Get arguments types from Constructor object (Java Reflection)

Question

I try to create an object out of class class (via java reflection) - user choose a constructor and I need to read variables for this constructor (it supposed to be some of the primitive types).
As you can see now my code works only for integer params.

How can I dynamically figure out type of the current argument and read it from the keyboard?

 public static Object chooseConstr(Class cls) throws IllegalAccessException, InstantiationException, InvocationTargetException {
    Scanner keyboard = new Scanner(System.in);

    Constructor[] constructors = cls.getDeclaredConstructors();
    System.out.println("Choose constructor from the list: ");
    System.out.println(Arrays.toString(constructors));

    int constr = keyboard.nextInt();
    constr--;


    Object[] arguments=new Object[constructors[constr].getParameterCount()];

    for(int i=0; i<arguments.length; i++){
        System.out.println("Write argument #"+(i+1));
        arguments[i]=keyboard.nextInt();
    }


    Object object = constructors[constr].newInstance(arguments);

    System.out.println("Object created!");
    return object;
}

Jorn Vernee · Accepted Answer · 2016-04-05 20:02:11Z

2

This will be tricky. Of course scanner has some basic methods to read in some types, but if you want to read other kinds of arguments you'll have to figure out a way to read them yourself.

For this, you can use Constructor.getParameterTypes():

...
Object[] arguments = new Object[constructors[constr].getParameterCount()];

Class<?>[] pTypes = constructors[constr].getParameterTypes();

for (int i = 0; i < arguments.length; i++) {
    System.out.println("Write argument #" + (i + 1) + ". Of type: "+pTypes[i].getName());

    if(pTypes[i].equals(int.class)) {               
        arguments[i] = keyboard.nextInt(); // read an int
    } else if(pTypes[i].equals(String.class)) {
        arguments[i] = keyboard.next(); // read a String
    } else {
        // custom read code for other types
    }
}
...

Note that the above is not complete, scanner has more methods to read other types too, which I didn't show here.

The best strategy for reading other types of arguments, is probably to read them as a string and convert them to the corresponding objects through some factory method.

edited Apr 5, 2016 at 20:02

answered Apr 5, 2016 at 19:53

Jorn Vernee

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1 Comment

Jorn Vernee Over a year ago

I just noticed that you only need it for primitives, so this should be simpler than I said. Just note that it can works for other types too.

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