Java Collection - 如何创建不可修改的地图

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

public class Main {

  public static void main(String[] args) {
    Map<String, String> map = getMap();
    map.put("a", "b");
  }

  public static final Map<String, String> getMap() {
    Map<String, String> map = new HashMap<String, String>();
    map.put("x", "y");
    return Collections.unmodifiableMap(map);
  }
}