Question

I tried using reflection to use a custom List View for an App with target api level 7. The necessary fileds are only available from api level 9 so I attempted to fix that via reflection.

I need to find the protected Method View.overScrollBy( int,int,int,int,int,int,int,int,boolean). When i call

View.getDeclaredMethods() 

and iterate over the Method[] array i find it, but when I try

View.class.getDeclaredMethod(String name, Class...< ? > paramTypes)

I get a NoSuchMethodException. I compared the hard coded Method Name and parameterType values with the values extracted from the method (found via iteration) and they are identical…

private boolean initCompatibility() 
{
Method[] methods = View.class.getDeclaredMethods();
try {
    // The name of the Method i am looking for;
    String OVERSCROLL_S = "overScrollBy";
    for (Method meth : methods) {
        if (meth.getName().equals(OVERSCROLL_S)) {
            mListView_overScrollBy = meth;
            break;
            // method found
        }
    }

    // Params for getDeclaredMethod(…)
    String methodName = "overScrollBy";
    Class[] methodParams =  {  Integer.TYPE, Integer.TYPE, Integer.TYPE, 
            Integer.TYPE, Integer.TYPE, Integer.TYPE, Integer.TYPE, 
            Integer.TYPE, Boolean.TYPE };

    // works
    Method test =  View.class.getDeclaredMethod(methodName,methodParams);
    // fails
    View.class.getDeclaredMethod(mListView_overScrollBy.getName(), 
               mListView_overScrollBy.getParameterTypes());

    /*
    * I also tried this way around and again the first worked and the second
    * failed, so the input arguments are not the problem...
    * View.class.getDeclaredMethod( mListView_overScrollBy.getName(), 
    *                           mListView_overScrollBy.getParameterTypes() );
    * Method test =  View.class.getDeclaredMethod(methodName,methodParams);
    */

    return true;
    } catch (SecurityException e) {
        e.printStackTrace();
        return false;
    } catch (NoSuchMethodException e) {
        e.printStackTrace();
        return false;
    }
}

I do not understand why the call always works the first time and the does not the second time. Interestingly it also fails when i call only once for View.class.getDeclaredMethod(String name, Class…< ? > paramTypes) and it does not make any difference whether i use the hard coded input values or the one extracted from the method I am looking for…

Does anybody know what the problem is? Thanks

Answer 1

This is very interesting, but it is not Android-specific, I think.

I wrote this small test in plain Java:

    public class ReflectionTest {

        public static void main(String[] args){
            Method[] m = ReflectionTest.class.getDeclaredMethods();
            for (Method method : m) {
                System.out.println(method.getName());
            }               

            try {
                Method m1 = ReflectionTest.class.getDeclaredMethod("d0", int.class, boolean.class);
                if(m1 != null){
                    System.out.println("m1 found!");
                }
            } catch (Exception e) {
                e.printStackTrace();
            } 

            try {
                Method m2 = ReflectionTest.class.getDeclaredMethod("d0", Integer.TYPE, Boolean.TYPE);
                if(m2 != null){
                    System.out.println("m2 found!");
                }
            } catch (Exception e) {
                e.printStackTrace();
            } 

            try {
                Class<?>[] carr = m[1].getParameterTypes();
                Method m3 = ReflectionTest.class.getDeclaredMethod("d0", carr);
                if(m3 != null){
                    System.out.println("m3 found!");
                }
            } catch (Exception e){
                e.printStackTrace();
            } 
        }

        public void d0(int a, boolean b){

        }
    }

In eclipse, if I debug it, the three m1,m2 and m3 are printed. However, if I run it, a NoSuchMethodException is thrown when trying to get m3.

UPDATES:

• Tested running with jre 7 under linux, and all three m1,m2 and m3 were printed. Perhaps is a problem with jre6? Or is eclipse run configuration?
• Changed carr declaration to use method 0 instead of 1: Class<?>[] carr = m[0].getParameterTypes(); as Gray suggested. Now it runs ok but throws exception in debug mode. This means different method order for the returned array m.
• Update #2 confirmed, I’ve included a for loop to print the method names. In run mode the order of the method array is reversed compared to debug mode.