在Java中，何时应该使用“Object o”而不是泛型？

Question

For example, I could write either of these:

class example <T>
{
    ...

    public void insert (T data)
    {
        ...
    }
}

or

class example
{
    ...

    public void insert (Object o)
    {
        ...
    }
}

Is there a signficant difference between the 2 in terms of performance? With generics I could restrict the type of the parameter and with the second approach I guess it wouldn't be necessary to define the type of the object as it is created.

Also, with the second approach I could basically insert anything into the class, right? Whereas with generics every element in the class would be of the same type.

Anything else I'm missing?

Answer 1

The only reason to write the latter is if you must target an earlier JVM. Generics are implemented by type-erasure, so they have no runtime impact - only added compile time checking which will improve your code.

Of course if you need a collection which holds any old object, or a mix of several which don't have a common superclass, you need the plain Object variation (but then your class can still be generic and instantiated with new ...<Object>).

Answer 2

I think you pretty much nailed it. There is no performance difference. Generics are rationalized away ( Type Erasure ) when the code is compiled, and don't exist anymore at runtime. They just add casts when needed and do type-checking as you stated. Neal Gafter wrote a nice overview of how they work, of the current problems with Generics and how they could be solved in the next version of Java: http://gafter.blogspot.com/2006/11/reified-generics-for-java.html

Answer 3

There shouldn't be a performance difference.

However, Java does not offer parameter variance, so there are situations where you will be overriding pre-generics functions such as equals, compareTo, etc. where you will have to use Objects.

Answer 4

Some of the encounters where I had to use 'Object' instead of Generics were those of compulsion than of a choice. When working with pre-generic code or libraries built around pre-generic api, one has little choice. Dynamic proxies for example, Proxy.newProxy() returns Object type. Passing generic context (where a context can be anything) is another instance. Some of my friends argue that are as good as no-generics. As far as performance is concerned, there shouldn't be any overhead, considering type erasure.

Answer 5

Regarding performance, i agree with the people above.

Regarding this point of yours

"Also, with the second approach I could basically insert anything into the class, right? Whereas with generics every element in the class would be of the same type."

One more advantage of generics is there is a type check for assignment of the example instance itself.

Say for example you had an Example e1 of and another Example e2 of , type safety would be maintained and you would never be able to do e1=e2;

while with the object example, that would be possible.