java.lang包源碼閱讀之一：類目綜述以及源碼閱讀第一個類Boolean

java.lang包下直屬類，從Java Platform Standard Edition 7 Documentation中一查便知：

**Interfaces**
Appendable 
AutoCloseable 
CharSequence 
Cloneable 
Comparable 
Iterable
Readable
Runnable
Thread.UncaughtExceptionHandler 
**Classes**
Boolean 
Byte 
Character 
Character.Subset 
Character.UnicodeBlock 
Class 
ClassLoader 
ClassValue 
Compiler 
Double 
Enum 
Float 
InheritableThreadLocal 
Integer 
Long 
Math 
Number 
Object 
Package 
Process 
ProcessBuilder 
ProcessBuilder.Redirect 
Runtime 
RuntimePermission 
SecurityManager 
Short 
StackTraceElement 
StrictMath 
String 
StringBuffer 
StringBuilder 
System 
Thread 
ThreadGroup 
ThreadLocal 
Throwable 
Void 
**Enums**
Character.UnicodeScript 
ProcessBuilder.Redirect.Type 
Thread.State 
**Exceptions**
ArithmeticException 
ArrayIndexOutOfBoundsException 
ArrayStoreException 
ClassCastException 
ClassNotFoundException 
CloneNotSupportedException 
EnumConstantNotPresentException 
Exception 
IllegalAccessException 
IllegalArgumentException 
IllegalMonitorStateException 
IllegalStateException 
IllegalThreadStateException 
IndexOutOfBoundsException 
InstantiationException 
InterruptedException 
NegativeArraySizeException 
NoSuchFieldException 
NoSuchMethodException 
NullPointerException 
NumberFormatException 
ReflectiveOperationException 
RuntimeException 
SecurityException 
StringIndexOutOfBoundsException 
TypeNotPresentException 
UnsupportedOperationException 
**Errors**
AbstractMethodError 
AssertionError 
BootstrapMethodError 
ClassCircularityError 
ClassFormatError 
Error 
ExceptionInInitializerError 
IllegalAccessError 
IncompatibleClassChangeError 
InstantiationError 
InternalError 
LinkageError 
NoClassDefFoundError 
NoSuchFieldError 
NoSuchMethodError 
OutOfMemoryError 
StackOverflowError 
ThreadDeath 
UnknownError 
UnsatisfiedLinkError 
UnsupportedClassVersionError 
VerifyError 
VirtualMachineError 
**Annotation Types**
Deprecated 
Override 
SafeVarargs 
SuppressWarnings

大致上按照上面的分類閱讀，估計應該學別人的體例-------關鍵代碼分析，加上編譯經過可運行的用例。類不少，源碼之間繼承關係也挺複雜，閱讀源碼計劃的開始階段我決定從簡單作起，今天閱讀第一個類Boolean:

一、Boolean是包裝類，包裝了基本數據類型boolean；

二、除非要求一個新的instance，Boolean的構造方法Boolean(boolean value)極少用，用靜態工廠方法valueOf(boolean value)來獲取一個Boolean實例，由於Boolean有兩個常量靜態成員TRUE和FALSE，valueOf(boolean value)是從常量池中取得類的實例，用源碼的話是「this method is likely to yield significantly better space and time performance.」。

三、能夠借鑑的是hashCode()方法和equals(Object obj)方法，因爲只有兩個值Boolean的hashCode是寫定的，分別是1231和1237。