java Integer和int的拆箱与装箱

官网：http://docs.oracle.com/javase/tutorial/java/data/autoboxing.html

1、赋值：

a. 把int类型赋值给Integer类型：JVM会自动调用Integer.valueOf()方法

b. 把Integer类型赋值给int类型：JVM会自动调用intValue()方法

2、比较

a. int 与 int 比较：直接对两个变量的值进行比较

b. int 与 Integer 比较：会先把Integer拆装成int类型，然后进行比较

c. Integer 与 Integer比较：比较两个对象的引用地址

值得注意的是：对Integer对象，JVM会自动缓存-128~127范围内的值，所以所有在这个范围内的值相等的Integer对象都会共用一块内存，而不会开辟多个；超出这个范围内的值对应的Integer对象有多少个就开辟多少个内存。底层代码如下：

     /**
      * Returns an {@code Integer} instance representing the specified
      * {@code int} value.  If a new {@code Integer} instance is not
      * required, this method should generally be used in preference to
      * the constructor {@link #Integer(int)}, as this method is likely
      * to yield significantly better space and time performance by
      * caching frequently requested values.
      *
      * This method will always cache values in the range -128 to 127,
      * inclusive, and may cache other values outside of this range.
      *
      * @param  i an {@code int} value.
      * @return an {@code Integer} instance representing {@code i}.
      * @since  1.5
      */
     public static Integer valueOf(int i) {
         if (i >= IntegerCache.low && i <= IntegerCache.high)
             return IntegerCache.cache[i + (-IntegerCache.low)];
         return new Integer(i);
     }

3、测试代码：

     @Test
     public void test1() {
         Integer num1 = 100;
         Integer num2 = 100;
         System.out.println("test1 result is: " + (num1 == num2));    // true
     }

     @Test
     public void test2() {
         Integer a = 128;
         Integer b = 128;
         System.out.println("test2 result is: " + (a == b));   // false
     }

     @Test
     public void test3() {
         Integer a = 100;
         Integer b = new Integer(100);
         System.out.println("test3 result is: " + (a == b));   // false
     }