Scala Cookbook1.1 比较字符串的相等性

问题：比较两个字符串是否相等？

解决办法：使用==方法，使用该方法的好处是即使当字符串为null时，他也不会抛出NullPointerException。

val s1 = "hello"
val s2 = "Hello"
val s3 = "H" + "ello"
val s4 = null

println(s1 == s2)
println(s1 == s3)
println(s3 == s4)

如果不考虑字符的大小写，需要先将比较的字符串转化为全部大写。

val s1 = "hello"
val s2 = "Hello"

println(s1.toUpperCase == s2.toUpperCase)

但是这样如果比较的双方有一个为null，则会抛出NullPointerException。

原因分析

为什么==不会抛出NullPointerException?

这是应为==其实是一个函数，来自于scala.AnyRef.

看一下官方文档：

class AnyRef extends Any

class AnyRef is the root class of all reference types.All types except the value types descend from this class.

Linear supertypes
Any

value Members

	final def !=(arg0: Any):Boolen
		Test two objects for inequality
	final def ##(): Int
		Equivalent to x.hashCOde except for boxed numeric types and null
	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 refrence to the receiver object(this)
	      def equals(arg0: Any): Boolean
	      The equality method for reference types
	final def getClass(): Class[_]
		Returns the runtime class representation of the object
	      def hashCode(): Int
	      	The hashCode method for the reference types
	final def ne(arg0: AnyRef): Boolean
		Equivalent to !(this eq that)
	final def notify(): Unit
		Wakes up a single thread that is waiting on the receiver object's monitor.
	final def notifyAll(): Unit
		Wakes up all threads that are waiting for on the receiver object's monitor
	final def synchronized[T0](arg0: => T0): T0): T0

	       def toString(): String
	       	createa a String representation of this object
	final def wait(): Unit

	final def wait(arg0: Long, arg1:Int): Unit

	final def wait(arg0: Long): Unit

参考文献

1.Scala官方文档-AnyRef