import java.util.Map;
public final class MutableClass {
  private String field;
  private Map<String, String> fieldMap;
public MutableClass(String field, Map<String, String> fieldMap) {
  this.field = field;
  this.fieldMap = fieldMap;
}
public String getField() {
  return field;
}
public Map<String, String> getFieldMap() {
  return fieldMap;
}
}

import java.util.HashMap;
import java.util.Map;
public class App {
public static void main(String[] args) {
  Map<String, String> map = new HashMap<>();
  map.put("key", "value");
  // Инициализация нашего "иммутабельного" класса
  MutableClass mutable = new MutableClass("this is not immutable", map);
  // Можно легко добавлять элементы в map == изменение состояния
  mutable.getFieldMap().put("unwanted key", "another value");
  mutable.getFieldMap().keySet().forEach(e ->  System.out.println(e));
}
}
// Вывод в консоли
unwanted key
key

import java.util.HashMap;
import java.util.Map;
public class AlmostMutableClass {
  private String field;
  private Map<String, String> fieldMap;
public AlmostMutableClass(String field, Map<String, String> fieldMap) {
  this.field = field;
  this.fieldMap = fieldMap;
}
public String getField() {
  return field;
}
public Map<String, String> getFieldMap() {
  Map<String, String> deepCopy = new HashMap<String, String>();
  for(String key : fieldMap.keySet()) {
    deepCopy.put(key, fieldMap.get(key));
  }
  return deepCopy;
}
}

import java.util.HashMap;
import java.util.Map;
public class App {
public static void main(String[] args) {
  Map<String, String> map = new HashMap<>();
  map.put("good key", "value");
  // Инициализация нашего "иммутабельного" класса
  AlmostMutableClass almostMutable = new AlmostMutableClass("this is not immutable", map);
  // Мы не можем изменять состояние объекта
  // через добавление элементов в полученную map
  System.out.println("Result after modifying the map after we get it from the object");
  almostMutable.getFieldMap().put("bad key", "another value");
  almostMutable.getFieldMap().keySet().forEach(e -> System.out.println(e));
  System.out.println("Result of the object's map after modifying the initial map");
  map.put("bad key", "another value");
  almostMutable.getFieldMap().keySet().forEach(e -> System.out.println(e));
  }
}
// Вывод в консоли
Result after modifying the map after we get it from the object
good key
Result of the object's map after modifying the initial map
good key
bad key

public AlmostMutableClass(String field, Map<String, String> fieldMap) {
  this.field = field;
  Map<String, String> deepCopy = new HashMap<String, String>();
  for(String key : fieldMap.keySet()) {
    deepCopy.put(key, fieldMap.get(key));
  }
  this.fieldMap = deepCopy;
}
// Вывод в консоли
Result after modifying the map after we get it from the object
good key
Result of the object's map after modifying the initial map
good key