几个谜题，深入的了解java

在2009年的JavaOne大会上，Joshua Bloch和Neal Gafter又为我们带来的7道谜题，挺有意思的。大家不妨看看。
摘自：
Return of the Puzzlers: Schlock and Awe
Joshua Bloch, Google, Inc.; Neal Gafter, Microsoft
http://developers.sun.com/learning/javaoneonline/sessions/2009/pdf/TS-5186.pdf
1.Life's Persistent Questions

Java code

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public
 
class
 SimpleQuestion {


static
 
boolean
 yesOrNo(String s) {

        s 
=
 s.toLowerCase();

if
 (s.equals(
"
yes
"
) 
||
 s.equals(
"
y
"
) 
||
 s.equals(
"
t
"
)) {

            s 
=
 
"
true
"
;

        }

return
 Boolean.getBoolean(s);

    }


public
 
static
 
void
 main(String[] args) {

        System.out.println(yesOrNo(
"
true
"
) 
+
 
"
 
"
 
+
 yesOrNo(
"
Yes
"
));

    }

}

问题：程序打印什么？
如果熟悉Boolean.getBoolean（）这个方法的话，应该不会出错。方法的功能参考文档。

2.Instruments of Tortue

Java code

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import
 java.util.Arrays;

import
 java.util.Collection;

import
 java.util.HashSet;


public
 
class
 InstrumentedHashSet
<
E
>
 
extends
 HashSet
<
E
>
 {

private
 
int
 addCount 
=
 
0
;

    @Override

public
 
boolean
 add(E e){

        addCount
++
;

return
 
super
.add(e);

    }


    @Override

public
 
boolean
 addAll(Collection
<?
 
extends
 E
>
 c){

        addCount 
+=
 c.size();

return
 
super
.addAll(c);

    }


public
 
static
 
void
 main(String[] args) {

        InstrumentedHashSet
<
String
>
 s 
=
 
new
 InstrumentedHashSet
<
String
>
();

        s.addAll(Arrays.asList(
"
Accordion
"
,
"
Banjo
"
,
"
Kazoo
"
));

        System.out.println(s.addCount);

    }

}

问题：打印结果是什么？

这个看第一遍可能会出错，不过也算容易理解。

3.Iterator Titillator

Java code

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import
 java.util.Iterator;

import
 java.util.NoSuchElementException;


public
 
abstract
 
class
 AbstractIterator
<
T
>
 
implements
 Iterator
<
T
>
 {


    T next 
=
 nextElement();


public
 
boolean
 hasNext() {

return
 next 
!=
 
null
;

    }


public
 T next() {

if
 (next 
==
 
null
) {

throw
 
new
 NoSuchElementException();

        }

        T result 
=
 next;

        next 
=
 nextElement();

return
 result;

    }


public
 
void
 remove() {

throw
 
new
 UnsupportedOperationException();

    }


protected
 
abstract
 T nextElement();


private
 
static
 Iterator
<
Character
>
 test(
final
 String s) {

return
 
new
 AbstractIterator
<
Character
>
() {


private
 
int
 cursor 
=
 
0
;


protected
 Character nextElement() {

return
 cursor 
==
 s.length() 
?
 
null
 : s.charAt(cursor
++
);

            }

        };

    }


public
 
static
 
void
 main(String[] args) {

for
 (Iterator
<
Character
>
 i 
=
 test(
"
OPS
"
); i.hasNext();) {

            System.out.print(i.next());

        }

    }

}

问题：输出结果是什么？

理解如何正确的设计Iterator。

4.Search for the One

Java code

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import
 java.util.ArrayList;

import
 java.util.Collections;

import
 java.util.Comparator;

import
 java.util.List;



public
 
class
 Searching {

public
 
static
 
void
 main(String[] args) {

        String[] strings 
=
 { 
"
0
"
, 
"
1
"
, 
"
2
"
, 
"
3
"
, 
"
4
"
, 
"
5
"
};


        List
<
Integer
>
 integers 
=
 
new
 ArrayList
<
Integer
>
();

for
(String s : strings){

            integers.add(Integer.valueOf(s));

        }


        System.out.println(Collections.binarySearch(integers, 
1
,cmp));

    }


static
 Comparator
<
Integer
>
 cmp 
=
 
new
 Comparator
<
Integer
>
(){

public
 
int
 compare(Integer i,Integer j){

return
 i
<
j
?-
1
:(i
==
j
?
0
:
1
);

        }

    };

}

问题：打印结果是什么？

如果看过《Java Puzzlers》这本书的话应该容易发现问题。

5.Cogito Ergo Sum

Java code

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import
 java.util.LinkedHashMap;

import
 java.util.Map;


public
 
enum
 RomanNumeral {


    I(
1
), V(
5
), X(
10
), L(
50
), C(
100
), D(
500
), M(
1000
);

private
 
static
 Map
<
Integer, RomanNumeral
>
 map 
=
 
new
 LinkedHashMap
<
Integer, RomanNumeral
>
();

public
 
final
 
int
 val;


    RomanNumeral(
int
 val) {

this
.val 
=
 val;

        storeInMap();

    }


private
 
void
 storeInMap() {

        map.put(val, 
this
);

    }


public
 
static
 RomanNumeral fromInt(
int
 val) {

return
 map.get(val);

    }


public
 
static
 
void
 main(String[] args) {

int
 sum 
=
 
0
;

for
 (
int
 i 
=
 
0
; i 
<
 
1000
; i
++
) {

if
 (fromInt(i) 
!=
 
null
) {

                sum 
+=
 i;

            }

        }

        System.out.println(sum);

    }

}

问题：打印结果是什么？

如果理解java加载类和创建对象的顺序的话这个问题容易理解。

6.Thread Friendly

Java code

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public
 
class
 ThreadFriendly {

    ThreadLocal
<
Value
>
 threadLocalPart 
=
 
new
 ThreadLocal
<
Value
>
();

class
 Value{

final
 
int
 i;

        Value(
int
 i){

this
.i 
=
 i;

        }

    }


    ThreadFriendly setThreadVal(
int
 i){

        threadLocalPart.set(
new
 Value(i));

return
 
this
;

    }


int
 getThreadVal(){

return
 threadLocalPart.get().i;

    }


public
 
static
 
void
 main(String[] args) {

int
 sum 
=
 
0
;

for
(
int
 i 
=
 
-
500000
;i
<=
500000
;i
++
){

            sum
+=
 
new
 ThreadFriendly().setThreadVal(i).getThreadVal();

        }

        System.out.println(sum);

    }

}

问题：打印结果是什么？

理解内部类和ThreadLocal。

7.When Words Collide

Java code

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public
 
class
 PrintWords {

public
 
static
 
void
 main(String[] args) {

        System.out.println(

            Words.FIRST 
+
 
"
 
"
 
+
 Words.SECOND 
+
 
"
 
"
 
+
 Words.THIRD

        );

    }

}


public
 
class
 Words{

public
 
static
 
final
 String FIRST 
=
 
"
the
"
;

public
 
static
 
final
 String SECOND 
=
 
null
;

public
 
static
 
final
 String THIRD 
=
 
"
set
"
;

}

编译PrintWords.java文件。
修改Words.java文件为

Java code

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public
 
class
 Words{

public
 
static
 
final
 String FIRST 
=
 
"
physics
"
;

public
 
static
 
final
 String SECOND 
=
 
"
chemistry
"
;

public
 
static
 
final
 String THIRD 
=
 
"
biology
"
;

}

问题：再次编译运行PrintWords.java，打印结果是什么？

需要了解常量折叠现象，理解什么是常量。