Methods Practice

Practice

Methods can be challenging to understand, so this chapter provides examples of various methods in action, showing how they work, where to use them, and how to write and combine multiple methods effectively.

Example 1

Let's start with a method that takes an array of type int and reverses it so that the last value becomes the first and the first becomes the last:

Main


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package com.example;

public class Main {
    static int[] flipIntArray(int[] array) {
        int[] result = new int[array.length];
        int index = 0;
        for (int i = array.length - 1; i >= 0; i--) {
            result[index] = array[i];
            index++;
        }
        return result;
    }

    static void printIntArrayToTheConsole(int[] array) {
        for (int element : array) {
            System.out.print(element + " ");
        }
        System.out.println();
    }
    
    public static void main(String[] args) {
        // do not modify the code below
        int[] firstArray = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};
        int[] secondArray = {0, 2, 4, 6, 8, 10, 12, 14, 16};
        printIntArrayToTheConsole(flipIntArray(firstArray));
        printIntArrayToTheConsole(flipIntArray(secondArray));

    }
    	// do not modify the code above
}

First, let's pay attention to the method flipIntArray on line 4, which uses a loop that counts from the end of the array and stores its values in a newly created array of the same size. There is also an index variable on line 6 that increases to store the values in the result array;
Next, you can see the method on line 14 from the previous chapter, which I use to print the array in just one line of code;
Now, let's move on to something interesting. You can see on lines 25-26 that I am passing a method as a parameter inside another method. This is because I am using a method that returns int[] inside a method that takes int[] as a parameter, thus saving the memory allocated for creating variables. We can see that our methods are working and displaying the correct values.

Example 2

Suppose we need to write a method that calculates the factorial, taking an integer variable as input.

Main


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package com.example;

// do not modify the code below this comment

public class Main {
    // method to calculate the factorial of a number
    static int calculateFactorial(int input) {
        int result = 1;
        // loop to calculate factorial by multiplying each number from 1 to input
        for (int i = 1; i <= input; i++) {
            result = result * i;
        }
        // return the final result of the factorial calculation
        return result;  
    }
    
    public static void main(String[] args) {
        // initializing the first number for factorial calculation
        int first = 5;  
        // initializing the second number for factorial calculation
        int second = 7;  
        // initializing the third number for factorial calculation
        int third = 4;  
        // calculating and printing the factorial of the first number
        System.out.println("Factorial of the first number = " + calculateFactorial(first));  
        // calculating and printing the factorial of the second number
        System.out.println("Factorial of the second number = " + calculateFactorial(second));  
        // calculating and printing the factorial of the third number
        System.out.println("Factorial of the third number = " + calculateFactorial(third));  
    }
}

In this example, we have written a method that multiplies the accumulating variable by numbers from 1 to the given number in order to calculate the factorial. It's a straightforward approach, and you can see how we use this method multiple times in the main method, which saves space and improves code readability. Next, we use our method inside System.out.println because the method returns an int value.

Example 3

We will write a method that creates an abbreviation from an array of strings.

Main


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package com.example;

// do not modify the code below this comment

public class Main {
    // method to create abbreviation by taking first letter of each word
    static String makeAbbreviation(String[] input) {
        StringBuilder result = new StringBuilder();
        // iterate through each word and append the first letter to result
        for (String word : input) {
            result.append(word.toUpperCase().charAt(0));
        }
        // return the abbreviation as a string
        return result.toString();
    }

    public static void main(String[] args) {
        // initializing arrays with words for abbreviation
        String[] firstArray = {"united", "nations"};  
        String[] secondArray = {"North", "Atlantic", "Treaty", "Organization"};  
        String[] thirdArray = {"Oh", "my", "God"};  
        // printing abbreviation for the first array
        System.out.println(makeAbbreviation(firstArray));  
        // printing abbreviation for the second array
        System.out.println(makeAbbreviation(secondArray));  
        // printing abbreviation for the third array
        System.out.println(makeAbbreviation(thirdArray));  
    }
}

In this method, we use the StringBuilder class to call the append() method and add the uppercase first letter to an initially empty string. This way, we iterate through each element of the array, take the letter at the zeroth position (the first letter of each word), convert it to uppercase using the toUpperCase() method, and add it to our result using the append() method. Finally, we use the toString() method to convert our StringBuilder to String.

1. What is a return value in a method?

2. What does a `void` method return?

3. Can a method be used inside another method?

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Seção 2. Capítulo 5