通过一段程序来说明其差异：

public class OptionalTest {
    static String B(){
        System.out.println("B()...");
        return "B";
    }

    public static void main(String[] args) {

        System.out.println(Optional.of("A").orElse(B()));
    }
}

执行结果为：

B()...
A

Process finished with exit code 0

public class OptionalTest {
    static String B(){
        System.out.println("B()...");
        return "B";
    }

    public static void main(String[] args) {
        System.out.println(Optional.of("A").orElseGet(()->B()));
    }
}

执行结果为：

A

Process finished with exit code 0