自动装箱与拆箱

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。