Main.java

import java.util.HashMap;
import java.util.Map;

public class Main {

    private static class Test {

        Map<String, String> valuesMap = new HashMap<String, String>();

        public void process(String initialString) throws Exception {
            if (!initialString.matches("^(;\\w+@\\w+)+;$")) {
                throw new Exception("Incorrect line format regexp");
            }
            String[] pairs = initialString.split(";");
            for (String pair : pairs) {
                if (!pair.isEmpty()) {
                    String[] elements = pair.split("@");
                    if (elements.length != 2) {
                        throw new Exception("Incorrect line format");
                    } else {
                        valuesMap.put(elements[0], elements[1]);
                    }
                }
            }
        }
        public String getValue(String key) {
            return valuesMap.get(key);
        }
        public String getKey(String value) {

            for (String key : valuesMap.keySet()) {
                if (valuesMap.get(key).equals(value)) {
                    return key;
                }
            }
            return null;
        }

    }

    public static void main(String[] args) throws Exception {


        Test test = new Test();

        test.process(";key1@value2;key2@value;key2@value2;");


        System.out.println("Key for value2: " + test.getKey("value2"));
        System.out.println("Value for key2: " + test.getValue("key2"));
        System.out.println("Value for null: " + test.getValue(null));



    }

}