My if statement which worked before no longer works

I am trying to create an if else statement that checks multiple values, one after another, to see whether they are null or not.

The if else statement that I created worked perfectly earlier, but after restarting my pc it no longer works.

Any help is appreciated.

Here is my code:

    //Declare Strings
    String ProjectNo, Location, VP;

    //Store values from text boxes into strings
    ProjectNo = txtProjectNum.getText();
    Location = txtLocation.getText();
    VP = txtVPNo.getText();


    if(ProjectNo == null){

        JOptionPane.showMessageDialog(null, "No fields can be left empty!");

    }

    else if (Location == null){

        JOptionPane.showMessageDialog(null, "No fields can be left empty!");

    }

    else if (VP == null){

        JOptionPane.showMessageDialog(null, "No fields can be left empty!");

    }

    else{

        JOptionPane.showMessageDialog(null, "All fields have been filled in!");

    }