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SCJP考题中的陷阱 |
最近在网上闲逛时偶然发现此文章,基本上指出了SCJP的出题陷阱,读过后觉得其列的还比较详细,故贴与此,希望能对打算考SCJP的朋友有一些帮助.此文虽是E文,但句法简单,很容易理解,而且我在大部分后面加了一些注释"()",对新手可能会有帮助.
SCJP考题中的陷阱
原著 Ajith Kallambella
(1) Two public classes in the same file. (illegal)
(2) Main method calling a non-static method. (illegal)
(3) Methods with the same name as the constructor(s). (这种题常有)
(4) Thread initiation with classes that do not have a run() method. (常考之题)
(5) Local inner classes trying to access non-final vars. (illegal)
(6) Case statements with values out of permissible range. (byte,int, short, chat)
(7) Math class being an option for immutable classes !! (totally wrong!)
(8) instanceOf is not same as instanceof.
(9) Private constructors. (legal)
(10) An assignment statement which looks like a comparison. (比如说if(a=true),对于这种题眼睛亮一点)
(11) System.exit() in try-catch-finally blocks. (finally 不会执行)
(12) Order of try-catch-finally blocks matters. (若顺序错的话: error: No try before catch)
(13) main() can be declared final. (OK)
(14) -0.0 == 0.0 is true.
(15) A class without abstract methods can still be declared abstract.
(16) RandomAccessFile descends from Object and implements DataInput and DataOutput.
(17) Map does not implement Collection.
(18) Dictionary is a class, not an interface.
(19) Collection is an Interface where as Collections is a helper class. (这题我倒没见过,但还真容易看混)
(20) Class declarations can come in any order.
(也就是说: class Child extends Parents{}
class Parents{}
这种顺序是可以的.)
(21) Forward references to variables gives compiler error.
(22) Multi dimensional arrays can be sparce.
(这句话是说: 多维数组中子数组不一定必须有一定个数的元素,比如我们把一个二维数组看成一个矩阵,那么行与列中的元素可以不完整,可以不对齐.)
(23) Arrays, whether local or class-level, are always initialized.
(24) Strings are initialized to null, not empty string.
(25) An empty string is NOT the same as a null string.
(26) A declaration cannot be labelled.
(27) "continue" must be in a loop(for, do, while). It cannot appear in case constructs.
(28) Primitive array types can never be assigned to each other, eventhough the primitives themselves can be assigned.
(也就是说: ArrayofLongPrimitives = ArrayofIntegerPrimitives 会编译出错,但 longvar = intvar 是合法的)
(29) A constructor can throw any exception.
(30) Initilializer blocks are executed in the order of declaration.
(31) Instance initializer(s) gets executed ONLY IF the objects are constructed.
(32) All comparisons involving NaN and a non-Nan would always result false. (对大多数朋友来说这可是个盲点噢)
(33) Default type of a numeric literal with a decimal point is double.
(我在这里把Java成员变量默认初始化原则写一下:
成员变量类型 取值
byte 0
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