15.3: Encapsulation and Generalization
- Page ID
- 15251
In Section 6.2, we presented a way of writing programs called incremental development. In this section we present another program development process called “encapsulation and generalization”. The steps are:
- Write a few lines of code in
main
or another method, and test them. - When they are working, wrap them in a new method, and test again.
- If it’s appropriate, replace literal values with variables and parameters.
The second step is called encapsulation; the third step is generalization.
To demonstrate this process, we’ll develop methods that display multiplication tables. Here is a loop that displays the multiples of two, all on one line:
int i = 1; while (i <= 6) { System.out.printf("%4d", 2 * i); i = i + 1; } System.out.println();
The first line initializes a variable named i
, which is going to act as a loop variable: as the loop executes, the value of i
increases from 1 to 6; when i
is 7, the loop terminates.
Each time through the loop, we display the value 2 * i
padded with spaces so it’s four characters wide. Since we use System.out.printf
, the output appears on a single line.
After the loop, we call println
to print a newline and complete the line. Remember that in some environments, none of the output is displayed until the line is complete.
The output of the code so far is:
2 4 6 8 10 12
The next step is to “encapsulate” this code in a new method. Here’s what it looks like:
public static void printRow() { int i = 1; while (i <= 6) { System.out.printf("%4d", 2 * i); i = i + 1; } System.out.println(); }
Next we replace the constant value, 2
, with a parameter, n
. This step is called “generalization” because it makes the method more general (less specific).
public static void printRow(int n) { int i = 1; while (i <= 6) { System.out.printf("%4d", n * i); i = i + 1; } System.out.println(); }
Invoking this method with the argument 2 yields the same output as before. With the argument 3, the output is:
3 6 9 12 15 18
And with argument 4, the output is:
4 8 12 16 20 24
By now you can probably guess how we are going to display a multiplication table: we’ll invoke printRow
repeatedly with different arguments. In fact, we’ll use another loop to iterate through the rows.
int i = 1; while (i <= 6) { printRow(i); i = i + 1; }
And the output looks like this:
1 2 3 4 5 6 2 4 6 8 10 12 3 6 9 12 15 18 4 8 12 16 20 24 5 10 15 20 25 30 6 12 18 24 30 36
The format specifier \%4d
in printRow
causes the output to align vertically, regardless of whether the numbers are one or two digits.
Finally, we encapsulate the second loop in a method:
public static void printTable() { int i = 1; while (i <= 6) { printRow(i); i = i + 1; } }
One of the challenges of programming, especially for beginners, is figuring out how to divide up a program into methods. The process of encapsulation and generalization allows you to design as you go along.