A Touch of Java

Java programs consist of many statements:

• assignments statements
• conditional statements
• iterative statements (loops)

Now, we examine assignment statements.

Variables

Java programs abstract the concept of memory addresses with variables. A variable reserves a segment of memory for a given type of data.

We'll use three types, corresponding to the ones we saw earlier.

	 char     character
	 int      integer
	 double   floating-point number

Variable declarations

Before using variables, we must first declare them to reserve memory and to assign type.

  int variable_name0;

The int gives the type of variable. variable_name0 is the name of the variable (consisting of letters, digits, and underscores, but beginning with a letter). The semicolon ends the statement.

Other examples:

  char character;
  int k;
  double root_n;

Assignment statements

To give a value to a variable, we use an assignment statement.

  k = 2;

Here k is the variable to be changed. The value to give it is 2. And we see yet another statement-terminating semicolon.

The right-hand side can be any expression. Some expressions:

   4        9 + k         30 / 4          12 - 3 * 4
   k        3 * k        30.0 / 4        (12 - 3) * 4
  -k        6 - 9         20 % 4

So we can also write:

  k = 9 * k * k + 6;

Arrays

We reserve long blocks of memory with arrays. To declare an array of 30 characters, we write

  char[] arr = new char[30];

We use an index in brackets to specify array elements in assignments.

  arr[0] = 'S';
  arr[k / 2] = '0';

Note: Arrays are indexed from 0. So arr[30] doesn't exist.

Multi-dimensional arrays

Arrays of arrays, too, are OK, as are arrays of arrays of arrays (and deeper). An array of arrays is a two-dimensional array.

  double[][] m = new double[4][5];

Access is as before.

  m[k - 39][4] = k / 3 / 2;
  m[0][0] = m[3][4] / k;