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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<Integer> 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<Integer>`	`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 `beginIndex`and 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。

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原文地址：https://www.cnblogs.com/upuphe/p/12523514.html