自动拆箱、装箱

自动拆箱、装箱

拆箱的情况

• 调用intValue()
• 与基本类型做算术运算和==时

装箱情况

• 直接赋值都会装箱调用valueOf()

  • Integer i =4;

• 源码

  public class tt {
      public static void main(String[] args) {
          int i = 3;
          int a = 1;
          Integer b = 2;
          Integer c = 3;
          Integer d = 3;
          Integer e = 321;
          Integer f = 321;
          Long g = 3l;
          System.out.println(i == c);
          System.out.println(c == d);
          System.out.println(e == f);
          System.out.println(c == (a + b));
          System.out.println(c.equals(a + b));
         System.out.println(g == (a + b));
          System.out.println(g.equals(a + b));
      }
  }
  

• 反编译后借用网上老哥的反编译截图
  

• return了一个新对象，IntegerCache.cache数组中的-128-127是在常量池（静态常量池）中的。

      /**
       * 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);
      }
  

• 包装类的“==”运算在遇到算术运算的情况下会自动拆箱

  • System.out.println(g == (a + b)); //true
  • 自动拆箱了
    

图一运行结果