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Exception in thread "main" java.lang.NullPointerException

 
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$ javac Vertex.java
$ javac DFS.java
$ java DFS

Exception in thread "main" java.lang.NullPointerException
at DFS.toplogicalSort(DFS.java:71)
at DFS.main(DFS.java:80)
 
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IntelliJ IDE VI Editor Ubuntu
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Albert, you forgot to ask a question.

Just out of curiosity, why are you only using positions 1 - 5 in your Vertex array? The first index in a Java array is 0.
 
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A constructor does not have return type void in the signature.
It should be:

 
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Does the API description for this method help?
 
Consider Paul's rocket mass heater.
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