Java variable type from string

Question

this is my code

Class<?> clazz;
    try {
        clazz = Class.forName("classes.Pizza");
        Object p2 = clazz.newInstance();
        System.out.println(p2.test);
    } catch (ClassNotFoundException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    } catch (InstantiationException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    } catch (IllegalAccessException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }

Error is "test cannot be resolved or is not a field" I want to get a string containing the class name and create a object with that type.. something like

String x = "Pizza"
x pizza1 = new x();

How do i do that ?

cast object to a specific type before you call a method on it. — LeTex
– LeTex, Commented Jun 7, 2015 at 13:31
Hint: what is the type of p2 available for compiler? Does this type have test field? — Pshemo
– Pshemo, Commented Jun 7, 2015 at 13:32
@user3770536 Then how do you know, that "the other class name" has the field test? Or any other field you like to access? — Tom
– Tom, Commented Jun 7, 2015 at 13:41
I have a project for school. Basically i created a class Products and other classes (Pizza, Calzone, etc) that have different attributes and i want to export them into txt. And on the import i want to get a attribute named Type which will help me determine what kind of object is that, pizza, calzone or etc to know what objects to create — John
– John, Commented Jun 7, 2015 at 14:03
That's a much better description for your problem. Clearly, you're not going to read or write any Java class to your text file: you only care about Pizza or Calzone. Those classes could implement a common interface with a method void import(String fields) and String export() to read their fields from a string or write fields to a string. They are responsible for correctly reading/writing their own fields, so you don't have to worry about that outside of those classes. I'm skipping over a lot of details here, but I feel like I'm already getting a bit off-topic... — Mattias Buelens
– Mattias Buelens, Commented Jun 7, 2015 at 16:00

maskacovnik · Accepted Answer · 2015-06-07 13:50:46Z

2

You have to cast Object to Pizza object:
This could cause ClassCastException too:

Class<?> clazz;
try {
    clazz = Class.forName("classes.Pizza");
    Object p2 = clazz.newInstance();
    System.out.println(((Pizza)p2).test);
} catch (ClassNotFoundException e) {
    e.printStackTrace();
} catch (InstantiationException e) {
    e.printStackTrace();
} catch (IllegalAccessException e) {
    e.printStackTrace();
} catch (ClassCastException e) {
    e.printStackTrace();
}

EDIT: You cant access the field test, when you dont know about it. So you can access the field:

Class<?> clazz;
try {
    clazz = Class.forName("classes.Pizza");
    Object p2 = clazz.newInstance();
    /*System.out.println(((Pizza)p2).test);*/
    System.out.println(clazz.getDeclaredField("test").get(p2));
} catch (ClassNotFoundException e) {
    e.printStackTrace();
} catch (InstantiationException e) {
    e.printStackTrace();
} catch (IllegalAccessException e) {
    e.printStackTrace();
} catch (NoSuchFieldException e) {
    e.printStackTrace();
}

Writed by javadoc only, not tested (and not sure about exception in this case)

edited Jun 7, 2015 at 13:50

answered Jun 7, 2015 at 13:29

maskacovnik

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4 Comments

John Over a year ago

yes. With cast it works.. but i want to create object according to the String x. I only have that string so i can't hardcode the cast to Pizza because that string can be another class

Mattias Buelens Over a year ago

@user3770536 How will you be sure that class has a test field? What should happen if the class with name x doesn't have a test field (e.g. what if x equals "java.lang.Object")?

maskacovnik Over a year ago

It will throw an NoSuchFieldException @MattiasBuelens

Mattias Buelens Over a year ago

@maskacovnik I know that's what will happen. I'm asking if that's what OP wants to happen. Sure, you can abuse reflection to get away with anything, but most of the time it's a band-aid solution for the wrong problem.

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