サンプル
public class NativeClass {

public NativeClass() {

}

public native int getQuantity();
public native Object getExternalValue(int index);

public List getResult() {
List list = new ArrayList(5);
for (int index = 0; index < getQuantity();index++){
list.add(getExternalValue(index));
}
return list;
}


}
解決策
修飾子を private に設定します。
public class NativeClass {

public NativeClass() {

}

private native int getQuantity();
private native Object getExternalValue(int index);

public List getResult() {
List list = new ArrayList(5);
for (int index = 0; index < getQuantity();index++){
list.add(getExternalValue(index));
}
return list;
}


}