What are the potential trips/traps in SCJP exam?
Answer: Provided by Ajith Kallambella (From Java Ranch: http://www.javaranch.com)
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Two public classes in the same file.
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Main method calling a non-static method.
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Methods with the same name as the constructor(s).
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Thread initiation with classes that dont have a run()
method.
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Local inner classes trying to access non-final vars.
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Case statements with values out of permissible range.
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Math class being an option for immutable classes !!
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instanceOf is not same as instanceof
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Private constructors
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An assignment statement which looks like a comparison if (
a=true)...
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System.exit() in try-catch-finally blocks.
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Uninitialized variable references with no path of proper
initialization.
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Order of try-catch-finally blocks matters.
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main() can be declared final.
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-0.0 == 0.0 is true.
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A class without abstract methods can still be declared
abstract.
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RandomAccessFile descends from Object and implements
DataInput and DataOutput.
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Map doesnot implement Collection.
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Dictionary is a class, not an interface.
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Collection is an Interface where as Collections is a
helper class.
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Class declarations can come in any order ( derived first,
base next etc. ).
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Forward references to variables gives compiler error.
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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.
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Arrays, whether local or class-level, are always
initialized,
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Strings are initialized to null, not empty string.
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An empty string is NOT the same as a null string.
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A declaration cannot be labelled.
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continue must be in a loop( for, do , while ). It cannot
appear in case constructs.
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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.
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A constructor can throw any exception.
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Initilializer blocks are executed in the order of
declaration.
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Instance initializer(s) gets executed ONLY IF the objects
are constructed.
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All comparisons involving NaN and a non-Nan would always
result false.
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Default type of a numeric literal with a decimal point is
double.
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integer (and long ) operations / and % can throw
ArithmeticException while float / and % will never, even in case of
division by zero.
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== gives compiler error if the operands are
cast-incompatible.
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You can never cast objects of sibling classes( sharing the
same parent ), even with an explicit cast.
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.equals returns false if the object types are different.It
does not raise a compiler error.
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No inner class can have a static member.
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File class has NO methods to deal with the contents of the
file.
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InputStream and OutputStream are abstract classes, while
DataInput and DataOutput are interfaces.
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