Functions, problem-solving and a coding exercise

Methods package reusable steps

A method (often called a function in other languages) is a named block of code that performs one task. Methods let you write logic once and reuse it, and they make programs easier to read and test.

public class Greeter {
    static void greet(String name) {
        System.out.println("Welcome to DevCrest, " + name + "!");
    }

    public static void main(String[] args) {
        greet("Arjun");
        greet("Meera");
    }
}

• static means the method belongs to the class, so main can call it directly. You'll learn about objects and non-static methods later.
• void means the method returns no value; it just performs an action.
• String name is a parameter: a variable that receives the value passed in, called an argument ("Arjun").

Returning a value

A method can calculate something and hand the result back with return:

static double average(int[] numbers) {
    int sum = 0;
    for (int n : numbers) {
        sum += n;
    }
    return (double) sum / numbers.length;
}

static boolean isEven(int n) {
    return n % 2 == 0;
}

public static void main(String[] args) {
    int[] marks = {70, 85, 90};
    double avg = average(marks);
    System.out.println("Average: " + avg);       // Average: 81.66666666666667
    System.out.println(isEven(10));              // true
}

The return type (double, boolean) is written before the method name, and every path through the method must return a value of that type. return also ends the method immediately.

A problem-solving process

Experienced developers rarely start by typing code. They follow steps like these:

1. Understand the problem. Restate it in your own words. What are the inputs and the expected output?
2. Work through examples by hand. Include an edge case, such as zero, a negative number or an empty array.
3. Plan the steps in plain language (pseudocode).
4. Write the code, one small method at a time.
5. Test with your examples and fix what fails.

Walkthrough: count the vowels in a word

Understand: input is a String; output is how many of its letters are a, e, i, o or u, ignoring case.

Examples: "DevCrest" has 2 vowels (e, e). "sky" has 0. "" has 0.

Plan:

1. Start a counter at 0.
2. Look at each character in the word.
3. If the lowercase character is a vowel, add one to the counter.
4. Return the counter.

Code:

public class Vowels {
    static boolean isVowel(char c) {
        char lower = Character.toLowerCase(c);
        return lower == 'a' || lower == 'e' || lower == 'i' || lower == 'o' || lower == 'u';
    }

    static int countVowels(String word) {
        int count = 0;
        for (int i = 0; i < word.length(); i++) {
            if (isVowel(word.charAt(i))) {
                count++;
            }
        }
        return count;
    }

    public static void main(String[] args) {
        System.out.println(countVowels("DevCrest")); // 2
        System.out.println(countVowels("sky"));      // 0
        System.out.println(countVowels(""));         // 0
    }
}

Notice how splitting the work into isVowel and countVowels keeps each method short and easy to check.