装箱 拆箱

http://jzinfo.iteye.com/blog/450590（这个写的很不错）

所谓装箱，就是把基本类型用它们相对应的引用类型包起来，使它们可以具有对象的特质，如我们可以把int型包装成Integer类的对象，或者把double包装成Double，等等。

所谓拆箱，就是跟装箱的方向相反，将Integer及Double这样的引用类型的对象重新简化为值类型的数据。

在官网上查看一下定义：http://docs.oracle.com/javase/tutorial/java/data/autoboxing.html

Autoboxing and Unboxing

Autoboxing is the automatic conversion that the Java compiler makes between the primitive types and their corresponding object wrapper classes. For example, converting an int to an Integer, a double to a Double, and so on. If the conversion goes the other way, this is called unboxing.

Here is the simplest example of autoboxing:

Character ch = 'a';

The rest of the examples in this section use generics. If you are not yet familiar with the syntax of generics, see the Generics (Updated) lesson.

Consider the following code:

List<Integer> li = new ArrayList<>();
for (int i = 1; i < 50; i += 2)
    li.add(i);

Although you add the int values as primitive types, rather than Integer objects, to li, the code compiles. Because li is a list of Integer objects, not a list of int values, you may wonder why the Java compiler does not issue a compile-time error. The compiler does not generate an error because it creates an Integer object from i and adds the object to li. Thus, the compiler converts the previous code to the following at runtime:

List<Integer> li = new ArrayList<>();
for (int i = 1; i < 50; i += 2)
    li.add(Integer.valueOf(i));

Converting a primitive value (an int, for example) into an object of the corresponding wrapper class (Integer) is called autoboxing. The Java compiler applies autoboxing when a primitive value is:

• Passed as a parameter to a method that expects an object of the corresponding wrapper class.
• Assigned to a variable of the corresponding wrapper class.

Consider the following method:

public static int sumEven(List<Integer> li) {
    int sum = 0;
    for (Integer i: li)
        if (i % 2 == 0)
            sum += i;
        return sum;
}

Because the remainder (%) and unary plus (+=) operators do not apply to Integer objects, you may wonder why the Java compiler compiles the method without issuing any errors. The compiler does not generate an error because it invokes the intValue method to convert an Integer to an int at runtime:

public static int sumEven(List<Integer> li) {
    int sum = 0;
    for (Integer i : li)
        if (i.intValue() % 2 == 0)
            sum += i.intValue();
        return sum;
}

Converting an object of a wrapper type (Integer) to its corresponding primitive (int) value is called unboxing. The Java compiler applies unboxing when an object of a wrapper class is:

• Passed as a parameter to a method that expects a value of the corresponding primitive type.
• Assigned to a variable of the corresponding primitive type.

The Unboxing example shows how this works:

import java.util.ArrayList;
import java.util.List;

public class Unboxing {

    public static void main(String[] args) {
        Integer i = new Integer(-8);

        // 1. Unboxing through method invocation
        int absVal = absoluteValue(i);
        System.out.println("absolute value of " + i + " = " + absVal);

        List<Double> ld = new ArrayList<>();
        ld.add(3.1416);    // Π is autoboxed through method invocation.

        // 2. Unboxing through assignment
        double pi = ld.get(0);
        System.out.println("pi = " + pi);
    }

    public static int absoluteValue(int i) {
        return (i < 0) ? -i : i;
    }
}

The program prints the following:

absolute value of -8 = 8
pi = 3.1416

Autoboxing and unboxing lets developers write cleaner code, making it easier to read. The following table lists the primitive types and their corresponding wrapper classes, which are used by the Java compiler for autoboxing and unboxing:

Primitive type	Wrapper class
boolean	Boolean
byte	Byte
char	Character
float	Float
int	Integer
long	Long
short	Short
double	Double

《Effective Java》中文版第2版第49条中有说到装箱拆箱的注意事项

比如说

public class Test {
    static Integer i;
    public static void main(String[] args) throws Exception {
        // TODO Auto-generated method stub
        if(i==1){
            System.out.println("Hello world");
        }
    }
}

结果，空指针异常

Exception in thread "main" java.lang.NullPointerException
    at Test.main(Test.java:6)

更详细的参看那本书吧