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Which methods are you calling? Do each of them have a println() at the start of the method?

To call a method, you must have a reference to an instance of the class with the method and use that:...

What was printed out when you added all the println() calls I suggested to the code?
You need to add lots of println() statements to see what the code is doing.
The code you posted looks like it...

Try debugging the code by adding println() statements that print out the value of shipment at all locations in the code where the value of the variable: shipment is changed. You need to see if and...

Just print the value of the shipment variable every time it is changed:
System.out.println("s1="+shipment);
change the digit to be unique in each call to println() so you can tell which println()...

Try debugging the code by adding some println() statements that print out the value of the shipment variable every time its value is changed and set to see why it has a null value.

Please copy and paste the full text of the error message.
If you know what line the variable with the null value is on, look at that line, find the variable with the null value and then backtrack in...