本文来自CSDN博客,转载请标明出处:http://blog.csdn.net/pfpfpfpfpf/archive/2009/08/05/4414224.aspx
在2009年的JavaOne大会上,Joshua Bloch和Neal Gafter又为我们带来的7道谜题,看了感觉自己的java基础很不扎实。
1.Life's Persistent Questions
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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")); } } 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?
开始毫不犹豫的以为是true,结果试验了都是false。。。
google了一下
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String aa = "aa";
System.setProperty(aa,"true");
boolean isRMS = Boolean.getBoolean(aa);
boolean isDW = Boolean.getBoolean("true");
System.out.println("isRMS="+isRMS);
System.out.println("isDW="+isDW);
String aa = "aa";
System.setProperty(aa,"true");
boolean isRMS = Boolean.getBoolean(aa);
boolean isDW = Boolean.getBoolean("true");
System.out.println("isRMS="+isRMS);
System.out.println("isDW="+isDW);
isRMS=true
isDW=false
结果是这个样子, public static boolean getBoolean(String name)当且仅当以参数命名的系统属性存在,且等于 "true" 字符串时,才返回 true。 如果没有以指定名称命名的属性或者指定名称为空或 null,则返回 false。
2.Instruments of Tortue
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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);
}
}
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?
应该是6,不要写3个,super.addAll()也是一个一个add()方法加进去的,所以add()方法也会再执行三次,一共是6次。
3.Iterator Titillator
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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());
}
}
}
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?
初始化顺序的问题,nextElement();里面虽然执行了cursor++, 但是执行到cursor = 0把cursor的值又归零了,那么第一次next是保存了o,但是cursor的值仍然是0,第二次不是初始化了,不会执行cursor=0,所以cursor=1,next缺还是o。
4.Search for the One
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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);
}
};
}
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?
-2,Integer类型比较时,并不是比较int型的数值,而是比较引用的内存地址,明显就不一样了。
5.Cogito Ergo Sum
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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);
}
}
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?
需要注意的是静态变量,也就是类变量,在类的preparation阶段进行赋值,此时都是赋默认的初值,并不是自己定义的初值,比如int型0,引用型null等,后面会进入Resolution阶段,然后才是初始化( Initialization),这个阶段会执行static初始化,把变量初始化为你指定的值。
6.Thread Friendly
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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);
}
}
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?
报错
Exception in thread "main" java.lang.OutOfMemoryError: Java heap space
这个不会了,转别人的分析
/**
其实这个细说起来还是很复杂的,需要理解ThreadLocal的原理和弱引用的相关知识。因为内部类对象存在一个指向外部类(包含内部类的类)ThreadFriendly对象的引用,而ThreadFriendly对象又存在到ThreadLocal对象的引用,导致Thread中ThreadLocal.ThreadLocalMap inheritableThreadLocals 中的key一直是强引用,无法释放,最终导致内存溢出。
解决方法:
static class Value{
final int i;
Value(int i){
this.i = i;
}
}
当然,也可以static ThreadLocal<Value> threadLocalPart = new ThreadLocal<Value>();
*/
7.When Words Collide
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public class test {
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";
}
public class test {
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
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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";
}
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?
这里需要注意的是,null并不是常量,将PrintWords.java编译之后,Words.FIRST,Words.THIRD都直接用"the"和"set"替换了,它们没有对Words类型的引用。但Words.SECOND 仍然保留对Words.SECOND的引用。
重编译Words.java文件之后,PrintWords.java中进行输出时,读取Words.SECOND时要去加载Words类。导致输出结果是the chemistry set。(因为the 和 set已经编译为字符串字面常量,不会引用Words类的内容)