Scala中==,eq與equals的區別

根據官方API的定義：

• final def ==(arg0: Any): Boolean
  The expression x == that is equivalent to if (x eq null) that eq null else x.equals(that).
• final def eq(arg0: AnyRef): Boolean
  Tests whether the argument (that) is a reference to the receiver object (this).
• def equals(arg0: Any): Boolean
  The equality method for reference types.

簡言之，equals方法是檢查值是否相等，而eq方法檢查的是引用是否相等。因此若是比較的對象是null那麼==調用的是eq，不是null的狀況調用的是equals。
看一個簡單的例子：
在java中若是要對兩個對象進行值比較，那麼必需要實現equals 和hashCode方法。而在scala中爲開發者提供了case class，默認實現了equals 和hashCode方法。

scala> case class Bread(brand:String, price:Int)
defined class Bread

scala> val b1 = Bread("BreadTalk", 50)
b1: Bread = Bread(BreadTalk,50)

scala> val b2 = Bread("BreadTalk", 60)
b2: Bread = Bread(BreadTalk,60)

scala> b1 eq b2
res2: Boolean = false

scala> b1 equals b2
res3: Boolean = true

而對於Array或者Map對象不能簡單點使用equals進行值比較，要經過sameElements方法，例如：

scala> val a1 = Array("x", "y")
a1: Array[String] = Array(x, y)

scala> val a2 = Array("x", "y")
a2: Array[String] = Array(x, y)

scala> a1 equals a2
res4: Boolean = false

scala> a1 eq a2
res5: Boolean = false

scala> a1 sameElements a2
res6: Boolean = true

scala> val m1 = Map(1->"x", 2->"y")
m1: scala.collection.immutable.Map[Int,String] = Map(1 -> x, 2 -> y)

scala> val m2 = Map(1->"x", 2->"y")
m2: scala.collection.immutable.Map[Int,String] = Map(1 -> x, 2 -> y)

scala> m1 sameElements m2
res7: Boolean = true

scala> val m3 = Map(1->"x", 2->"z")
m3: scala.collection.immutable.Map[Int,String] = Map(1 -> x, 2 -> z)

scala> m1 sameElements m3
res8: Boolean = false

若是Array中存的是對象，也是同樣的，例如

scala> case class Bread(brand:String, price:Int)
defined class Bread

scala> val b1 = Bread("BreadTalk", 50)
b1: Bread = Bread(BreadTalk,50)

scala> val b2 = Bread("BreadTalk", 50)
b2: Bread = Bread(BreadTalk,50)

scala> val b3 = Bread("BreadTalk", 60)
b3: Bread = Bread(BreadTalk,60)

scala> val a1 = Array(b1)
a1: Array[Bread] = Array(Bread(BreadTalk,50))

scala> val a2 = Array(b2)
a2: Array[Bread] = Array(Bread(BreadTalk,50))

scala> val a3 = Array(b3)
a3: Array[Bread] = Array(Bread(BreadTalk,60))

scala> a1 equals a2
res0: Boolean = false

scala> a1 sameElements a2
res1: Boolean = true

scala> a1 equals a3
res2: Boolean = false

scala> a1 sameElements a3
res3: Boolean = false

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