kotlin - Kotlin中的引用平等

Question

在教程示例中，我正在学习Kotlin:

fun main() {
    val a: Int = 100
    val boxedA: Int? = a
    val anotherBoxedA: Int? = a

    val b: Int = 1000
    val boxedB: Int? = b
    val anotherBoxedB: Int? = b

    println(boxedA === anotherBoxedA) // true
    println(boxedB === anotherBoxedB) // false
}

为什么两个比较的结果不同？

Answer 1

最可能是因为Integer.valueOf的JDK实现
https://docs.oracle.com/javase/7/docs/api/java/lang/Integer.html#valueOf(int)

Returns an Integer instance representing the specified int value. If a new Integer instance is not required, this method should generally be used in preference to the constructor 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.

如果您在Intellij中反编译该方法，则会发现

   public static final void main() {
      int a = 100;
      Integer boxedA = Integer.valueOf(a);
      Integer anotherBoxedA = Integer.valueOf(a);
      int b = 1000;
      Integer boxedB = Integer.valueOf(b);
      Integer anotherBoxedB = Integer.valueOf(b);
      boolean var6 = boxedA == anotherBoxedA;
      boolean var7 = false;
      System.out.println(var6);
      var6 = boxedB == anotherBoxedB;
      var7 = false;
      System.out.println(var6);
   }