[wsaryada]

A class cannot be static, but for you purposes you can set the attributes and methods to be static. This way you can access it without create an instance.

Code:

public class Constant {
    public static final String ERROR = "1";

    public static String calculate(int number, int number) {
        //
    }
}

public class Test {
    public static void main(String... args) {
        String error = Constant.ERROR;
        Constant.calculate(1, 10);
    }
}