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Here's a technique that will be helpful. Add a toString() method to the Node class so when you print a reference to it, it will show all the data in the Node object.

Another hint: add ID Strings...

If you have an interactive debugger try using that.
Otherwise add lots of printlns to show the value of all the variables as the code executes to see what it is doing.
If you understand what the...

Have you found out why the variable is null yet?

Why is the variable at line 46 null?

Please post the full text of the error message.

Look at the line where the error occurs and find the variable with the null value. Then backtrack in the code to see why that variable does not have...