Method calling

Question

Hello,
I have a problem in calling a method in say ClassA from ClassB. In my scenario, ClassA knows the name of the calling class, namely ClassB as it has been stored only as a string variable.
So, In ClassA, I have a declaration statement like
String name=classB;
And I also know the method in classB to be called, say classB.run( ). But, I cant create an object of classB and call its method directly from classA, as I have the classname (classB) only as a value of string variable in classA. This is because, the classname (classB) is known only during runtime, thatis in my case, the classname is a value of an attribute in a xml file that is collected during parsing and rendering stage.
So, can anyone please guide to further proceed with my work.
Thank you,
Shiv.

Answer 1

You can use the reflection API for this:
     * Instantiate Java object from a class by calling the constructor that matches the given parameters.
     * @param className
     * @param classes Constructor parameter types (class names).
     * @param values Constructor parameter values.
     * @return Java object.
     * @throws ClassNotFoundException if class can't be found in classpath.
     * @throws NoSuchMethodException if there's no constructor with the given types.
     * @throws InvocationTargetException if the constructor throws an exception.
     * @throws IllegalAccessException if access to the constructor is denied.
     * @throws InstantiationException if the class is abstract and therefore can't be instantiated.
     * @since 1.8.3
    static public Object getInstance(String className, Class[] classes, Object[] values) throws ClassNotFoundException, NoSuchMethodException, InvocationTargetException, IllegalAccessException, InstantiationException {
        Object result = null;
        if (className != null) {
            Class c = Class.forName(className);
            if (classes != null && values != null) { //constructor with arguments:
                Constructor cc = c.getConstructor(classes);
                result = cc.newInstance((Object[]) values);
            } else { //empty constructor:
                result = c.newInstance();
        }//else: input unavailable
        return result;
    }//getInstance()
     * Call a (non-static) Java method.
     * @param instance Java object created via the instantiate() method.
     * @param methodName Name of instance method.
     * @param classes Optional method parameter types.
     * @param values Optional method parameter values.
     * @return Result of method call.
     * @throws ClassNotFoundException if parameter type can't be found in classpath.
     * @throws NoSuchMethodException if <i>methodName</i> is invalid.
     * @throws InvocationTargetException if the method throws an exception.
     * @throws IllegalAccessException if access to the method is denied.
     * @since 1.8.3
    static public Object call(Object instance, String methodName,  Class[] classes, Object[] values) throws ClassNotFoundException, NoSuchMethodException, InvocationTargetException, IllegalAccessException {
        Object result = null;
        if (instance != null && methodName != null) {
            Method method = instance.getClass().getMethod(methodName, classes);
            result = method.invoke(instance, (Object[]) values);
        }//else: input unavailable
        return result;
    }//call()