filters not null fields of a bean (e.g. has getters for all of the fields)

Bean.java

public class Bean {

    private String a;
    private Integer b;
    private String c;
    private Integer d;

    public Bean( String a, Integer b, String c, Integer d ) {
        this.a = a;
        this.b = b;
        this.c = c;
        this.d = d;
    }

    public String getC() {
        return c;
    }

    public Integer getD() {
        return d;
    }

    public String getA() {
        return a;
    }

    public Integer getB() {
        return b;
    }
}

FieldFilter.java

import java.lang.reflect.Field;
import java.lang.reflect.Method;
import java.util.ArrayList;
import java.util.List;

public class FieldFilter {

    // should be somewhere in apache or just put it in other utils
    public static String capitalizeFirstLetter( String s ) {
        // if non empty / null
        return Character.toUpperCase( s.charAt( 0 ) ) + s.substring( 1 );
    }

    @SuppressWarnings( "unchecked" )
    public static boolean isExistingMethod( Class clazz, String name ) {

        try {
            clazz.getMethod( name );
            return true;
        }
        catch ( NoSuchMethodException nsme ) {
            return false;
        }
    }

    @SuppressWarnings( "unchecked" )
    public static List<String> notNullFields( Class clazz, Object obj ) {

        // use guava to create the list
        List<String> nullFields = new ArrayList<String>();

        for ( Field f: clazz.getDeclaredFields() ) {

            String getterName = "get" + FieldFilter.capitalizeFirstLetter ( f.getName() );

            if ( isExistingMethod( clazz, getterName ) ) {
                try {

                    Method method = clazz.getMethod( getterName );

                    Object value = method.invoke( obj, ( Object[] ) null );

                    if ( value == null )
                        nullFields.add( f.getName() );

                }
                catch ( Throwable t ) {
                    throw new RuntimeException( "could not read " + clazz + " field values", t );
                }
            }
        }

        return nullFields;
    }

    public static void main( String[] args ) {

        // should return [a, b]
        System.out.println(
                FieldFilter.notNullFields( Bean.class,
                        new Bean( null, null, "No", 42 ) ) );

        // should return [b, c]
        System.out.println(
                FieldFilter.notNullFields( Bean.class,
                        new Bean( "Y", null, null, 42 ) ) );

        // should return [a, b, c, d]
        System.out.println(
                FieldFilter.notNullFields( Bean.class,
                        new Bean( null, null, null, null ) ) );

        // should return [a, d]
        System.out.println(
                FieldFilter.notNullFields( Bean.class,
                        new Bean( null, 0, "", null ) ) );

    }
}