Decomposition means breaking a problem into pieces small enough to be obviously correct, then assembling the pieces. It is the difference between a program you can reason about and one you can only hope about. The target is simple: each method should do one thing, and its name should say what that thing is. If you cannot describe a method in a single short sentence without using the word "and", it is doing too much.
- You need a comment to explain what a section of the method does — that comment is the name of a method waiting to be written.
- The name contains "and":
readAndValidateAndAverageis three methods wearing a trench coat. - The body is long enough that you have to scroll to see all of it.
- You cannot test one part of it without running all of it.
- The same block of code also appears somewhere else in the program.
Top-down design means writing the outline before the details. Start with the whole job and split it into a handful of major steps. Ask of each step whether you could write it in a few clear lines; if not, split it again. You stop when every piece is small enough to write directly. The payoff is that main ends up reading like a summary of the program, with the detail tucked into the methods that need it.
import java.util.Scanner;
public class MarkReport {
public static void main(String[] args) {
Scanner input = new Scanner(System.in);
int[] marks = new int[5];
System.out.println("=== Mark Report ===");
for (int i = 0; i < marks.length; i++) {
System.out.print("Enter mark " + (i + 1) + ": ");
int value = input.nextInt();
while (value < 0 || value > 100) {
System.out.print("Marks must be 0 to 100. Try again: ");
value = input.nextInt();
}
marks[i] = value;
}
int total = 0;
for (int i = 0; i < marks.length; i++) {
total = total + marks[i];
}
double average = (double) total / marks.length;
int highest = marks[0];
for (int i = 1; i < marks.length; i++) {
if (marks[i] > highest) {
highest = marks[i];
}
}
int lowest = marks[0];
for (int i = 1; i < marks.length; i++) {
if (marks[i] < lowest) {
lowest = marks[i];
}
}
String level;
if (average >= 80) {
level = "Level 4";
} else if (average >= 70) {
level = "Level 3";
} else if (average >= 60) {
level = "Level 2";
} else if (average >= 50) {
level = "Level 1";
} else {
level = "Below Level 1";
}
System.out.println("--- Results ---");
System.out.println("Average: " + average);
System.out.println("Highest: " + highest);
System.out.println("Lowest: " + lowest);
System.out.println("Level: " + level);
}
}Step 1 — find the separate jobs
Read the long method and mark where one idea stops and the next begins. The blank lines are usually a good first guess, because you already put them there for exactly that reason; section comments are an even stronger clue. Ask as well which parts you could test on their own, since anything checkable with a handful of values is a natural method. In this program there are six distinct jobs.
- Read one mark and refuse anything outside 0 to 100.
- Read a whole set of marks into an array.
- Work out the average of an array.
- Find the highest value in an array.
- Find the lowest value in an array.
- Turn an average into an Ontario achievement level.
Step 2 — name each job, then write its signature
| Job | Signature | Why that return type |
|---|---|---|
| Read one valid mark | static int readValidMark(int number) | It produces a mark, so it returns one |
| Read all the marks | static int[] readAllMarks(int howMany) | It produces a filled array |
| Average an array | static double average(int[] marks) | An average is rarely a whole number |
| Find the highest | static int highest(int[] marks) | It produces one of the marks |
| Find the lowest | static int lowest(int[] marks) | Same shape, opposite comparison |
| Average to level | static String levelFor(double mark) | The level is text, not a number |
| Show the report | static void printReport(int[] marks) | It displays; it produces nothing |
Name methods with verbs, variables with nouns
A method does something, so its name should be an action: readValidMark, printReport, levelFor. A variable holds something, so its name should be a thing: total, marks, average. Methods that answer a yes or no question conventionally start with is or has, such as isValid or hasPassed, which makes them read naturally inside an if.
Step 3 — the decomposed program
import java.util.Scanner;
public class MarkReport {
static Scanner input = new Scanner(System.in);
public static int readValidMark(int number) {
System.out.print("Enter mark " + number + ": ");
int value = input.nextInt();
while (value < 0 || value > 100) {
System.out.print("Marks must be 0 to 100. Try again: ");
value = input.nextInt();
}
return value;
}
public static int[] readAllMarks(int howMany) {
int[] marks = new int[howMany];
for (int i = 0; i < marks.length; i++) {
marks[i] = readValidMark(i + 1);
}
return marks;
}
public static double average(int[] marks) {
int total = 0;
for (int i = 0; i < marks.length; i++) {
total = total + marks[i];
}
return (double) total / marks.length;
}
public static int highest(int[] marks) {
int best = marks[0];
for (int i = 1; i < marks.length; i++) {
if (marks[i] > best) {
best = marks[i];
}
}
return best;
}
public static int lowest(int[] marks) {
int worst = marks[0];
for (int i = 1; i < marks.length; i++) {
if (marks[i] < worst) {
worst = marks[i];
}
}
return worst;
}
public static String levelFor(double mark) {
if (mark >= 80) {
return "Level 4";
} else if (mark >= 70) {
return "Level 3";
} else if (mark >= 60) {
return "Level 2";
} else if (mark >= 50) {
return "Level 1";
}
return "Below Level 1";
}
public static void printReport(int[] marks) {
double avg = average(marks);
System.out.println("--- Results ---");
System.out.println("Average: " + avg);
System.out.println("Highest: " + highest(marks));
System.out.println("Lowest: " + lowest(marks));
System.out.println("Level: " + levelFor(avg));
}
public static void main(String[] args) {
System.out.println("=== Mark Report ===");
int[] marks = readAllMarks(5);
printReport(marks);
}
}What decomposition actually bought
levelFor can now be tested on its own with the values 85, 79, 50 and 12, without typing five marks in first. readValidMark is reused for every mark instead of being written once and trusted. Changing the report layout touches only printReport. And main states the whole program in three lines, so a reader knows what it does before reading any detail.
// Stubs: right signature, placeholder body, compiles and runs today
public static double average(int[] marks) {
return 0.0; // TODO: real calculation
}
public static String levelFor(double mark) {
return "Level ?"; // TODO: real thresholds
}
public static void printReport(int[] marks) {
System.out.println("printReport called"); // TODO: real layout
}