Java开发网 - SCJP考题中的陷阱

Topic: SCJP考题中的陷阱

1.SCJP考题中的陷阱

Posted by: nerd
Posted on: 2003-05-17 21:07

SCJP考题中的陷阱原著 Ajith Kallambella [from:chinajavaworld]
==========
(1) Two public classes in the same file. (illegal)
同一个文件里有两个public类。(非法)
(2) Main method calling a non-static method. (illegal)
在main(String[] args)方法内调用一个非静态方法。(非法)
(3) Methods with the same name as the constructor(s). (这种题常有)
与Constructor(s)有相同名字的方法。
(4) Thread initiation with classes that do not have a run() method. (常考之题
)
初始化了一个没有run()方法的线程类。
(5) Local inner classes trying to access non-final vars. (illegal)
内部类尝试访问非final变量(非法)
(6) Case statements with values out of permissible range. (byte,int, short,
chat)
选择语句case中，没有使用允许的值。如(byte,int,short,char)等
(7) Math class being an option for immutable classes !! (totally wrong!)
Math类作为不可改变类。(完全错误) (请提意见)
(8) instanceOf is not same as instanceof.
instanceOf 不是 instanceof。
(9) Private constructors. (legal)
私有的Constructor。 (合法)
(10) An assignment statement which looks like a comparison.
一个赋值语句看起来像比较语句。
比如说if(a=true),和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)
在try-catch-final块中的退出语句。 (finally不会执行)
(12) Order of try-catch-finally blocks matters. (若顺序错的话: error: No try
before catch)
try-catch-final块的顺序问题。
(13) main() can be declared final. (OK)
main()方法可以声明为 final.
(14) -0.0 == 0.0 is true.
如题。
(15) A class without abstract methods can still be declared abstract
没有抽象方法的类，仍然可以定义为抽象类。
(16) RandomAccessFile descends from Object and implements DataInput and Data
Output.
RandomAccessFile 类继承Object，并且实现了DataInput和DataOutput接口。
(17) Map does not implement Collection.
Map 并不实现 Collection.
(18) Dictionary is a class, not an interface.
Dictionary 是一个类，不是接口。
(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)
Collection是一个接口，但 Collections却是一个辅助类。
(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.)
可以以任何顺序申明类。
(21) Forward references to variables gives compiler error.
把 reference 给变量，会产生编译错误。 (请提意见)
(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.
数组，无论是当前的，还是类等级的，都会被初始化。
(24) Strings are initialized to null, not empty string.
String 是被初始化为 null,不是空字符。
(25) An empty string is NOT the same as a null string.
一个空字符串不是一个null字符。
(26) A declaration cannot be labelled.
一个声明语句不能被标记。
(27) "continue" must be in a loop(for, do, while). It cannot appear in case
constructs.
“continue”已经要在一个循环里(如for,do,while)，它不能在case语句中出现。

(28) Primitive array types can never be assigned to each other, eventhough t
he primitives themselves can be assigned.
(也就是说: ArrayofLongPrimitives = ArrayofIntegerPrimitives 会编译出错,但
longvar = intvar 是合法的)
Primitive(int,char,long等)数组是不能互相赋值的，即使它们本身可以。
(29) A constructor can throw any exception.
一个Constructor可以抛出任何异常。
(30) Initilializer blocks are executed in the order of declaration.
初始化块是按照声明的顺序执行的。
(31) Instance initializer(s) gets executed ONLY IF the objects are construct
ed.
实例初始化语句块只有在它建立后才会被执行。
(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)
所有关于 NaN(Not a Number) 和 non-NaN 的比较，都返回false.
这条很重要。
(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 wh
ile float / and % will never, even in case of division by zero.
integer和long 操作 /和% 的话，会抛出ArithmeticException，但是 float形不
会，即使是除以0。
(35) == gives compiler error if the operands are cast-incompatible.
==会产生编译错误，如果两边不兼容的话。
(36) You can never cast objects of sibling classes( sharing the same parent
), even with an explicit cast.
你永远不可能转化具有同一个超类的类的对象，即使是刻意转化。class A
class sonA extends A

class daughterA extens A
对这种情况：
sonA 和 daughterA 之间不能相互转化。
即：sonA son = (sonA) daughterA();是非法的。
而：sonA son = (sonA) A();A father = (A) sonA();是合法的。
(37) .equals returns false if the object types are different.It does not rai
se a compiler error.
equals() 返回 false 如果对象类型不同，但不产生编译错误。
(38) No inner class can have a static member.(but static inner class can)
没有内部类可以拥有静态成员。(但静态内部类可以)
(39) File class has NO methods to deal with the contents of the file.(also t
he existing directory)
File类没有任何处理文件内容的方法。(当然，存在的目录也一样)
(40) InputStream and OutputStream are abstract classes, while DataInput and
DataOutput are interfaces.
InputStream 和 OutputStream 是抽象类，但是 DataInput 和 DataOutput是接
口。

2.Re:SCJP考题中的陷阱 [Re: nerd]	Copy to clipboard
Posted by: richzh Posted on: 2003-06-03 20:48 不错！谢谢！

3.Re:SCJP考题中的陷阱 [Re: nerd]	Copy to clipboard
Posted by: yamakasy Posted on: 2003-06-03 22:20 好像有点OLD了！

4.Re:SCJP考题中的陷阱 [Re: nerd]	Copy to clipboard
Posted by: nerd Posted on: 2003-06-05 02:58 还好不是toooooooooooooold

5.Re:SCJP考题中的陷阱 [Re: nerd]	Copy to clipboard
Posted by: 小新 Posted on: 2003-06-13 14:41 xie xie la