Backend

src/main/java/CaesarShift.java

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+import java.util.ArrayList;
+
+public class CaesarShift {
+    private static final String ALPHABET = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"; //Encapsulate and make the alphabet immutable
+    private static int key = 25; //Initialize and encapsulate the shift key
+    private static String encryptedMsg; //Create and encapsulate the message container
+    private static String decryptedMsg;
+
+    public String encrypt(String encryptMsg){
+        String upCased = encryptMsg.toUpperCase();
+        char[] upCasedArrs = upCased.toCharArray();//split the string to a character array
+        ArrayList<Character> encryptedChars = new ArrayList<Character>();
+
+        //loop through the character array
+        for(Character character : upCasedArrs ){
+            int index = ALPHABET.indexOf(character.toString());//get the character rank in the alphabet
+            int encryptedCharIndex = Math.floorMod((index+key),26);//shift the character using the key and get the new characters rank in the alphabet
+            encryptedChars.add(ALPHABET.charAt(encryptedCharIndex));//get the character from the alphabet rank and add it to the char array
+            encryptedMsg = encryptedChars.toString().replaceAll("\\[|\\]|\\s","").replaceAll(",","");//convert and cleanup the char array to a string
+        }
+        return encryptedMsg;
+    }
+
+    public String decrypt(String decryptMsg) {
+        String upCased = decryptMsg.toUpperCase();
+        char[] upCasedArrs = upCased.toCharArray();//split the string to a character array
+        ArrayList<Character> decryptedChars = new ArrayList<Character>();
+
+        //loop through the character array
+        for (Character character : upCasedArrs) {
+            int index = ALPHABET.indexOf(character.toString());//get the character rank in the alphabet
+            int decryptedCharIndex = Math.floorMod((index - key), 26);//shift the character using the key and get the new characters rank in the alphabet
+            decryptedChars.add(ALPHABET.charAt(decryptedCharIndex));//get the character from the alphabet rank and add it to the char array
+            decryptedMsg = decryptedChars.toString().replaceAll("\\[|\\]|\\s", "").replaceAll(",", "");//convert and cleanup the char array to a string
+        }
+        return decryptedMsg;
+
+    }
+
+}

src/test/java/CaesarShiftTest.java

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+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+class CaesarShiftTest {
+    @Test
+    public void caesarShift_isEncryptUpcasedTest_Boolean() {
+        CaesarShift encryptCaseCheck = new CaesarShift();
+        assertEquals(true, Character.isUpperCase(encryptCaseCheck.encrypt("a").charAt(0)));
+    }
+
+    @Test
+    void caesarShift_encryptStringTest_String() {
+        CaesarShift encryptString = new CaesarShift();
+        assertEquals("GDKKN", encryptString.encrypt("hello"));
+    }
+
+    @Test
+    public void caesarShift_isDecryptMsgUpcasedTest_String() {
+        CaesarShift caseTest = new CaesarShift();
+        assertEquals(true, Character.isUpperCase(caseTest.decrypt("a").charAt(0)));
+    }
+
+    @Test
+    public void caesarShift_decryptStringTest_String(){
+        CaesarShift decryptStringTest = new CaesarShift();
+        assertEquals("Z", decryptStringTest.decrypt("Y"));
+    }
+
+
+}