Scala中==,eq与equals的区别

根据官方API的定义：

• 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.

简言之，equals方法是检查值是否相等，而eq方法检查的是引用是否相等。所以如果比较的对象是null那么==调用的是eq，不是null的情况调用的是equals。

看一个简单的例子：

在java中如果要对两个对象进行值比较，那么必须要实现equals 和hashCode方法。而在scala中为开发者提供了case class，默认实现了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

而对于Array或者Map对象不能简单点使用equals进行值比较，要通过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

如果Array中存的是对象，也是一样的，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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