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Commit 8952e53

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新增"如何取得android唯一码"、"为什么以下用随机生成的文字会得出 "hello world""
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‎README.md

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@@ -36,6 +36,7 @@ stackoverflow-Java-top-qa
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* [反射是什么及其用途](https://github.com/giantray/stackoverflow-java-top-qa/blob/master/contents/What-is-reflection-and-why-is-it-useful.md.md)
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* [为什么不能用string类型进行switch判断](https://github.com/giantray/stackoverflow-java-top-qa/blob/master/contents/Why-can't-I-switch-on-a-String.md)
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> 编程技巧
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* [去掉烦人的"!=null"(判空语句](https://github.com/giantray/stackoverflow-java-top-qa/blob/master/contents/avoiding-null-statements-in-java.md)
@@ -44,15 +45,16 @@ stackoverflow-Java-top-qa
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* [初始化静态map](https://github.com/giantray/stackoverflow-java-top-qa/blob/master/contents/How-can-I-Initialize-a-static-Map.md)
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* [给3个布尔变量,当其中有2个或者2个以上为true才返回true](https://github.com/giantray/stackoverflow-java-top-qa/blob/master/contents/Check-if-at-least-two-out-of-three-booleans-are-true.md)
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* [Java中打印一个数组最简单的方法是什么](https://github.com/giantray/stackoverflow-java-top-qa/blob/master/contents/What's-the-simplest-way-to-print-a-Java-array.md)
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*[为什么以下用随机生成的文字会得出 "hello world"?](https://github.com/giantray/stackoverflow-java-top-qa/blob/master/contents/why-does-this-code-using-random-strings-print-hello-world.md)
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> 网络
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* [如何使用java.net.URLConnection接收及发送HTTP请求](https://github.com/giantray/stackoverflow-java-top-qa/blob/master/contents/using-java-net-urlconnection-to-fire-and-handle-http-requests.md)
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> 性能
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* [LinkedList、ArrayList各自的使用场景,如何确认应该用哪一个呢?](https://github.com/giantray/stackoverflow-java-top-qa/blob/master/contents/when-to-use-linkedlist-over-arraylist.md)
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* [LinkedList、ArrayList各自的使用场景,如何确认应该用哪一个呢?](https://github.com/giantray/stackoverflow-java-top-qa/blob/master/contents/w
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* hen-to-use-linkedlist-over-arraylist.md)
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* [StringBuilder和StringBuffer有哪些区别呢](https://github.com/giantray/stackoverflow-java-top-qa/blob/master/contents/stringbuilder-and-stringbuffer.md)
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* [为什么处理排序的数组要比非排序的快](https://github.com/giantray/stackoverflow-java-top-qa/blob/master/contents/Why-is-processing-a-sorted-array-faster-than-an-unsorted-array.md)
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* [如何测试 private 方法,变量或者内部类](https://github.com/giantray/stackoverflow-java-top-qa/blob/master/contents/How_to_test_a_class_that_has_private_methods,_fields_or_inner_classes.md)
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> Android
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* [在Android里面下载文件,并在ProgressDialog显示进度](https://github.com/troyliu0105/stackoverflow-java-top-qa/blob/master/contents/Download_a_file_with_Android_and_showing_the_progress_in_a_ProgressDialog.md)
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*[在Android里面下载文件,并在ProgressDialog显示进度](https://github.com/giantray/stackoverflow-java-top-qa/blob/master/contents/Download_a_file_with_Android_and_showing_the_progress_in_a_ProgressDialog.md)
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* [如何取得android唯一码?](https://github.com/giantray/stackoverflow-java-top-qa/blob/master/contents/is-there-a-unique-android-device-id.md)
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### 待翻译问题链接(还剩x问题)
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- [Why is subtracting these two times (in 1927) giving a strange result?](http://stackoverflow.com/questions/6841333/why-is-subtracting-these-two-times-in-1927-giving-a-strange-result)
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- [Proper use cases for Android UserManager.isUserAGoat()?](http://stackoverflow.com/questions/13375357/proper-use-cases-for-android-usermanager-isuseragoat)
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- [Creating a memory leak with Java [closed]](http://stackoverflow.com/questions/6470651/creating-a-memory-leak-with-java)
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- [Why is char[] preferred over String for passwords?](http://stackoverflow.com/questions/8881291/why-is-char-preferred-over-string-for-passwords)
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- [Why is printing "B" dramatically slower than printing "#"?](http://stackoverflow.com/questions/21947452/why-is-printing-b-dramatically-slower-than-printing)
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- [Is there a unique Android device ID?](http://stackoverflow.com/questions/2785485/is-there-a-unique-android-device-id)
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- [Why does this code using random strings print "hello world"?](http://stackoverflow.com/questions/15182496/why-does-this-code-using-random-strings-print-hello-world)
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- [How can I create an executable jar with dependencies using Maven?](http://stackoverflow.com/questions/574594/how-can-i-create-an-executable-jar-with-dependencies-using-maven)
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- [How to avoid Java code in JSP files?](http://stackoverflow.com/questions/3177733/how-to-avoid-java-code-in-jsp-files)
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- [Why is executing Java code in comments with certain Unicode characters allowed?](http://stackoverflow.com/questions/30727515/why-is-executing-java-code-in-comments-with-certain-unicode-characters-allowed)
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- [Dealing with "java.lang.OutOfMemoryError: PermGen space" error](http://stackoverflow.com/questions/88235/dealing-with-java-lang-outofmemoryerror-permgen-space-error)
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- ["implements Runnable" vs. "extends Thread"](http://stackoverflow.com/questions/541487/implements-runnable-vs-extends-thread)
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- [Android SDK installation doesn't find JDK](http://stackoverflow.com/questions/4382178/android-sdk-installation-doesnt-find-jdk)
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- [Initialization of an ArrayList in one line](http://stackoverflow.com/questions/1005073/initialization-of-an-arraylist-in-one-line)
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- [Java inner class and static nested class](http://stackoverflow.com/questions/70324/java-inner-class-and-static-nested-class)
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- ['Must Override a Superclass Method' Errors after importing a project into Eclipse](http://stackoverflow.com/questions/1678122/must-override-a-superclass-method-errors-after-importing-a-project-into-eclips)
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- [Fastest way to determine if an integer's square root is an integer](http://stackoverflow.com/questions/295579/fastest-way-to-determine-if-an-integers-square-root-is-an-integer)

‎contents/Declare-array-in-Java.md

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##如何取得android唯一码?
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###问题
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每一个android装置都有唯一ID吗?如果有?怎么用java最简单取得呢?
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###回答1(最佳)
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如何取得android唯一码?
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好处:
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1.不需要特定权限.
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2.在99.5% Android装置(包括root过的)上,即API => 9,保证唯一性.
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3.重装app之后仍能取得相同唯一值.
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伪代码:
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```
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if API => 9/10: (99.5% of devices)
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return unique ID containing serial id (rooted devices may be different)
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else
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return unique ID of build information (may overlap data - API < 9)
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```
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代码:
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```java
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/**
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* Return pseudo unique ID
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* @return ID
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*/public static String getUniquePsuedoID() {
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// If all else fails, if the user does have lower than API 9 (lower
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// than Gingerbread), has reset their device or 'Secure.ANDROID_ID'
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// returns 'null', then simply the ID returned will be solely based
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// off their Android device information. This is where the collisions
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// can happen.
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// Thanks http://www.pocketmagic.net/?p=1662!
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// Try not to use DISPLAY, HOST or ID - these items could change.
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// If there are collisions, there will be overlapping data
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String m_szDevIDShort = "35" + (Build.BOARD.length() % 10) + (Build.BRAND.length() % 10) + (Build.CPU_ABI.length() % 10) + (Build.DEVICE.length() % 10) + (Build.MANUFACTURER.length() % 10) + (Build.MODEL.length() % 10) + (Build.PRODUCT.length() % 10);
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// Thanks to @Roman SL!
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// http://stackoverflow.com/a/4789483/950427
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// Only devices with API >= 9 have android.os.Build.SERIAL
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// http://developer.android.com/reference/android/os/Build.html#SERIAL
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// If a user upgrades software or roots their device, there will be a duplicate entry
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String serial = null;
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try {
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serial = android.os.Build.class.getField("SERIAL").get(null).toString();
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// Go ahead and return the serial for api => 9
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return new UUID(m_szDevIDShort.hashCode(), serial.hashCode()).toString();
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} catch (Exception exception) {
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// String needs to be initialized
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serial = "serial"; // some value
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}
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// Thanks @Joe!
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// http://stackoverflow.com/a/2853253/950427
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// Finally, combine the values we have found by using the UUID class to create a unique identifier
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return new UUID(m_szDevIDShort.hashCode(), serial.hashCode()).toString();}
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```
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###回答2
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好处:
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1.不需要特定权限.
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2.在100% Android装置(包括root过的)上,保证唯一性.
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坏处
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1.重装app之后不能取得相同唯一值.
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```java
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private static String uniqueID = null;
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private static final String PREF_UNIQUE_ID = "PREF_UNIQUE_ID";
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public synchronized static String id(Context context) {
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if (uniqueID == null) {
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SharedPreferences sharedPrefs = context.getSharedPreferences(
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PREF_UNIQUE_ID, Context.MODE_PRIVATE);
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uniqueID = sharedPrefs.getString(PREF_UNIQUE_ID, null);
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if (uniqueID == null) {
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uniqueID = UUID.randomUUID().toString();
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Editor editor = sharedPrefs.edit();
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editor.putString(PREF_UNIQUE_ID, uniqueID);
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editor.commit();
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}
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}
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return uniqueID;
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}
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```
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###回答3(需要有电话卡)
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好处:
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1.重装app之后仍能取得相同唯一值.
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代码:
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```java
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final TelephonyManager tm = (TelephonyManager) getBaseContext().getSystemService(Context.TELEPHONY_SERVICE);
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final String tmDevice, tmSerial, androidId;
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tmDevice = "" + tm.getDeviceId();
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tmSerial = "" + tm.getSimSerialNumber();
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androidId = "" + android.provider.Settings.Secure.getString(getContentResolver(), android.provider.Settings.Secure.ANDROID_ID);
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UUID deviceUuid = new UUID(androidId.hashCode(), ((long)tmDevice.hashCode() << 32) | tmSerial.hashCode());
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String deviceId = deviceUuid.toString();
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```
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谨记:要取得以下权限
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```
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<uses-permission android:name="android.permission.READ_PHONE_STATE" />
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```
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stackoverflow链接:
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http://stackoverflow.com/questions/2785485/is-there-a-unique-android-device-id
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##为什么以下用随机生成的文字会得出 "hello world"?
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###问题
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为什么以下用随机生成的文字会得出"hello world".
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有人能解释一下吗?
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```
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System.out.println(randomString(-229985452) + " " + randomString(-147909649));
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public static String randomString(int i)
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{
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Random ran = new Random(i);
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StringBuilder sb = new StringBuilder();
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while (true)
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{
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int k = ran.nextInt(27);
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if (k == 0)
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break;
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sb.append((char)('`' + k));
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}
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return sb.toString();
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}
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```
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###回答1(最佳)
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在JAVA 里面,随机类的实现不是真正的随机,是伪随机.
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就是说如果随机类的种子是一样的话,他们会生成同一组的数字。
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比如说这个问题:
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new Random(-229985452).nextInt(27)
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首6个生成的数字一定是:
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8
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5
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12
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12
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15
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0
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而 `new Random(-147909649).nextInt(27)` 首6个生成的数字一定是:
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23
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15
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18
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12
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4
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0
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而把每一个数目字加 <code>`</code> (which is 96),就会得到了相应的英文字母:
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8 + 96 = 104 --> h
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5 + 96 = 101 --> e
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12 + 96 = 108 --> l
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12 + 96 = 108 --> l
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15 + 96 = 111 --> o
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23 + 96 = 119 --> w
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15 + 96 = 111 --> o
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18 + 96 = 114 --> r
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12 + 96 = 108 --> l
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4 + 96 = 100 --> d

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