Add tests for BoundaryGenerator

k9mail-library/src/main/java/com/fsck/k9/mail/BoundaryGenerator.java

@@ -4,20 +4,23 @@ package com.fsck.k9.mail;
 import java.util.Locale;
 import java.util.Random;
 
+import android.support.annotation.VisibleForTesting;
+
 
 public class BoundaryGenerator {
-    private static final BoundaryGenerator INSTANCE = new BoundaryGenerator();
+    private static final BoundaryGenerator INSTANCE = new BoundaryGenerator(new Random());
 
 
-    private Random random;
+    private final Random random;
 
 
     public static BoundaryGenerator getInstance() {
         return INSTANCE;
     }
 
-    private BoundaryGenerator() {
-        this.random = new Random();
+    @VisibleForTesting
+    BoundaryGenerator(Random random) {
+        this.random = random;
     }
 
     public String generateBoundary() {

k9mail-library/src/test/java/com/fsck/k9/mail/BoundaryGeneratorTest.java

@@ -0,0 +1,46 @@
+package com.fsck.k9.mail;
+
+
+import java.util.Random;
+
+import org.junit.Test;
+import org.mockito.stubbing.OngoingStubbing;
+
+import static org.junit.Assert.assertEquals;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.when;
+
+
+public class BoundaryGeneratorTest {
+    @Test
+    public void generateBoundary_allZeros() throws Exception {
+        Random random = createRandom(0);
+        BoundaryGenerator boundaryGenerator = new BoundaryGenerator(random);
+
+        String result = boundaryGenerator.generateBoundary();
+
+        assertEquals("----000000000000000000000000000000", result);
+    }
+
+    @Test
+    public void generateBoundary() throws Exception {
+        Random random = createRandom(0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22,
+                23, 24, 25, 26, 27, 28, 35);
+        BoundaryGenerator boundaryGenerator = new BoundaryGenerator(random);
+
+        String result = boundaryGenerator.generateBoundary();
+
+        assertEquals("----0123456789ABCDEFGHIJKLMNOPQRSZ", result);
+    }
+
+    private Random createRandom(int... values) {
+        Random random = mock(Random.class);
+        
+        OngoingStubbing<Integer> ongoingStubbing = when(random.nextInt(36));
+        for (int value : values) {
+            ongoingStubbing = ongoingStubbing.thenReturn(value);
+        }
+        
+        return random;
+    }
+}