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。