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SCJP考题中的陷阱
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| 2005-08-22 来源:CSDN 作者: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
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.
当然以上这些都是从题目中总结出来的技巧,其实书上也都有,大家还是多看看书噢. 又大致翻译了一下，有意见可以提出来大家讨论
(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 DataOutput.
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.)
如jiayuan所说。
(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 the 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。
解释见jiayuan所说。
(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。(请提意见)
(31) Instance initializer(s) gets executed ONLY IF the objects are constructed.
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 while 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。(请提意见)
(37) .equals returns false if the object types are different.It does not raise 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 the 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。
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