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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
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Java code
-
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
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Java code
-
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
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Java code
-
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
- class="code">
Java code
-
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[] class="tags" href="/tags/STRING.html" title=string>strings = { " 0 " , " 1 " , " 2 " , " 3 " , " 4 " , " 5 " };
List < Integer > integers = new ArrayList < Integer > ();
for (String s : class="tags" href="/tags/STRING.html" title=string>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
- class="code">
Java code
-
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
- class="code">
Java code
-
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
- class="code">
Java code
-
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
- class="code">
Java code
-
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。
