Traps in SCJP exam

What are the potential trips/traps in SCJP exam?

Answer: Provided by Ajith Kallambella (From Java Ranch: http://www.javaranch.com)

• 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 dont have a run() method.


• Local inner classes trying to access non-final vars.


• Case statements with values out of permissible range.


• Math class being an option for immutable classes !!


• instanceOf is not 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 doesnot 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 gives compiler error.


• Multi dimensional arrays can be sparce ie., 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, eventhough the primitives themselves can be assigned. ie., ArrayofLongPrimitives = ArrayofIntegerPrimitives gives compiler error eventhough longvar = intvar is perfectly valid.


• A constructor can throw any exception.


• Initilializer 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 would always result 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.