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1. Check for any obvious complier errors first like two else statements , accessing ofprivate variable/method , accessing a variable declared in a for loop outside the for loop. 2. If you try to unbox a null Integer value to int , it will result in a NullPointerException. 3. If method having the same name as Class name and having a return type is not a constructor but a ordinary method. 4. Map is the only interface that does not extend Collection. List , Set and Queue extends the Collection interface. Collection is a interface , Collections is Class which extends Object class. Map: key , value pair Hashtable , HashMap , LinkedHashMap , TreeMap Set : unique values HashSet , TreeSet , LinkedHashSet List : List of values allows duplicates, accessed by index ArrayList , Vector , LinkedList Queue : stack or queue. to-do list. priority queue is sorted based on the natural order or based on the object of Comparator or Comparable.LinkedList , PriorityQueue The twins clase...