March 26, 2009

(TIPS & TRICKS) What are the potential trips/traps in the SCJP exam?

(TIPS & TRICKS) What are the potential trips/traps in the SCJP exam?

  • Two public classes in the same file.
  • Main method calling a non-static method.
  • Methods with the same name as the constructor(s).
  • Thread initiation with classes that don't have a run () method.
  • Local inner classes trying to access non-final variables.
  • Case statements with values out of permissible range.
  • Math class being an option for immutable classes!!
  • instanceOf is not the same as instanceof
  • Private constructors
  • An assignment statement which looks like a comparison if( a=true)...
  • System.exit() in try-catch-finally blocks.
  • Uninitialized variable references with no path of proper initialization.
  • Order of try-catch-finally blocks matters.
  • main () can be declared final.
  • -0.0 == 0.0 is true.
  • A class without abstract methods can still be declared abstract.
  • RandomAccessFile descends from Object and implements DataInput and DataOutput.
  • Map does not implement Collection.
  • Dictionary is a class, not an interface.
  • Collection is an Interface where as Collections is a helper class.
  • Class declarations can come in any order (derived first, base next etc.).
  • Forward references to variables give compiler error.
  • Multi dimensional arrays can be sparse i.e. if you imagine the array as a matrix, every row need not have the same number of columns.
  • Arrays, whether local or class-level, are always initialized
  • Strings are initialized to null, not empty string.
  • An empty string is NOT the same as a null string.
  • A declaration cannot be labelled.
  • Continue must be in a loop (for, do, while). It cannot appear in case constructs.
  • Primitive array types can never be assigned to each other, even though the primitives themselves can be assigned. i.e., ArrayofLongPrimitives = ArrayofIntegerPrimitives gives compiler error even though longvar = intvar is perfectly valid.
  • A constructor can throw any exception.
  • Initializer blocks are executed in the order of declaration.
  • Instance initializer(s) gets executed ONLY IF the objects are constructed.
  • All comparisons involving NaN and a non-Nan always result in false.
  • Default type of a numeric literal with a decimal point is double.
  • Integer (and long) operations / and % can throw ArithmeticException while float / and % will never, even in case of division by zero.
  • == gives compiler error if the operands are cast-incompatible.
  • You can never cast objects of sibling classes (sharing the same parent), even with an explicit cast.
  • .equals() returns false if the object types are different. It does not raise a compiler error.
  • No inner class can have a static member.
  • File class has NO methods to deal with the contents of the file.
  • InputStream and OutputStream are abstract classes, while DataInput and DataOutput are interfaces.


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