diff --git a/src/xs_ibdpcompsci_javaintro/calculator.java b/src/xs_ibdpcompsci_javaintro/calculator.java
new file mode 100644
index 0000000..e55e653
--- /dev/null
+++ b/src/xs_ibdpcompsci_javaintro/calculator.java
@@ -0,0 +1,114 @@
+package xs_ibdpcompsci_javaintro;
+import java.util.*;
+
+class RPN_Calculator {
+	static void handle_error(int ERROR_TYPE) {
+		String ERROR_MESSAGE = "";
+		
+		switch (ERROR_TYPE) {
+			case -1:
+				// Delete the previous line to allow users to edit it. 
+				System.out.println("\033[A");
+				break;
+			case -2: 
+				ERROR_MESSAGE = "Memory exceeded.";
+				break;
+			case -3: 
+				// Division by zero. 
+				ERROR_MESSAGE = "Division by zero";
+				break;
+			default:
+				ERROR_MESSAGE = "Error in defining the error.";
+				break;
+		};
+		System.out.println(ERROR_MESSAGE);
+	}
+
+
+    static ArrayList<Object> request_input() {
+		
+		// Initialize variables. 
+		ArrayList<Object> USER_INPUTS = new ArrayList<Object>();
+		String USER_INPUT_CURRENT = "";
+		Boolean USER_INPUT_MORE = true;
+		int USER_INPUT_LENGTH = 0;
+		
+		// Loop for input. 
+		do {
+			Scanner input = new Scanner(System.in);
+			
+			USER_INPUT_CURRENT = input.nextLine();
+			USER_INPUT_LENGTH = USER_INPUTS.size();
+
+			if (USER_INPUT_CURRENT.matches("-?\\d+(\\.\\d+)?")) {
+				USER_INPUTS.add(Double.parseDouble(USER_INPUT_CURRENT));
+			} else if (USER_INPUTS.size() >= 2) {
+				if (USER_INPUT_CURRENT.matches("[-+\\/*×÷]") && (String.valueOf(USER_INPUTS.get(USER_INPUT_LENGTH-1)).matches("-?\\d+(\\.\\d+)?"))) {
+					System.out.println(USER_INPUTS.get(USER_INPUT_LENGTH-1));
+					USER_INPUTS.add(USER_INPUT_CURRENT);
+				} else if (USER_INPUT_CURRENT.isBlank()) {
+					if (String.valueOf(USER_INPUTS.get(USER_INPUT_LENGTH-1)).isBlank()) {
+						System.exit(0);
+					} else if (String.valueOf(USER_INPUTS.get(USER_INPUT_LENGTH-1)).matches("-?\\d+(\\.\\d+)?")) {
+						handle_error(-1);
+					} else {
+						USER_INPUT_MORE = false;
+					};
+				} else {
+					handle_error(-1);
+				};
+			} else {
+				// The user inputs a wrong code. Let the user edit it. 
+				handle_error(-1);
+			};
+			
+		} while (USER_INPUT_MORE == true);
+
+		
+		return (USER_INPUTS);
+	}
+	
+	static double calculate(ArrayList<Object> ENTRIES_LIST) {
+		// Calculate length. 
+		int ENTRIES_LIST_LENGTH = ENTRIES_LIST.size();
+		String OPERATION = "";
+		double OPERATION_RESULT = (double) ENTRIES_LIST.get(0);
+
+		if (ENTRIES_LIST_LENGTH > 1) {
+			for (int ENTRY_CURRENT = 0; ENTRY_CURRENT < ENTRIES_LIST_LENGTH; ENTRY_CURRENT++) {
+				if (String.valueOf(ENTRIES_LIST.get(ENTRY_CURRENT)).matches("[-+\\/*×÷]")) {
+					OPERATION = String.valueOf(ENTRIES_LIST.get(ENTRY_CURRENT));
+					
+					if (OPERATION == "+") {
+						OPERATION_RESULT += (float) ENTRIES_LIST.get(ENTRY_CURRENT - 1);
+					} else if (OPERATION == "-") {
+						OPERATION_RESULT -= (float) ENTRIES_LIST.get(ENTRY_CURRENT - 1);
+					} else if (OPERATION == "*" || OPERATION == "×") {
+						OPERATION_RESULT *= (float) ENTRIES_LIST.get(ENTRY_CURRENT - 1);
+					} else if (OPERATION == "/" || OPERATION == "÷") {
+						if ((double) ENTRIES_LIST.get(ENTRY_CURRENT - 1) != 0) {
+							OPERATION_RESULT /= (float) ENTRIES_LIST.get(ENTRY_CURRENT - 1);
+						} else {
+							handle_error(-3);
+							break;
+						};
+					};
+				};
+			};
+		};
+
+		return (OPERATION_RESULT);
+	};
+	
+	
+	public static void main(String[] args) {
+		// Initialize the input numbers. 
+		ArrayList<Object> NUMBERS_INPUT = new ArrayList<Object>();
+		double NUMBER_RESULT;
+		while (true) {
+			NUMBERS_INPUT = request_input();
+			NUMBER_RESULT = calculate(NUMBERS_INPUT);
+			System.out.println(NUMBER_RESULT);
+		}
+    }
+}
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