Retrieving class variables through usage of `.class`

Question

Is it possible to access a public variable in a class while if the only available access to that class is its .class.
As an example if I have A_VARIABLE in both ClassA and ClassB, if I put the reference to each class Object into an HashSet, am I able to retrieve the A_VARIABLE for each item in the HashSet.

public class ClassA{
    public static final String A_VARIABLE = "ABC";
}

public class ClassB{
    public static final String A_VARIABLE = "123";
}


public class ClassC{
    private void someMethod(){
        HashSet<Class> someClassHashSet = new HashSet<>();
        someClassHashSet.add(ClassA.class);
        someClassHashSet.add(ClassB.class);

        for (Class someClass : someClassHashSet){
            System.out.println("The value is: " + /*someClass.A_VARIABLE*/);
        }
    }
}

Which I would like to print:

The Value is: ABC
The Value is: 123

ClassA.A_VARIABLE (it's a constant field) and an equals on the class should do the trick — user180100
– user180100, Commented Oct 19, 2015 at 5:48
@RC wouldn't this mean that he needs to cast his Class objects...? — schneida
– schneida, Commented Oct 19, 2015 at 5:53

schneida · Accepted Answer · 2015-10-19 05:51:21Z

3

Yes, you can do that, you'll need reflection which is slower than direct access but still used widely:

for (Class someClass : someClassHashSet){
        Object value = null;
        try {
            value = someClass.getField("A_VARIABLE").get(null);
        } catch (IllegalAccessException e) {
            e.printStackTrace();
        } catch (NoSuchFieldException e) {
            e.printStackTrace();
        }
        System.out.println("The value is: " + value);
}

You see that this code gets the value of a field, that you only reference by a string. The null in .get(null) is because your fields are static. If they were not, you would have to provide an instance of which you'd like to get the value.

Note: Your someMethod() is missing a return value, eg void.

answered Oct 19, 2015 at 5:51

schneida

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Comments

Rahul Yadav · Accepted Answer · 2015-10-19 05:55:42Z

1

You can do it following way

 for (Class someClass : someClassHashSet){
         if (ClassA.class.equals(someClass)) {
            System.out.println("The value is: " + ClassA.A_VARIABLE );  
          } else if (ClassB.class.equals(someClass)) {
            System.out.println("The value is: " + ClassB.A_VARIABLE );
          }
 }

answered Oct 19, 2015 at 5:55

Rahul Yadav

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