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”"

Author: giantray

README.md

@@ -36,6 +36,7 @@ stackoverflow-Java-top-qa
 * [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)
 * [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)
 
+
 > 编程技巧
 
 * [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
 * [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)
 * [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)
 * [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)
-
+* [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)
 
 > 网络
 
 * [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)
 
 > 性能
 
-* [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)
+* [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
+* hen-to-use-linkedlist-over-arraylist.md)
 * [StringBuilder和StringBuffer有哪些区别呢](https://github.com/giantray/stackoverflow-java-top-qa/blob/master/contents/stringbuilder-and-stringbuffer.md)
 * [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)
 
@@ -61,24 +63,21 @@ stackoverflow-Java-top-qa
 * [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)
 
 > Android
-
-* [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)
+* [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)
+* [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)
 
 ### 待翻译问题链接(还剩x问题)
 - [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)
 - [Proper use cases for Android UserManager.isUserAGoat()?](http://stackoverflow.com/questions/13375357/proper-use-cases-for-android-usermanager-isuseragoat)
 - [Creating a memory leak with Java [closed]](http://stackoverflow.com/questions/6470651/creating-a-memory-leak-with-java)
 - [Why is char[] preferred over String for passwords?](http://stackoverflow.com/questions/8881291/why-is-char-preferred-over-string-for-passwords)
 - [Why is printing “B” dramatically slower than printing “#”?](http://stackoverflow.com/questions/21947452/why-is-printing-b-dramatically-slower-than-printing)
-- [Is there a unique Android device ID?](http://stackoverflow.com/questions/2785485/is-there-a-unique-android-device-id)
-- [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)
 - [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)
 - [How to avoid Java code in JSP files?](http://stackoverflow.com/questions/3177733/how-to-avoid-java-code-in-jsp-files)
 - [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)
 - [Dealing with “java.lang.OutOfMemoryError: PermGen space” error](http://stackoverflow.com/questions/88235/dealing-with-java-lang-outofmemoryerror-permgen-space-error)
 - [“implements Runnable” vs. “extends Thread”](http://stackoverflow.com/questions/541487/implements-runnable-vs-extends-thread)
 - [Android SDK installation doesn't find JDK](http://stackoverflow.com/questions/4382178/android-sdk-installation-doesnt-find-jdk)
-- [Initialization of an ArrayList in one line](http://stackoverflow.com/questions/1005073/initialization-of-an-arraylist-in-one-line)
 - [Java inner class and static nested class](http://stackoverflow.com/questions/70324/java-inner-class-and-static-nested-class)
 - ['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)
 - [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

@@ -23,4 +23,3 @@ String[] myStringArray = new String[]("a", "b", "c");
 
 
 
-

contents/is-there-a-unique-android-device-id.md

@@ -0,0 +1,115 @@
+##如何取得android唯一码？
+
+###问题
+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？
+
+###回答1（最佳）
+
+如何取得android唯一码？
+好处：
+1.不需要特定权限.
+2.在99.5% Android装置（包括root过的）上，即API => 9，保证唯一性.
+3.重装app之后仍能取得相同唯一值.
+
+伪代码：
+
+```
+if API => 9/10: (99.5% of devices)
+
+return unique ID containing serial id (rooted devices may be different)
+
+else
+
+return unique ID of build information (may overlap data - API < 9)
+```
+
+代码:
+
+```java
+
+/**
+ * Return pseudo unique ID
+ * @return ID
+ */public static String getUniquePsuedoID() {
+    // If all else fails, if the user does have lower than API 9 (lower
+    // than Gingerbread), has reset their device or 'Secure.ANDROID_ID'
+    // returns 'null', then simply the ID returned will be solely based
+    // off their Android device information. This is where the collisions
+    // can happen.
+    // Thanks http://www.pocketmagic.net/?p=1662!
+    // Try not to use DISPLAY, HOST or ID - these items could change.
+    // If there are collisions, there will be overlapping data
+    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);
+
+    // Thanks to @Roman SL!
+    // http://stackoverflow.com/a/4789483/950427
+    // Only devices with API >= 9 have android.os.Build.SERIAL
+    // http://developer.android.com/reference/android/os/Build.html#SERIAL
+    // If a user upgrades software or roots their device, there will be a duplicate entry
+    String serial = null;
+    try {
+        serial = android.os.Build.class.getField("SERIAL").get(null).toString();
+
+        // Go ahead and return the serial for api => 9
+        return new UUID(m_szDevIDShort.hashCode(), serial.hashCode()).toString();
+    } catch (Exception exception) {
+        // String needs to be initialized
+        serial = "serial"; // some value
+    }
+
+    // Thanks @Joe!
+    // http://stackoverflow.com/a/2853253/950427
+    // Finally, combine the values we have found by using the UUID class to create a unique identifier
+    return new UUID(m_szDevIDShort.hashCode(), serial.hashCode()).toString();}
+```
+###回答2
+好处：
+1.不需要特定权限.
+2.在100% Android装置（包括root过的）上，保证唯一性.
+
+坏处
+1.重装app之后不能取得相同唯一值.
+
+```java
+private static String uniqueID = null;
+private static final String PREF_UNIQUE_ID = "PREF_UNIQUE_ID";
+
+public synchronized static String id(Context context) {
+    if (uniqueID == null) {
+        SharedPreferences sharedPrefs = context.getSharedPreferences(
+                PREF_UNIQUE_ID, Context.MODE_PRIVATE);
+        uniqueID = sharedPrefs.getString(PREF_UNIQUE_ID, null);
+        if (uniqueID == null) {
+            uniqueID = UUID.randomUUID().toString();
+            Editor editor = sharedPrefs.edit();
+            editor.putString(PREF_UNIQUE_ID, uniqueID);
+            editor.commit();
+        }
+    }
+    return uniqueID;
+}
+```
+
+###回答3（需要有电话卡）
+
+好处：
+1.重装app之后仍能取得相同唯一值.
+
+代码：
+
+```java
+    final TelephonyManager tm = (TelephonyManager) getBaseContext().getSystemService(Context.TELEPHONY_SERVICE);
+    final String tmDevice, tmSerial, androidId;
+    tmDevice = "" + tm.getDeviceId();
+    tmSerial = "" + tm.getSimSerialNumber();
+    androidId = "" + android.provider.Settings.Secure.getString(getContentResolver(), android.provider.Settings.Secure.ANDROID_ID);
+    UUID deviceUuid = new UUID(androidId.hashCode(), ((long)tmDevice.hashCode() << 32) | tmSerial.hashCode());
+    String deviceId = deviceUuid.toString();
+```
+
+谨记：要取得以下权限
+```
+<uses-permission android:name="android.permission.READ_PHONE_STATE" />
+```
+stackoverflow链接：
+http://stackoverflow.com/questions/2785485/is-there-a-unique-android-device-id

contents/why-does-this-code-using-random-strings-print-hello-world.md

@@ -0,0 +1,64 @@
+##为什么以下用随机生成的文字会得出 “hello world”?
+
+###问题
+为什么以下用随机生成的文字会得出"hello world".
+有人能解释一下吗?
+
+```
+System.out.println(randomString(-229985452) + " " + randomString(-147909649));
+
+public static String randomString(int i)
+{
+   Random ran = new Random(i);
+   StringBuilder sb = new StringBuilder();
+   while (true)
+   {
+       int k = ran.nextInt(27);
+        if (k == 0)
+           break;
+           
+       sb.append((char)('`' + k));
+   }
+   return sb.toString();
+}
+```
+###回答1（最佳）
+在JAVA 里面，随机类的实现不是真正的随机,是伪随机.
+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。
+
+比如说这个问题:
+
+    new Random(-229985452).nextInt(27)
+
+首6个生成的数字一定是:
+
+    8
+    5
+    12
+    12
+    15
+    0
+
+
+而 `new Random(-147909649).nextInt(27)` 首6个生成的数字一定是:
+
+    23
+    15
+    18
+    12
+    4
+    0
+
+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:
+
+    8  + 96 = 104 --> h
+    5  + 96 = 101 --> e
+    12 + 96 = 108 --> l
+    12 + 96 = 108 --> l
+    15 + 96 = 111 --> o
+    
+    23 + 96 = 119 --> w
+    15 + 96 = 111 --> o
+    18 + 96 = 114 --> r
+    12 + 96 = 108 --> l
+    4  + 96 = 100 --> d