Java开发网 - 【经验】 SCJP考题中的陷阱<转贴>

Topic: 【经验】 SCJP考题中的陷阱<转贴>

1.【经验】 SCJP考题中的陷阱<转贴>

Posted by: yamakasy
Posted on: 2002-07-22 21:16

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 Moon

. (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)

Devil

Case statements with values out of permissible range. (byte,int, short, chat)

(7) Math class being an option for immutable classes !! (totally wrong!)

Musical Note

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 Moon

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
                       short 0
                       int 0
                       long 0L
                       char '\u0000'
                       float 0.0F
                       double 0.0D
                       boolean false
                       所有引用类型 null )

(34) integer (and long ) operations / and % can throw ArithmeticException while float / and % will never, even in case of division by zero.

(35) == gives compiler error if the operands are cast-incompatible.

(36) You can never cast objects of sibling classes( sharing the same parent ), even with an explicit cast.

(37) .equals returns false if the object types are different.It does not raise a compiler error.

(38) No inner class can have a static member.(but static inner class can)

(39) File class has NO methods to deal with the contents of the file.(also the existing directory)

(40) InputStream and OutputStream are abstract classes, while DataInput and DataOutput are interfaces.