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 Questionspublic 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
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 Titillatorimport 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
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
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
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 Collidepublic 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。
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 Questionspublic 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
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 Titillatorimport 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
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
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
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 Collidepublic 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。
解决方案 »
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第一题getBoolean(String name)
当且仅当以参数命名的系统属性存在,且等于 "true" 串时,返回为 true。这样写的话,返回true了 public static void main(String[] args) {
Properties p = System.getProperties();
p.setProperty("test", "true");
System.out.println("Boolean.getBoolean():" + Boolean.getBoolean("test"));
}
java类那么多肯定得查呀,我看了你给出的第7个例子,我没明白什么意思。
通常这种情况在自己实践的过程种中会慢慢积累,每次遇到一个去翻翻书看看,牢记, 做成自己的DEMO
用处很大的,我时新手,象这种“偏”的小知识点,我每次遇到我都收集做成DEMO
原作者的演示材料:
http://developers.sun.com/learning/javaoneonline/sessions/2009/pdf/TS-5186.pdf
或者我的分析总结:
http://blog.csdn.net/ZangXT/archive/2009/07/29/4391628.aspx
为啥重新编译 PrintWords 输出没有变化??这个可以使用 常量堆叠来理解;因为 PrintWords.class 此时存放的是 the null set.但重新 编译 Words 为啥能够使得 PrintWords的执行结果产生变化啊??此时 PrintWords.class 根本没有任何变化的(因为俺一直用UltraEdit打开着,没有任何更新提示).
学习.
false false
第7题
the chemistry set
第1题:这个题没意思,反正我没有这样用过Boolean,不知大伙用过每。
第2题:多态,陷阱题目
第3题:明知道输出的是OOPS,但是还是不知道怎么输出的,debug了半天,因为用了匿名内部类和泛型,也不好调试,但最后还是想明白了:先初始化父类成员next. 调用了子类的nextElement方法,虽然nextElement更改了子类的成员cursor,但是在紧接着初始化子类成员的时候,cursor又还原了。
给个例子解释一下这个问题:
public abstract class Test {
int a = change();
public abstract int change();
}
//----------
public class Test1 extends Test{
int b = 111;
public int change() {
b = 333;
return 456;//a
}
public void change1(){
b = 888;
}
public static void main(String[] args){
Test1 t = new Test1();
System.out.println(t.b);
t.change1();
System.out.println(t.b);
}
}第4题:愣是没看明白,直到看到大家的提示;不过倒是了解了一下Collections的binarySearch方法。
第5题:枚举的, 没看。
第6题:以前写过Hibernate SessionFactory,所以了解一点ThreadLocal. i=500000会出错.i=5000似乎可以.
第7题:不了解有什么变化.