Growing数客联盟

Scala中使用var和val定义mutable集合与immut

2016-09-24  本文已影响1773人  Woople
scala> val map = scala.collection.mutable.Map(1->"x", 2->"y")
map: scala.collection.mutable.Map[Int,String] = Map(2 -> y, 1 -> x)

//map变量不能再指向其他对象
scala> map = scala.collection.mutable.Map(1->"x", 2->"y")
<console>:12: error: reassignment to val
       map = scala.collection.mutable.Map(1->"x", 2->"y")
           ^

scala> map += (3->"z")
res0: map.type = Map(2 -> y, 1 -> x, 3 -> z)
scala> val map = scala.collection.immutable.Map(1->"x", 2->"y")
map: scala.collection.immutable.Map[Int,String] = Map(1 -> x, 2 -> y)

scala> map += (3->"z") //修改内容会报错
<console>:13: error: value += is not a member of scala.collection.immutable.Map[Int,String]
       map += (3->"z")
           ^

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

scala> map += (3->"z") //并没有任何的错误

scala> 

其实原来immutable对象的内容并没有改变,而是因为执行map += (3->"z")之后在堆内存中又新建了一个对象,这时候的变量map的引用已经改变,指向了不同的对象,可以通过eq方法做个简单的验证

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

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

scala> original_map eq map
res0: Boolean = true

scala> map += (3->"z")

scala> original_map eq map //引用已经改变
res2: Boolean = false

scala> println(original_map) //最初定义的immutable.Map内容并没有改变
Map(1 -> x, 2 -> y)

scala> println(map)
Map(1 -> x, 2 -> y, 3 -> z)
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