int	to double	to char	to boolean
   from String	n/a	// From String to int: // use "parse" method String s = "42"; int i = Integer.parseInt(s); System.out.print(i); // prints 42	// From String to double: // use "parse" method String s = "42.0"; double d = Double.parseDouble(s); System.out.print(d); // prints 42.0	// From String to char: // use "charAt" method String s = "ABCD"; char c = s.charAt(0); System.out.print(c); // prints A	// From String to boolean: // use "parse" method String s = "true"; boolean b = Boolean.parseBoolean(s); System.out.print(b); // prints true
   from int	// From int to String: // String concatenation int i = 65; String s = "" + i; System.out.print(s); // prints 65 (as a String)	n/a	// From int to double: // Widening, so assign int i = 3; double d = i; System.out.print(d); // prints 3.0	// From int to char: // Narrowing, so cast int i = 65; char c = (char) i; System.out.print(c); // prints A (UNICODE 65)	n/a
   from double	// From double to String: // String concatenation double d = 3.7; String s = "" + d; System.out.print(s); // prints 3.7 (as a String)	// From double to int: // Narrowing, so cast double d = 3.7; int i = (int) d; System.out.print(i); // prints 3	n/a	n/a	n/a
   from char	// From char to String: // String concatenation char c = 'A'; String s = "" + c; System.out.print(s); // prints A (as a String)	// From char to int: // Widening, so assign char c = 'A'; int i = c; System.out.print(i); // prints 65 (UNICODE 'A')	// From char to double: // Widening, so assign char c = 'A'; double d = c; System.out.print(d); // prints 65.0 // UNICODE 'A' as a double	n/a	n/a
   from boolean	// From boolean to String: // String concatenation boolean b = true; String s = "" + b; System.out.print(s); // prints true (as a String)	n/a	n/a	n/a	n/a

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