Add Check Password exercise

Author: phrdang

src/com/codefortomorrow/intermediate/chapter11/practice/CheckPassword.java

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+package com.codefortomorrow.intermediate.chapter11.practice;
+
+/*
+Some websites impose certain rules for passwords.
+Write a method that checks whether a string is a valid password.
+
+Suppose the password rules are as follows:
+■ A password must have at least eight characters.
+■ A password consists of only letters and digits.
+■ A password must contain at least two digits.
+
+Write a program that prompts the user to enter a password and displays
+Valid Password if the rules are followed or Invalid Password otherwise.
+To do this, use a method called isValid which returns true if the password
+is valid and false otherwise.
+
+You may also find it helpful to write a method called
+isAlphanumeric which returns true if the given string
+is only made up of letters and digits.
+
+You may also find it helpful to write a method called
+getNumberOfDigits which returns the number of digits
+in a given string.
+
+You may also find it helpful to use the Character.isDigit(char)
+and Character.isDigit(char) methods. You can read more about them here:
+https://www.tutorialspoint.com/java/java_characters.htm
+
+If you don't want to use those methods, you can also use regular
+expressions (aka regex) and the matches(regex) String method.
+You can read about that here:
+https://www.tutorialspoint.com/java/java_string_matches.htm
+https://www.vogella.com/tutorials/JavaRegularExpressions/article.html
+
+Adapted from Exercise 6.18, Introduction to Java Programming (Comprehensive),
+10th ed. by Y. Daniel Liang
+*/
+
+public class CheckPassword {
+    public static void main(String[] args) {
+        // test checkPassword method here
+    }
+    
+    // write checkPassword method here
+}

src/com/codefortomorrow/intermediate/chapter11/solutions/CheckPassword.java

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+package com.codefortomorrow.intermediate.chapter11.solutions;
+
+import java.util.Scanner;
+
+/*
+Some websites impose certain rules for passwords.
+Write a method that checks whether a string is a valid password.
+
+Suppose the password rules are as follows:
+■ A password must have at least eight characters.
+■ A password consists of only letters and digits.
+■ A password must contain at least two digits.
+
+Write a program that prompts the user to enter a password and displays
+Valid Password if the rules are followed or Invalid Password otherwise.
+To do this, use a method called isValid which returns true if the password
+is valid and false otherwise.
+
+You may also find it helpful to write a method called
+isAlphanumeric which returns true if the given string
+is only made up of letters and digits.
+
+You may also find it helpful to write a method called
+getNumberOfDigits which returns the number of digits
+in a given string.
+
+You may also find it helpful to use the Character.isDigit(char)
+and Character.isDigit(char) methods. You can read more about them here:
+https://www.tutorialspoint.com/java/java_characters.htm
+
+If you don't want to use those methods, you can also use regular
+expressions (aka regex) and the matches(regex) String method.
+You can read about that here:
+https://www.tutorialspoint.com/java/java_string_matches.htm
+https://www.vogella.com/tutorials/JavaRegularExpressions/article.html
+
+Adapted from Exercise 6.18, Introduction to Java Programming (Comprehensive),
+10th ed. by Y. Daniel Liang
+*/
+
+public class CheckPassword {
+    public static void main(String[] args) {
+        // prompt user for a password
+        Scanner input = new Scanner(System.in);
+        System.out.print("Password: ");
+        String password = input.nextLine();
+        input.close();
+
+        // print whether it's valid
+        if (isValid(password)) {
+            System.out.println("Valid Password");
+        } else {
+            System.out.println("Invalid Password");
+        }
+    }
+    
+    /**
+     * Check if a password is valid.
+     * A valid password must:
+     * - Contain at least 8 characters
+     * - Be alphanumeric
+     * - Contain at least 2 digits
+     * @param password  password to check if valid
+     * @return true if the password is valid
+     */
+    public static boolean isValid(String password) {
+        boolean atLeastEightChars = password.length() >= 8;
+        boolean isAlphanumeric = isAlphanumeric(password);
+        boolean atLeastTwoDigits = getNumberOfDigits(password) >= 2;
+
+        return atLeastEightChars && isAlphanumeric && atLeastTwoDigits;
+    }
+
+    /**
+     * Check if a String is alphanumeric
+     * (contains only letters and numbers)
+     * @param str  String to check if alphanumeric
+     * @return true if the String is alphanumeric
+     */
+    public static boolean isAlphanumeric(String str) {
+        for (int i = 0; i < str.length(); i++) {
+            boolean isDigit = Character.isDigit(str.charAt(i));
+            boolean isLetter = Character.isLetter(str.charAt(i));
+            if (!isDigit && !isLetter) {
+                return false;
+            }
+        }
+        return true;
+
+        // alternate solution using regular expressions
+        // matches lowercase and uppercase letters, and digits
+        // the + indicates the pattern must match 1 or more times
+        // (need to check each char in the string)
+        // return str.matches("[a-zA-Z0-9]+");
+    }
+
+    /**
+     * Returns the number of digits
+     * contained in the given String
+     * @param str  String to get number of digits
+     * @return the number of digits contained in the given String
+     */
+    public static int getNumberOfDigits(String str) {
+        int numberOfDigits = 0;
+        for (int i = 0; i < str.length(); i++) {
+            if (Character.isDigit(str.charAt(i))) {
+                numberOfDigits++;
+            }
+        }
+        return numberOfDigits;
+    }
+}