What is the best way to know if all the variables in a Class are null?

This can be done fairly easily using a Lombok generated equals and a static EMPTY object:

import lombok.Data;

public class EmptyCheck {
    public static void main(String[] args) {
        User user1 = new User();

        User user2 = new User();
        user2.setName("name");

        System.out.println(user1.isEmpty()); // prints true
        System.out.println(user2.isEmpty()); // prints false
    }

    @Data
    public static class User {
        private static final User EMPTY = new User();

        private String id;
        private String name;
        private int age;

        public boolean isEmpty() {
            return this.equals(EMPTY);
        }
    }
}

Prerequisites:

• Default constructor should not be implemented with custom behavior as that is used to create the EMPTY object
• All fields of the class should have an implemented equals (built-in Java types are usually not a problem, in case of custom types you can use Lombok)

Advantages:

• No reflection involved
• As new fields added to the class, this does not require any maintenance as due to Lombok they will be automatically checked in the equals implementation
• Unlike some other answers this works not just for null checks but also for primitive types which have a non-null default value (e.g. if field is int it checks for 0, in case of boolean for false, etc.)

If you want this for unit testing I just use the hasNoNullFieldsOrProperties() method from assertj

assertThat(myObj).hasNoNullFieldsOrProperties();

Another non-reflective solution for Java 8, in the line of paxdiabo's answer but without using a series of if's, would be to stream all fields and check for nullness:

return Stream.of(id, name)
        .allMatch(Objects::isNull);

This remains quite easy to maintain while avoiding the reflection hammer.

Try something like this:

public boolean checkNull() throws IllegalAccessException {
    for (Field f : getClass().getDeclaredFields())
        if (f.get(this) != null)
            return false;
    return true;            
}

Although it would probably be better to check each variable if at all feasible.