Question

I came across these lines of code:

import org.apache.shiro.util.Factory;
import org.apache.shiro.config.IniSecurityManagerFactory;
import org.apache.shiro.mgt.SecurityManager;

.....

Factory<SecurityManager> factory = new IniSecurityManagerFactory("classpath:shiro.ini");
        SecurityManager securityManager = factory.getInstance();

Can you explain this code in particular what Factory<SecurityManager> means and why there is Generics reference <SecurityManager> and why do I need it?

Answer 1

Generics are compile time markers to provide your compiler typing informations and templates. They don’t exist in runtime. You could even ignore generic information in your code:

Factory factory = new IniSecurityManagerFactory("classpath:shiro.ini");

In this case you get a small warning from your IDE (Eclipse) because of unchecked type, but you can supress it with SupressWarning annotation.

@SuppressWarnings("unchecked")
Factory factory = new IniSecurityManagerFactory("classpath:shiro.ini");

Your code will be still compiled without any problem.
For an example you have an Calculate class:

class Calculate<T extends Number> {
     private T value;
     public T add(T value1, T value2) {
          return 
     }
}

that’s a template and you can use your calculation utility with every type extends Number.

Calculate<Integer> calculator = new  Calculate<Integer>();
calculator.add(1 , 2);

or a double version;

Calculate<Double> calculator = new  Calculate<Double>();
calculator.add(1.2d , 2.3d);

Generics make your code flexible and let you write such templates.

Factory is a idiom in Software Engineering to create objects using these factory classes. I would make a research about Factory Pattern, Dependency Inversion , Holywood Principle.
There are many threads around here about (Abstract) Factory Pattern or even on Wikipedia.