为什么 Scala 的 == 在 Int 的情况下不同？

Question

后台不是应该调用equals吗？

scala> 1 equals 1l
res2: Boolean = false

scala> 1 == 1l
res3: Boolean = true

Answer 1

来自 Programming in Scala 2nd Edition，第 11 章

1 The only cases where == is does not directly call equals is for Java’s boxed numeric classes such as Integer or Long . In Java, a new Integer(1) does not equal a new Long(1) even though for primitive values 1 == 1L . Since Scala is a more regular language than Java it was necessary correct this discrepancy by special-casing the == method for these classes. Likewise, the ## method provides a Scala version of hashing that is the same as Java’s hashCode , except for boxed numeric types, where it works consistently with == . For in- stance new Integer(1) and new Long(1) hash the same with ## even though their Java hashCode s are different.

Answer 2

正如@MichaelLang 所建议的，this 答案可以帮助您，具体来说：

comparing two primitives (boxed or unboxed) with == should always give the result you would have gotten by comparing those values as unboxed primitives. When you call equals directly, you are skipping all that softening logic and instead treated to java's theory that two boxed values of different types are always unequal.

如果分析字节码：

1 == 1l 产生

         0: aload_0       
         1: invokespecial #19                 // Method java/lang/Object."<init>":()V
         4: aload_0       
         5: putstatic     #21                 // Field MODULE$:L;
         8: aload_0       
         9: iconst_1      
        10: putfield      #17                 // Field res0:Z
        13: return

直接比较两个原始 1 的值，但是如果您检查：

1 等于 1l 然后

     0: aload_0       
     1: invokespecial #19                 // Method java/lang/Object."<init>":()V
     4: aload_0       
     5: putstatic     #21                 // Field MODULE$:L;
     8: aload_0       
     9: iconst_1      
    10: invokestatic  #27                 // Method scala/runtime/BoxesRunTime.boxToInteger:(I)Ljava/lang/Integer;
    13: lconst_1      
    14: invokestatic  #31                 // Method scala/runtime/BoxesRunTime.boxToLong:(J)Ljava/lang/Long;
    17: invokevirtual #35                 // Method java/lang/Object.equals:(Ljava/lang/Object;)Z
    20: putfield      #17                 // Field res0:Z
    23: return

您可以看到将 1l 基元装箱到 Long 对象中，然后执行等式。

为什么 Scala 的 == 在 Int 的情况下不同？

Why is Scala's == different in case of Int?

scala

scala-2.10