Question

See the bottom for the solution.

I’m trying to write some generic handling code, but in 1 of the sub-classes, it requires a Class that is more specific.

So the base class as a field of type Class, and in the subclass I’m trying to cast that Class object to type Class which is a subclass of org.​apache.​hadoop.​hbase.​mapreduce.Mapper.

I get the following error from Netbeans:

"Incompatible types
required: java.lang.Class<org.apache.hadoop.hbase.mapreduce.TableMapper>
found: java.lang.class<capture#3 of ? extends org.apache.hadoop.mapreduce.Mapper>"

when I try the following code

Class<TableMapper> tableMapperClass = null;
if( mapperClass.equals(TableMapper.class) ) {
    tableMapperClass = TableMapper.class.asSubclass(mapperClass);

    //do stuff
}

and I get:

incompatible types
required: java.lang.Class<org.apache.hadoop.hbase.mapreduce.TableMapper>
found: java.lang.Class<capture#8 of ? extends org.apache.hadoop.hbase.mapreduce.TableMapper>

for

Class<TableMapper> tableMapperClass = null;
if( mapperClass.equals(TableMapper.class) ) {
    tableMapperClass = mapperClass.asSubclass(TableMapper.class);

    //do stuff
}

Ok, got the answer from my co-worker, looks like this should work:

Class<? extends TableMapper> tableMapperClass = null;
if( mapperClass.equals(TableMapper.class) ) {
    tableMapperClass = mapperClass.asSubclass(TableMapper.class);

    //do stuff
}

Answer 1

If you want to assign a class to tableMapperClass that is actually a subclass of TableMapper, then you need to change the type of the variable. Instead, use:

Class<? extends TableMapper> tableMapperClass = null;

and now you can assign TableMapper.class or any subclass to this variable. When you write Class<TableMapper> you are promising that the variable will be exactly TableMapper.class or null.

Another example:

Class<Number> number = Integer.class; // does not compile
Number number = new Integer(1); // compiles fine
Class<? extends Number> number = Integer.class; // also ok

Note that you can do different things with Class<Number> then you can with Class<? extends Number>. For example, you can call a constructor of Class<Number>, because you know the class. You can’t do that with Class<? extends Number> because the constructors are not defined at compile time.

Similarly with, say, List<Number> vs List<? extends Number>. You can call list.add(7) on a variable of type List<Number>, but you can’t do that on a List<? extends Number> because you don’t know the type of the second list. It might be a List<Double> for example, in which case adding an Integer is not allowed.

Generics are weird. 🙂 Hope this helps.