java primes

.gitignore

@@ -0,0 +1,5 @@
+*/.idea
+.idea
+*.iml
+*.class
+out

primes/README.md

@@ -0,0 +1,8 @@
+# Primes
+
+Find all primes from 0 to 10 000 000.  
+Printing time is not included.
+
+| Language | Time   |
+| ---------| -------|
+| Java     | 1836ms |

primes/java/src/DefaultPrimes.java

@@ -0,0 +1,33 @@
+import java.util.ArrayList;
+import java.util.List;
+
+public class DefaultPrimes implements Primes {
+
+    // (will make the code 15% slower)
+    @SuppressWarnings("ForLoopReplaceableByForEach")
+    public List<Integer> primes(int max) {
+        List<Integer> primesList = new ArrayList<>(10000);
+
+        primesList.add(2);
+
+        for (int i = 3; i < max; i++) { //go through all numbers
+            boolean isPrime = true;
+            for (int j = 0; j < primesList.size(); j++) { //test each number
+                int prime = primesList.get(j);
+                if (prime > Math.sqrt(i)) {
+                    break;
+                }
+                if (i % prime == 0) { //i = test number, j = some known prime
+                    isPrime = false;
+                    break;
+                }
+            }
+
+            if (isPrime) {
+                primesList.add(i);
+            }
+        }
+
+        return primesList;
+    }
+}

primes/java/src/Main.java

@@ -0,0 +1,25 @@
+import java.util.ArrayList;
+import java.util.List;
+
+public class Main {
+    public static void main(String[] args) {
+        Primes primes = new DefaultPrimes();
+        List<Long> times = new ArrayList<>();
+        List<Integer> values = null;
+
+        for (int i = 0; i < 10; i++) {
+            System.out.println("iteration " + i + " started");
+            long startTime = System.currentTimeMillis();
+            values = primes.primes(10_000_000);
+            long totalTime = System.currentTimeMillis() - startTime;
+            times.add(totalTime);
+        }
+
+        double average = times.stream()
+                .mapToLong(l -> l)
+                .average()
+                .getAsDouble();
+
+        System.out.println("Average time after 10 times: " + average + "ms");
+    }
+}

primes/java/src/Primes.java

@@ -0,0 +1,5 @@
+import java.util.List;
+
+public interface Primes {
+    List<Integer> primes(int max);
+}