Scala判断字符串是否相等

在Scala中可以通过==判断字符串实例是否相等，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，特别感谢Z尽际的提醒，增加了val s4 = Array("H", "e", "l", "l", "o").mkString("") 这个例子，s1，s2，s3编译之后会变成同一个对象的引用，比较字符串相等没有意义，增加s4更能说明上面的结论；

==在AnyRef中是一个函数，其定义如下：

/** 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会判断当前String对象是否为空，所以null也可以使用==来比较，并且两个null字符串比较会得到true的结果也不会抛出NullPointerException； eg.

scala> s4 = null
:11: error: not found: value s4
       s4 = null
       ^
: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字符串做一些函数操作，则会抛出NullPointerException，eg.

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

如果要忽略大小写比较字符串，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

\033[1q Scala不推荐使用null，可以使用Option代替

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