Writing A Test Class Using (invoke) Method

I had to write a class called Thermometer, that has one instance variable (an integer) for the temperature in Fahrenheit. I had to include the following methods

-a constructor that initializes the temperature to 60

-there is a method to change the temperature

-there is a method to display the temperature

-there is a method to reset the teperature to 60

Here is the code for that.

public class Thermometer {
private int temp;
private int thermometer;
public Thermometer() {
thermometer = 60;

[code]....

Now I get to the issue. I have to write a test class called thermometer to test the thermometer class. I need to test each method while displaying the temperature after it. My professor said I should use the invoke method but didn't go into much more detail than that.