java.lang包中不能被继承的类：
public final class Byte
public final class Character
public final class Double
public final class Float
public final class Integer
public final class Long
public final class Short
public static final class Character.UnicodeBlock
public final class Compile
public final class ProcessBuilder
public final class RuntimePermission
public final class StackTraceElement
public final class StrictMath
public final class Class
public final class Math
public final class System
public final class Void
public final class String
public final class StringBuffer
public final class StringBuilder