diff --git a/padlock-impl/src/test/java/com/cleverthis/interview/padlock/PadlockTest.java b/padlock-impl/src/test/java/com/cleverthis/interview/padlock/PadlockTest.java
new file mode 100644
index 0000000..b935727
--- /dev/null
+++ b/padlock-impl/src/test/java/com/cleverthis/interview/padlock/PadlockTest.java
@@ -0,0 +1,75 @@
+package com.cleverthis.interview.padlock;
+
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+class PadlockTest {
+
+    @Test
+    void testInstantiationRejectNonPositiveNumpadSize() {
+        assertThrows(IllegalArgumentException.class,
+                () -> new PadlockImpl(0));
+        assertThrows(IllegalArgumentException.class,
+                () -> new PadlockImpl(-500));
+    }
+
+    @Test
+    void testInstantiationAcceptPositiveNumpadSize() {
+        assertDoesNotThrow(() -> new PadlockImpl(1));
+        assertDoesNotThrow(() -> new PadlockImpl(500));
+    }
+
+    @Test
+    void testInstantiationRest() {
+        PadlockImpl padlock = new PadlockImpl(5, true);
+        for (int i = 0; i < 5; i++) {
+            // ensure input buffer is uninitialized
+            // should return null when first set
+            assertNull(padlock.writeInputBuffer(i, i));
+        }
+        // should return false
+        assertFalse(padlock.isPasscodeCorrect());
+    }
+
+    @Test
+    void testRejectInvalidInput() {
+        PadlockImpl padlock = new PadlockImpl(5, true);
+        for (int i = 0; i < 3; i++) {
+            padlock.writeInputBuffer(i, i);
+        }
+        // partially initialized, should fail
+        assertThrows(IllegalStateException.class, padlock::isPasscodeCorrect);
+        for (int i = 0; i < 2; i++) {
+            padlock.writeInputBuffer(i + 3, i);
+        }
+        // now we have duplicated value, should fail too
+        assertThrows(IllegalStateException.class, padlock::isPasscodeCorrect);
+        for (int i = 0; i < 2; i++) {
+            padlock.writeInputBuffer(i + 3, i + 3);
+        }
+        // now we fixed everything, we should have no error
+        assertDoesNotThrow(padlock::isPasscodeCorrect);
+    }
+
+    @Test
+    void testRejectInvalidInputBufferAddressAndValue() {
+        PadlockImpl padlock = new PadlockImpl(5, true);
+        // test address
+        assertThrows(ArrayIndexOutOfBoundsException.class, () -> padlock.writeInputBuffer(-1, 1));
+        assertThrows(ArrayIndexOutOfBoundsException.class, () -> padlock.writeInputBuffer(-10, 1));
+        assertThrows(ArrayIndexOutOfBoundsException.class, () -> padlock.writeInputBuffer(5, 1));
+        assertThrows(ArrayIndexOutOfBoundsException.class, () -> padlock.writeInputBuffer(50, 1));
+        // test value
+        assertThrows(IllegalArgumentException.class, () -> padlock.writeInputBuffer(1, -1));
+        assertThrows(IllegalArgumentException.class, () -> padlock.writeInputBuffer(1, -10));
+        assertThrows(IllegalArgumentException.class, () -> padlock.writeInputBuffer(1, 5));
+        assertThrows(IllegalArgumentException.class, () -> padlock.writeInputBuffer(1, 50));
+        // test both
+        assertThrows(IllegalArgumentException.class, () -> padlock.writeInputBuffer(-10, -1));
+        assertThrows(IllegalArgumentException.class, () -> padlock.writeInputBuffer(5, -10));
+        assertThrows(IllegalArgumentException.class, () -> padlock.writeInputBuffer(50, 5));
+        assertThrows(IllegalArgumentException.class, () -> padlock.writeInputBuffer(-1, 50));
+    }
+
+}
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