Thread: Envrionment Variable & Properties

I was doing a program asking the following: Write a Java program to get the value of the environment variable USERNAME.

Si I wrote the following code :

// getting username using System.getProperty in Java
        String user = System.getProperty("user.name");
        System.out.println(user); //print sid

After checking solution of exercise they wrote the following code :

// get the value of the specified environment variable USERNAME
        System.out.println(System.getenv("USERNAME")); // print null

My question is what is the difference between environment variable and property?

I did but I want to have someone else explanation. I got my own understanding but it seems not that good since what I suggested as an answer (my code) is not correct