Master mind code gen

MasterMindGame.java

@@ -1,4 +1,5 @@
 import java.util.Optional;
+import java.util.*;
 
 /**
  * A code-breaking game where the app selects a sequence of symbols, and
@@ -22,3 +23,18 @@ public Optional<Integer> play() {
         return Optional.empty();
     }
 }
+
+class CodeGenerator {
+        public static String generateCode() {
+            Random random = new Random();
+            String numbers = "0123456789";
+            String code = "";
+
+            for (int i=0; i < 4; i++) {
+                int index = random.nextInt(numbers.length());
+                code = code + numbers.charAt(index);
+            }
+
+            return code;
+        }
+    }

MasterMindGameTest.java

@@ -0,0 +1,45 @@
+import org.junit.jupiter.api.Test;
+import static org.junit.jupiter.api.Assertions.*;
+
+import java.util.*;
+import java.util.regex.*;
+
+public class MasterMindGameTest {
+
+    private static final String SYMBOLS = "0123456789";
+    private static final Pattern CODE_PATTERN = 
+        Pattern.compile("[" + Pattern.quote(SYMBOLS) + "]{4}");
+
+
+@Test
+void isExactlyFourSymbols() {
+
+    String code = CodeGenerator.generateCode();
+    assertEquals(4, code.length(), "Code should be 4 symbol long");
+
+}
+
+@Test
+void areFromSymbolSet() {
+
+    String code = CodeGenerator.generateCode();
+    assertTrue(CODE_PATTERN.matcher(code).matches(), "Code must be from this symbol set: " + SYMBOLS);
+
+}
+
+@Test
+void isNotMoreThanTwiceTheSize() {
+
+    Map<String, Integer> counts = new HashMap<>();
+    int sampleSize = 100;
+
+    for (int i = 0; i < sampleSize; i++) {
+        String code = CodeGenerator.generateCode();
+        counts.merge(code, 1, Integer::sum);
+    }
+
+    counts.forEach((code, count) ->
+        assertTrue(count <= 2, "Code '" + code + "' appeared " + count + " times"));
+}
+
+}