Hi,
We have intensive usages of java generics in the code base including SPI
model classes and extensiblity interfaces. But I noticed that there are some
inconsistencies which mix the legacy and parameterized types (either known
type or unknown wildcard).
Assuming we have an interface or class MyType<T>, the following method
styles will produce different results as illustrated in the sample test
below.
Style 1: MyType getMyType(...); // Legacy, all the generic information will
not be honored
Style 2: MyType<?> getMyType(...); // unknown parameter type
Style 3 : MyType<String> getMyType(...); // know parameter type
Please read the following example to understand the differencies.
Thanks,
Raymond
package test;
import java.util.ArrayList;
import java.util.Collection;
import java.util.HashMap;
import java.util.Map;
public class TestGenerics {
/**
* @param args
*/
public static void main(String[] args) {
Collection<?> a = new ArrayList<String>();
Collection b = new ArrayList();
Collection<Object> c = new ArrayList<Object>();
String value = "ABC";
a.add(value); // Illegal
b.add(value); // legal but with a warning
c.add(value); // legal
c = a; // illegal
c = b;// legal but with a warning
b = a; // legal
a = c; // legal
Test<String> t1 = new Test<String>();
Test t2 = t1;
Test<?> t3 = t1;
Collection result = null;
result = t1.getMap().get(value); // returns Collection<String>
result = t2.getMap().get(value); // illegal, returns Object
result = t3.getMap().get(value); // returns Collection<?>
}
public static class Test<T> {
public Map<String, Collection<T>> getMap() {
return new HashMap<String, Collection<T>>();
}
}
}
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