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
-
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
-
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
-
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
-
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
-
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
-
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
-
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
-
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。