Variables and Types

Variable

Data in memory is referred to by name. This is called a variable. You can think of a variable name as the human-usable equivalent of a memory address.

Type

Every variable has a type (what kind of thing it is). Eg, int, String, ...

Declaration

Every variable must be declared once before using it. This tells the compiler that you're going to use it, and what kind of a thing it is.

   String fullName;

Value

Every variable has a value. You can use the value in a calculation, or assign a new value to a variable. All variables must have a value assigned to them before they can be used.

   fullName = "Michael Mortimer Maus";

Assignment statement

An assignment statement has this form.

   variable = expression;

Where expression can be one of the following:

• A constant.

     fullName = "Brenda Begoode";

• Another variable.

     fullName = nameEntered;

• A more complex expression using method calls and operators.

     fullName = JOptionPane.showInputDialog(null, "Enter your name - or else!");

Types

• Numbers -- integer and floating-point. (Wu p87...)
• Strings
• Many others.

Numeric types

• Integers - numbers without a decimal point
  • byte, short, int, long
  • We will use int for integer variables.
  • int range is -2147483648 to +2147483647 .

  •    int age;
       age = 24;

• Floating-point - numbers with a decimal point
  • float, double
  • We will use double for floating-point variables.
  • double range is about +/- 10³⁰⁸.
  • double precision is about 15 significant digits.

  •    double temperature;
       temperature = 65.377;

More than one variable in same declaration

It's possible to declare more than one variable in the same declaration.

   double height, weight;

But I would prefer that you put them in separate declarations. It makes documenting them easier, and avoids errors in certain cases.

   double height;  // Height in centimeters.
   double weight;  // Weight in kilograms.

Declaring and initializing in one statement.

Before you can use a variable, you have to declare it and initialize it.

   String fullName;
   fullName = "nobody";

It is common to initialize a variable in the declaration.

   String fullName = "nobody";

Numeric operators

• You can perform calculations on numbers with these common operations.
  • Addition (+) 8 + 3 is 11
  • Subtraction (-) 8 - 3 is 5
  • Multiplication (*) 8 * 2 is 16
  • Division (/) 8 / 2 is 4
  • Remainder (%) 5 % 3 is 2
• Warning: integer division produces integer result. 3 / 2 is 1, not 1.5
• If either operand is floating-point, the result is floating-point. This applies to all numeric operations (not just division).
  • 3 / 2 is 1 (integer)
  • 3.0 / 2 is 1.5 (floating-point)
  • 3 / 2.0 is 1.5 (floating-point)
  • 3.0 / 2.0 is 1.5 (floating-point)

Precedence and the order of operations

• Multiplication, Division, and Remainder are performed before Addition and Subtraction

      a + b * c   is    a + (b * c)

• Use parentheses to control order of evaluation.
• Equal precedence operations done left-to-right. a+b+c is (a+b)+c

String Concatenation

• Concatenation - Creates a new string by following left operand by right operand.
• If one of the operands is String, it will convert the other to String if necessary.
• Example

      String s;
      s = "abc" + "def";    // same as s = "abcdef";
      s = "abc" + 4;        // same as s = "abc4"
      s = "abc" + (3 + 4);  // same as s = "abc7"
      s = "abc" + 3 + 4;    // same as s = "abc34"

Common String methods (Wu p61...)

String s1 = "goodbye";
String s2;
int    i;

• Note: characters in a String are numbered starting at 0.
• substring

      s2 = s1.substring(0,3);  // same as s2 = "goo"
      s2 = s1.substring(1, 5); // same as s2 = "oodb"

• length

      i = s1.length();  // same as i = 7
      s2 = s1.substring(i-1, i);  // last char in s1 (assuming i=s1.length()

• searching with indexOf

      i = s1.indexOf("b");   // same as i = 4
      i = s1.indexOf("bye"); // same as above
      i = s1.indexOf("B");   // i = -1  Uppercase not same as lowercase.

End