java包装器和自动装箱

java一共有9种包装器：Integer,Long,Float,Double,Short,Byte,Character,Void,Boolean,，这些对象包装器又有着与之对应的基本类型.对象包装器是不可变的，也是final的。我们什么时候会用到这些对象包装器呢？实际上，在声明一个数组列表时，由于其中的类型参数不能是基本类型，因此我们只能采用对象包装器。如下面的声明：

ArrayList list=new ArrayList<>;

　　这里的Integer不能用int代替，这也是许多初学者容易犯的错误。但是我们在往上面的数组列表添加元素时，就可以直接添加int类型元素，如下面的语句：

list.add(5);

　　这里实际上编译器会进行自动装箱操作（autoboxing），即自动变为list.add(Integer.valueof(5)。相反的，当将一个Integer对象赋给一个int时，又会采用自动拆箱。并且，如果我们在一个条件表达式中混用Integer类型和Double类型，这里Integer就会自动拆箱，转变为int，然后再自动装箱，变为Double。

　　关于自动装箱和自动拆箱的实现，这里实际上是在编译器在生成字节码时就执行了，而非虚拟机执行的。我们在使用对象包装类时还可以调用一些它的基本方法，这也使得我们编写代码更加便捷：

　　

Modifier and Type	Method	Description
static int	bitCount​(int i)	Returns the number of one-bits in the two's complement binary representation of the specified  int value.
byte	byteValue()	Returns the value of this  Integer as a  byte after a narrowing primitive conversion.
static int	compare​(int x, int y)	Compares two  int values numerically.
int	compareTo​(Integer anotherInteger)	Compares two  Integer objects numerically.
static int	compareUnsigned​(int x, int y)	Compares two  int values numerically treating the values as unsigned.
static Integer	decode​(String nm)	Decodes a  String into an  Integer.
Optional	describeConstable()	Returns an  Optional containing the nominal descriptor for this instance, which is the instance itself.
static int	divideUnsigned​(int dividend, int divisor)	Returns the unsigned quotient of dividing the first argument by the second where each argument and the result is interpreted as an unsigned value.
double	doubleValue()	Returns the value of this  Integer as a  double after a widening primitive conversion.
boolean	equals​(Object obj)	Compares this object to the specified object.
float	floatValue()	Returns the value of this  Integer as a  float after a widening primitive conversion.
static Integer	getInteger​(String nm)	Determines the integer value of the system property with the specified name.
static Integer	getInteger​(String nm, int val)	Determines the integer value of the system property with the specified name.
static Integer	getInteger​(String nm, Integer val)	Returns the integer value of the system property with the specified name.
int	hashCode()	Returns a hash code for this  Integer.
static int	hashCode​(int value)	Returns a hash code for an  int value; compatible with  Integer.hashCode().
static int	highestOneBit​(int i)	Returns an  int value with at most a single one-bit, in the position of the highest-order ("leftmost") one-bit in the specified  int value.
int	intValue()	Returns the value of this  Integer as an  int.
long	longValue()	Returns the value of this  Integer as a  long after a widening primitive conversion.
static int	lowestOneBit​(int i)	Returns an  int value with at most a single one-bit, in the position of the lowest-order ("rightmost") one-bit in the specified  int value.
static int	max​(int a, int b)	Returns the greater of two  int values as if by calling  Math.max.
static int	min​(int a, int b)	Returns the smaller of two  int values as if by calling  Math.min.
static int	numberOfLeadingZeros​(int i)	Returns the number of zero bits preceding the highest-order ("leftmost") one-bit in the two's complement binary representation of the specified  int value.
static int	numberOfTrailingZeros​(int i)	Returns the number of zero bits following the lowest-order ("rightmost") one-bit in the two's complement binary representation of the specified  int value.
static int	parseInt​(CharSequence s, int beginIndex, int endIndex, int radix)	Parses the  CharSequence argument as a signed  int in the specified  radix, beginning at the specified  beginIndexand extending to  endIndex - 1.
static int	parseInt​(String s)	Parses the string argument as a signed decimal integer.
static int	parseInt​(String s, int radix)	Parses the string argument as a signed integer in the radix specified by the second argument.
static int	parseUnsignedInt​(CharSequence s, int beginIndex, int endIndex, int radix)	Parses the  CharSequence argument as an unsigned  int in the specified  radix, beginning at the specified beginIndex and extending to  endIndex - 1.
static int	parseUnsignedInt​(String s)	Parses the string argument as an unsigned decimal integer.
static int	parseUnsignedInt​(String s, int radix)	Parses the string argument as an unsigned integer in the radix specified by the second argument.
static int	remainderUnsigned​(int dividend, int divisor)	Returns the unsigned remainder from dividing the first argument by the second where each argument and the result is interpreted as an unsigned value.
Integer	resolveConstantDesc​(MethodHandles.Lookup lookup)	Resolves this instance as a  ConstantDesc, the result of which is the instance itself.
static int	reverse​(int i)	Returns the value obtained by reversing the order of the bits in the two's complement binary representation of the specified  int value.
static int	reverseBytes​(int i)	Returns the value obtained by reversing the order of the bytes in the two's complement representation of the specified  int value.
static int	rotateLeft​(int i, int distance)	Returns the value obtained by rotating the two's complement binary representation of the specified  int value left by the specified number of bits.
static int	rotateRight​(int i, int distance)	Returns the value obtained by rotating the two's complement binary representation of the specified  int value right by the specified number of bits.
short	shortValue()	Returns the value of this  Integer as a  short after a narrowing primitive conversion.
static int	signum​(int i)	Returns the signum function of the specified  int value.
static int	sum​(int a, int b)	Adds two integers together as per the + operator.
static String	toBinaryString​(int i)	Returns a string representation of the integer argument as an unsigned integer in base 2.
static String	toHexString​(int i)	Returns a string representation of the integer argument as an unsigned integer in base 16.
static String	toOctalString​(int i)	Returns a string representation of the integer argument as an unsigned integer in base 8.
String	toString()	Returns a  String object representing this  Integer's value.
static String	toString​(int i)	Returns a  String object representing the specified integer.
static String	toString​(int i, int radix)	Returns a string representation of the first argument in the radix specified by the second argument.
static long	toUnsignedLong​(int x)	Converts the argument to a  long by an unsigned conversion.
static String	toUnsignedString​(int i)	Returns a string representation of the argument as an unsigned decimal value.
static String	toUnsignedString​(int i, int radix)	Returns a string representation of the first argument as an unsigned integer value in the radix specified by the second argument.
static Integer	valueOf​(int i)	Returns an  Integer instance representing the specified  int value.
static Integer	valueOf​(String s)	Returns an  Integer object holding the value of the specified  String.
static Integer	valueOf​(String s, int radix)	Returns an  Integer object holding the value extracted from the specified  String when parsed with the radix given by the second argument.

　　以上就是Integer所有的方法说明和其返回值说明，其余包装类也有着类似的方法。