在Scala中可以通過==判斷字符串實例是否相等，eg.

scala> val s1 = "Hello"
s1: String = Hello

scala> val s2 = "Hello"
s2: String = Hello

scala> val s3 = "H" + "ello"
s3: String = Hello

scala> val s4 = Array("H", "e", "l", "l", "o").mkString("")  
s4: String = Hello

scala> s1 == s2
res0: Boolean = true

scala> s1 == s3
res1: Boolean = true

scala> s1 == s4
res2: Boolean = true

scala> s2 == s3
res3: Boolean = true

scala> s1.eq
eq   equals   equalsIgnoreCase

scala> s1.equals(s2)
res4: Boolean = true

scala> s1.equals(s3)
res5: Boolean = true

scala> s1.equals(s4)
res6: Boolean = true

scala> s1.eq(s2)
res7: Boolean = true

scala> s1.eq(s3)
res8: Boolean = true

scala> s1.eq(s4)
res9: Boolean = false

在此，特別感謝Z盡際的提醒，增加了val s4 = Array("H", "e", "l", "l", "o").mkString("") 這個例子，s1，s2，s3編譯之后會變成同一個對象的引用，比較字符串相等沒有意義，增加s4更能說明上面的結(jié)論；

==在AnyRef中是一個函數(shù)，其定義如下：

/** The expression `x == that` is equivalent to `if (x eq null) that eq null else x.equals(that)`.
   *
   *  @param    arg0  the object to compare against this object for equality.
   *  @return         `true` if the receiver object is equivalent to the argument; `false` otherwise.
   */
  final def ==(that: Any): Boolean =
    if (this eq null) that.asInstanceOf[AnyRef] eq null
    else this equals that

其中if (this eq null) that.asInstanceOf[AnyRef] eq null會判斷當前String對象是否為空，所以null也可以使用==來比較，并且兩個null字符串比較會得到true的結(jié)果也不會拋出NullPointerException； eg.

scala> s4 = null
<console>:11: error: not found: value s4
       s4 = null
       ^
<console>:12: error: not found: value s4
       val $ires0 = s4
                    ^

scala> val s4:String = null
s4: String = null

scala> val s5:String = null
s5: String = null

scala> s4 == s5
res2: Boolean = true

但是，如果使用null字符串做一些函數(shù)操作，則會拋出NullPointerException，eg.

scala> s4.isEmpty
java.lang.NullPointerException
  ... 28 elided

如果要忽略大小寫比較字符串，可以使用java的equalsIgnoreCase方法，eg.

scala> val s6 = "Amgen"
s6: String = Amgen

scala> val s7 = "amgen"
s7: String = amgen

scala> s6 == s7
res4: Boolean = false

scala> s6.equalsIgnoreCase(s7)
res5: Boolean = true

<font color=#0099ff size=7 face="黑體">\033[1q Scala不推薦使用null，可以使用Option代替 </font>

<font color=red>Markdwon測試</font>
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