写在之前
以下是《Java8编程入门官方教程》中的一些知识，如有错误，烦请指正。涉及的程序如需下载请移步http://down1.tupwk.com.cn/qhwkdownpage/978-7-302-38738-1.zip
自动装箱
简化了在基本类型和对象之间转换所需的代码。
类型封装器
主要有：Double、Float、Long、Integer、Short、Byte、Character、Boolean，它们包含在java.lang中。把值封装在对象中的过程称为装箱。
Integer iOb = new Integer(100); //手动装箱
从类型封装器中提取值的过程称为拆箱。
int i = iOb.intValue();//手动拆箱
自动装箱是指在需要某种类型的数据对象时，把该基本类型自动装箱到其等效的类型封装器的过程。不需要显式地构造对象。
自动拆箱是指当需要某个装箱对象的值时，从类型封装其实把装箱对象的值自动地提取出来的过程。
Integer iOb =100; //自动装箱
int i = iOb; //auto-unbox
除了赋值的情况，自动装箱在基本类型必须转化为对象时会自动发生，自动拆箱在对象必须转化为基本类型时会自动发生。
class AutoBox2 {
// 方法接收一个 Integer.
static void m(Integer v) {
System.out.println("m() received " + v);
}
//返回一个int.
static int m2() {
return 10;
}
// 返回一个 Integer.
static Integer m3() {
return 99; // autoboxing 99 into an Integer.
}
public static void main(String args[]) {
// Pass an int to m(). Because m() has an Integer
// parameter, the int value passed is automatically boxed.
m(199);
// Here, iOb recieves the int value returned by m2().
// This value is automatically boxed so that it can be
// assigned to iOb.
Integer iOb = m2();
System.out.println("Return value from m2() is " + iOb);
// Next, m3() is called. It returns an Integer value
// which is auto-unboxed into an int.
int i = m3();
System.out.println("Return value from m3() is " + i);
// Next, Math.sqrt() is called with iOb as an argument.
// In this case, iOb is auto-unboxed and its value promoted to
// double, which is the type needed by sqrt().
iOb = 100;
System.out.println("Square root of iOb is " + Math.sqrt(iOb));
}
}
下面看一下表达式之中，数值对象被自动拆箱，表达式结果必要时重新装箱的情况。
class AutoBox3 {
public static void main(String args[]) {
Integer iOb, iOb2;
int i;
iOb = 99;
System.out.println("Original value of iOb: " + iOb);
// The following automatically unboxes iOb,
// performs the increment, and then reboxes
// the result back into iOb.
++iOb;
System.out.println("After ++iOb: " + iOb);
// Here, iOb is unboxed, its value is increased by 10
// and the result is boxed and stored back in iOb.
iOb += 10;
System.out.println("After iOb += 10: " + iOb);
// Here, iOb is unboxed, the expression is
// evaluated, and the result is reboxed and
// assigned to iOb2.
iOb2 = iOb + (iOb / 3);
System.out.println("iOb2 after expression: " + iOb2);
// The same expression is evaluated, but the
// result is not reboxed.
i = iOb + (iOb / 3);
System.out.println("i after expression: " + i);
}
}
注意：使用封装类型做计算，因为涉及到自动装箱和拆箱，比使用基本类型，代码的效率要低很多。因为每次装箱和拆箱都增加了基本类型没有的开销。
