Add Is Prime exercise

Author: phrdang

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

@@ -0,0 +1,21 @@
+package com.codefortomorrow.intermediate.chapter11.practice;
+
+/*
+Write a method called isPrime which returns
+true if the given integer is prime and false otherwise.
+Bonus points for writing JavaDoc comments for the method.
+
+Test your isPrime method in the main method by
+calling it at least 3 times and printing the result.
+
+Solution from Listing 6.7, Introduction to Java Programming (Comprehensive),
+10th ed. by Y. Daniel Liang
+*/
+
+public class IsPrime {
+    public static void main(String[] args) {
+        // test isPrime method here
+    }
+
+    // write isPrime method here
+}

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

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+package com.codefortomorrow.intermediate.chapter11.solutions;
+
+/*
+Write a method called isPrime which returns
+true if the given integer is prime and false otherwise.
+Bonus points for writing JavaDoc comments for the method.
+
+Test your isPrime method in the main method by
+calling it at least 3 times and printing the result.
+
+Solution from Listing 6.7, Introduction to Java Programming (Comprehensive),
+10th ed. by Y. Daniel Liang
+*/
+
+public class IsPrime {
+    public static void main(String[] args) {
+        System.out.println(isPrime(5)); // true
+        System.out.println(isPrime(1)); // false
+        System.out.println(isPrime(56)); // false
+    }
+
+    /**
+     * Check whether number is prime
+     * @param number  number to check if prime
+     * @return true if the number is prime
+     */
+    public static boolean isPrime(int number) {
+        if (number <= 1) {
+            return false;
+        }
+
+        // only need to check divisors up to number / 2
+        // because after that the factors "repeat"
+        for (int divisor = 2; divisor <= number / 2; divisor++) {
+            if (number % divisor == 0) { // primes only divisible by 1 and itself
+                return false; // number isn't a prime
+            }
+        }
+        return true; // number is a prime
+    }
+}