Java-使用方法参数定义HashMap类型

Question

I want a method argument to define the class a HashMap will accept. Something along the lines of:

private HashMap hashMap = new HashMap();

public boolean createMap(Object obj) {
    return hashMap = new HashMap<String, obj.getClass()>() != null;
}

Answer 1

You can't do what you're trying to do: you can't "define the class a HashMap will accept" because at runtime a HashMap will accept all classes.

What you could do, on the other hand, is:

private Map<String, ?> map;

public <V> void createMap(Class<V> clazz) {
   map = Collections.checkedMap(new HashMap<String, V>(),
      String.class, clazz);
}

...which will actually enforce the restrictions you're trying to create, with reflection. In this case, your hashMap member should have type Map<String, Object> , though it will enforce

Answer 2

define a typed function that accepts the type you desire as value for the map

public <T> Map<String, T> checkedStringKeyMap(Class<T> type) {
    return Collections.checkedMap(new HashMap<String, T>(), String.class, type);
}

then create your map as follows

Map<String, Person> persons = checkedStringKeyMap(Person.class);