Examples - leangen/geantyref GitHub Wiki

Examples

Check if a type is a supertype of another type, including type parameters

You use TypeToken to refer to specific types, and GenericTypeReflector.isSuperType to check if a type is a supertype of another. Given the following interface and classes:

interface Processor {
    void process(T t);
}

class StringProcessor implements Processor<String> {
    public void process(String s) {
        System.out.println("processing " + s);
    }
}

class IntegerProcessor implements Processor<Integer> {
    public void process(Integer i) {
        System.out.println("processing " + i);
    }
}

We can check that a certain class is the right kind of Processor:

/**
* Returns true if processorClass extends Processor<String>
*/
public boolean isStringProcessor(Class<? extends Processor<?>> processorClass) {
    // Use TypeToken to get an instanceof a specific Type
    Type type = new TypeToken<Processor<String>>(){}.getType();
    // Use GenericTypeReflector.isSuperType to check if a type is a supertype of another return
    GenericTypeReflector.isSuperType(type, processorClass);
}

For example:

  • isStringProcessor(StringProcessor.class) returns true, because StringProcessor extends Processor<String>.
  • isStringProcessor(IntegerProcessor.class) return false, because IntegerProcessor doesn't extend Processor<String> but Processor<Integer>

Get the exact return type of a method, including resolved generic type variables

Given the following classes:

abstract class Collector { public List list() { ... } public void add(T item) { ... } }

class StringCollector extends Collector<String> {
}

The return type of the Collector.list() method is List<T>. For StringCollector, this would be List<String>. But, if we simply get the return type from Java, we get List<T>:

Method listMethod = StringCollector.class.getMethod("list");
Type returnType = listMethod.getGenericReturnType();

Then returnType is List<T>, not what we want. To get the exact return type, use GenericTypeReflector.getExactReturnType:

Type exactReturnType = GenericTypeReflector.getExactReturnType(listMethod, StringLister.class);

Now exactReturnType is List<String>.

Get the exact types the parameters of a method, including resolved generic type variables

Continuing with the sample above, if you ask Java, the parameter for the add method has type T:

Method addMethod = StringCollector.class.getMethod("add", Object.class);

// returns [T]
Type[] parameterTypes = addMethod.getGenericParameterTypes();

You can get the exact parameter type using GenericTypeReflector.getExactParameterTypes:

// returns [String]
Type[] exactParameterTypes = GenericTypeReflector.getExactParameterTypes(addMethod, StringCollector.class);
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