Add SimpleSubstitutionCipherTest (#3857)

src/main/java/com/thealgorithms/ciphers/SimpleSubstitutionCipher.java

@@ -80,16 +80,4 @@ public class SimpleSubstitutionCipher {
 
         return decoded.toString();
     }
-
-    /**
-     * TODO remove main and make JUnit Testing
-     */
-    public static void main(String[] args) {
-        String a = encode(
-            "defend the east wall of the castle",
-            "phqgiumeaylnofdxjkrcvstzwb"
-        );
-        String b = decode(a, "phqgiumeaylnofdxjkrcvstzwb");
-        System.out.println(b);
-    }
 }

src/test/java/com/thealgorithms/ciphers/SimpleSubstitutionCipherTest.java

@@ -0,0 +1,48 @@
+package com.thealgorithms.ciphers;
+
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+public class SimpleSubstitutionCipherTest {
+
+    @Test
+    void testEncode() {
+        // Given
+        String message = "HELLOWORLD";
+        String key = "phqgiumeaylnofdxjkrcvstzwb";
+
+        // When
+        String actual = SimpleSubstitutionCipher.encode(message, key);
+
+        // Then
+        assertEquals("EINNDTDKNG", actual);
+    }
+
+    @Test
+    void testDecode() {
+        // Given
+        String message = "EINNDTDKNG";
+        String key = "phqgiumeaylnofdxjkrcvstzwb";
+
+        // When
+        String actual = SimpleSubstitutionCipher.decode(message, key);
+
+        // Then
+        assertEquals("HELLOWORLD", actual);
+    }
+
+    @Test
+    void testIsTextTheSameAfterEncodeAndDecode() {
+        // Given
+        String text = "HELLOWORLD";
+        String key = "phqgiumeaylnofdxjkrcvstzwb";
+
+        // When
+        String encodedText = SimpleSubstitutionCipher.encode(text, key);
+        String decodedText = SimpleSubstitutionCipher.decode(encodedText, key);
+
+        // Then
+        assertEquals(text, decodedText);
+    }
+}