I'm calling a method by reflection, and its return type is generic. I don't want the return value to be null, so I in that case I want to assign a default value of that generic type.
That is, after calling the method by reflection, I'd like to execute something like this:
T resultNotNull = (T) reflectionMethodThatCanReturnNull.invoke(anObject);
// If it's null, let's assign something assignable.
if (resultNotNull == null) {
if (resultNotNull.getClass().isAssignableFrom(Long.class)) {
resultNotNull = (T) Long.valueOf(-1);
} else if (resultNotNull.getClass().isAssignableFrom(String.class)) {
resultNotNull = (T) "NOTHING";
}
}
But, of course, if 'resultNotNull == null', we can't call 'resultNotNull.getClass()' without getting an exception.
Any ideas on how to assign a default value depending on the generic type?
The whole code would be something like this:
public class ReflectionTest {
public class Class1ok {
public Long method () { return Long.valueOf(1); }
}
public class Class1null {
public Long method () { return null; } // It returns null
}
public class Class2ok {
public String method () { return "SOMETHING"; }
}
public static void main(String[] args) {
ReflectionTest test = new ReflectionTest();
Long notNullValue1
= test.methodReturnNotNull(Class1ok.class); // 1
Long notNullValue2
= test.methodReturnNotNull(Class1null.class); // -1, not null
String notNullValue3
= test.methodReturnNotNull(Class2ok.class); // "SOMETHING"
}
public <T> T methodReturnNotNull(Class theClass) {
T resultNotNull = null;
try {
Object theObject = theClass.newInstance();
Method methodThatCanReturnNull = theClass.getMethod("method");
resultNotNull = (T) methodThatCanReturnNull.invoke(theObject);
} catch (Exception e) {
// Meh.
}
// If it's null, assign something assignable.
if (resultNotNull == null) {
if (resultNotNull.getClass().isAssignableFrom(Long.class)) {
resultNotNull = (T) Long.valueOf(-1);
} else if (resultNotNull.getClass().isAssignableFrom(String.class)) {
resultNotNull = (T) "NOTHING";
}
}
return resultNotNull;
}
}
You can use getReturnType();
private static final Map<Class, Object> replacementForNull = new HashMap<>();
static {
replacementForNull.put(String.class, "NOTHING");
replacementForNull.put(Long.class, 1L);
}
Method reflectionMethodThatCanReturnNull = ...
T resultNotNull = (T) reflectionMethodThatCanReturnNull.invoke(anObject);
// If it's null, let's assign something assignable.
if (resultNotNull == null) {
Class returnType = reflectionMethodThatCanReturnNull.getReturnType();
resultNotNull = replacementForNull.get(returnType);
}
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