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Intro to Java

Exercises: Reusing Methods

Prime numbers

Methods are reusable pieces of code. The last set of exercise requires all the previous ones to be resolved and reused.

User input

Write a method that asks the user for a number in a range (between two numbers), verifies if the user answer is valid, so if it is between the two numbers defining the range, and return its value. If the number is not valid, it should inform the user of the mistake and ask again, until the user types a valid number.

HINTS

  • Name the method askNumberBetween.
  • It shall have two parameters, min and max, both of type int and return an int.
  • You may need to use a Scanner inside to read from System.in.

Checking Prime numbers

Define a method that checks if a number is a prime number. Prime numbers are numbers divisible only by 1 and themselves. For more information, please see https://en.wikipedia.org/wiki/Prime_number.

Testing if a number is prime is a very well known problem, with many solutions proposed in history. Some solution are so advanced that require a Quantum Computer to be implemented. This computer exists only in the most advanced research laboratories in the world. But we don’t need to this technology to solve the problem. We’ll implement a much simpler solution.

HINTS

  • Name the method isPrimeNumber.
  • It shall have one parameter, n, the number to be tested, and return a boolean.
  • To check if a is divisible by b, you can check if the reminder of the division, a % b, is zero.
  • We implement a simple test, following the definition of prime number:
    • Try to find a number between 2 and n - 1 that is a divisor of n.
    • If this number is found, return false, meaning than n is not a prime number.
    • If this number is not found, return true, meaning that n is prime.

EXTRA

The test can be improved, without changing it dramatically. Try to find out how.

Finding a prime number manually

Reusing the methods defined above, implement this and the following two programs.

Ask the user for a number between 1 and 1.000.000.000. Check if the number is prime or not. If it is, inform the user and terminate the program. If it is not, ask the user to try again, until they find a prime number.

Do you need some large prime number to test this program? Implement the next programs to find them!

Printing prime numbers

Ask the user for two numbers, a and b. a must be between 1 and 1.000.000. b must be between a and 1.000.000. Print all the prime numbers you find between a and b, inclusive.

Collecting prime numbers

Ask the user for a number n between 1 and 50.000. Print the first n prime numbers you can find, starting from 2.

Password check

1. Minimum character count

Write a method public static boolean isStringLongerThan(String word, int lowerBound) which returns true if word has at least lowerBound characters.

2. Contains letters and digits

A password is just allowed to contain letters and digits. Write a method public static boolean containsOnlyLettersAndDigits(String word) which returns true if a word just contains letters and digits.

Examples:

  • true for: 12Polizei, 1234Password and 13Zwölf11
  • false for: P4$$w0rd, G€h€!m and P@ssword

HINT: to check if a character is a letter or digit you can use the method Character.isLetterOrDigit(character).

3. At least n digits

Write a method public static boolean containsEnoughDigits(String word, int lowerBound) which checks if a string has at least lowerBound digits.

HINT: to check if a character is a digit you can use the method Character.isDigit(character).

Check password validity

Write a method public static boolean isValidPassword(String password) which checks if a password is a valid.

A password is valid if:

  • it is minimum 10 characters long
  • it just contains of letters and digits
  • it has at least 2 numbers

HINT: reuse the methods you implemented in 1, 2 and 3.