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

class Helper {
  public static <K, V> V get(Map<K, V> m, K key, V def) {
    V v = m.get(key);
    return (v != null) ? v : def;
  }
}

public class Main {

  public static void main(String[] args) {

    Map<String, Integer> m = new HashMap<String, Integer>();

    m.put("A", 42);
    m.put("B", 3);
    m.put("C", 1);

    System.out.println(Helper.get(m, "forty-two", -1));
    System.out.println(Helper.get(m, "A", -1));
    System.out.println(Helper.get(m, "B", -1));
    System.out.println(Helper.get(m, "something_else", -1));

  }
}

運行結(jié)果如下: