Re: HashMap get/put
Mike Schilling wrote:
Wojtek wrote:
If I have the following:
HashMap<MyKey,MyValue> map = new HashMap<MyKey,MyValue>();
MyKey myKey = new MyKey();
Then I can put in a value by:
map.put(myKey, new MyValue() );
The compiler enforces the use of these two types.
However to do a get I can do the following and the compiler does not
complain:
map.get(myKey); // this is right
map.get(myKey.toString()); // this is wrong yet legal
map.get(new Long(20)); // this is wrong yet legal
All of these are legal according to the compiler. Why is it that the
compiler does not enforce type checking on the get()?
Or rather, why does the spec not say get(K key) instead of
get(Object
o)
http://java.sun.com/javase/6/docs/api/java/util/HashMap.html#get(java.lang.Object)
I can picture (vaguely) looking something up with an object that's
equal to the original key but not of the same type. This makes at
least some sense with HashMaps, since it's possible, for objects T1 a
and T2 b, where T1 and T2 are any types at all, that a.equals(b) and
a.hashCode() == b.hashCode().
Here's an example program in which a get call succeeds despite the type
of the get argument being of a different type from the key. Uncommenting
the "//map.put(probe, "Probe");" line creates a compile time error.
import java.util.ArrayList;
import java.util.Arrays;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.Stack;
public class MapGetTest {
Map<Stack,String> map = new HashMap<Stack,String>();
String[] data1 = {"aaa","bbb"};
String[] data2 = {"ccc","ddd"};
public static void main(String[] args) {
new MapGetTest().test();
}
private MapGetTest(){
Stack<String> stack1 = new Stack<String>();
stack1.addAll(Arrays.asList(data1));
Stack<String> stack2 = new Stack<String>();
stack2.addAll(Arrays.asList(data2));
map.put(stack2, "Stack2");
}
private void test(){
List<String> probe = new ArrayList<String>();
probe.add("ccc");
probe.add("ddd");
System.err.println(map.get(probe));
//map.put(probe, "Probe");
}
}