Re: java.util.Properties extending from HashMap<Object, Object> instead of HashMap<String, String>

Patricia Shanahan <>
Fri, 04 Apr 2008 14:30:12 -0700
Zig wrote:

On Fri, 04 Apr 2008 13:16:47 -0400, Owen Jacobson
<> wrote:

On Apr 4, 12:14 pm, Rakesh <> wrote:

I was curious if there is a reason why java.util.Properties extends
from HashMap<Object, Object> instead of HashMap<String, String>
(seems more intuitive to me).

A variation of that would be -
   how do we get about converting Properties to Map<String, String> .

Backwards compatability. In Java 1.4 and earlier,
Properties.get(Object) returned Object; changing that signature for
1.5 would break any code that used that method without an immediate
cast to String.


I don't think this is the reason. Particularly, making a method
signature generic does not change it's runtime signature.

Even if Properties were made as "Properties extends HashMap<String,String>"
Properties.get will inherit the Map.get signature, which is just:


and thus runtime compatibility is preserved. At the compiler's option,
additional synthetic bridge methods may be created in order to support
covariant return types, but even then, a method with a Ljava/lang/Object
return type must exist at runtime.

Additionally, the parameter "key" of Map.get is not generified: all Maps
accept java.lang.Object as a key, under the premis that if you pass in a
different type then it can't possibly have a previous mapping.

Thus code calling "Properties.get(new Object())" would still be
compatible at compile time.

Lastly, all Strings are assignable as Objects. Thus code calling "Object
x=Properties.get(key)" will also still be compile-time compatible.

I think the real reason Properties does not extend
Hashtable<String,String> is that, while discouraged, it is still valid
to have Properties with non-string key/values. An existing Java 1.4 API
could something like:

public Properties createProps() {
    Properties props=new Properties();
    props.setProperty("name", "foo");
    props.put("version", new Integer());
    return props;

If this method existed, and you were tasked with calling this method
such as:

Integer version=(Integer) createProps().get("version");

This code would likely not compile in Java 5 if properties was made as
Hashtable<String,String>, and users would be "locked in" to using Java
1.4 as long as they continued to use this API.



The thing I don't understand is why, before 1.5, Properties extended
anything other than Object. Extending Hashtable or HashMap before
generics created features, such as the ability to add a mapping from an
Integer to a Float, that make no sense for Properties.


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