how to get a constant in java with class

Question

basically I need to get a constant for a class however I have no instance of the object but only its class. In PHP I would do constant(XYZ); Is there a similar way

Answer 1

If you don't want to actually load or initialize the class, ClassGraph can do this:

String fieldVal;
try (ScanResult scanResult =
        new ClassGraph().whitelistClasses(className).enableFieldInfo().scan()) {
    fieldVal = (String) scanResult.getClassInfo(className).getFieldInfo("s")
            .getConstantInitializerValue();
}

N.B. this works only for constant initializer values (values that can be computed without object instantiation, with the exception of String), assigned to static final fields. The compiler can produce a constant at compiletime for simple arithmetic and simple string concatenation.

(disclaimer, I am the author of ClassGraph)

Simply using reflection to access static fields is simpler, if you don't mind loading the class, as given in this other answer: https://stackoverflow.com/a/4076792/3950982

Answer 2

To get private constant like:

private static final String BAR = "test";

you need to set it accessible first:

Field barField = Foo.class.getDeclaredField("BAR");
barField.setAccessible(true);
String bar = (String) barField.get(null);

Answer 3

If this constant is metadata about the class, I'd do this with annotations:

First step, declare the annotation:

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
@interface Abc {
    String value(); 
}

Step two, annotate your class:

@Abc("Hello, annotations!")
class Zomg {

}

Step three, retrieve the value:

String className = "com.example.Zomg";
Class<?> klass = Class.forName(className);
Abc annotation = klass.getAnnotation(Abc.class);
String abcValue = annotation.value();
System.out.printf("Abc annotation value for class %s: %s%n", className, abcValue);

Output is:

Abc annotation value: Hello, annotations!

Answer 4

You might look for sth. like
Foo.class.getDeclaredField("THIS_IS_MY_CONST").get(null); or Class.forName("Foo").getDeclaredField("THIS_IS_MY_CONST").get(null); (thanks f-o-o)

Gets the value of a String constant (THIS_IS_MY_CONST) in class Foo.

Update use null as argument for get thanks acdcjunior

Answer 5

I am not sure what you want to get out. But it shouldn't be too difficult to show you an example.

Lets say you have a Class Foo with property bar.

Class Foo {
    private final String bar = "test";
    public String getBar() {return bar;}
}

Now to get this through reflection you would:

Class fooClass = Foo.class;
Object fooObj = fooClass.newInstance();
Method fooMethod = fooClass.getMethod("getBar");
String bar = (String) fooMethod.invoke(fooObj);

Now you will get value of method getBar() in bar variable

Answer 6

Maybe I don't understand what you need, but did you try with final static attributes and static methods?

Final means that it can't be changed once set, so you get a constant. Static means it's accessible even if there aren't any objects of the class.