Kotlin开发Android笔记3：Kotlin中类与函数

—–类的定义—–

如果你想定义一个类，你只需要使用    class   关键字。

<code class="hljs php has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//如定义一个TestKotlin类</span><span class="hljs-class" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">class</span>   <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">TestKotlin</span>{</span>}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li></ul>

它有一个默认唯一的构造器。当然可以创建额外的构造器，通常情况下我们只需要这个默认的构造器。你只需要在类名后面写上它的构造参数，而且如果这个类没有任何内容可以省略大括号：

<code class="hljs scala has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-class" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">class</span>   <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">Student</span><span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">(name:   String, age:Int)</span></span><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//那么构造函数的函数体在哪呢？你可以写在   init    块中：</span><span class="hljs-class" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">class</span>   <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">Teacher</span><span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">(name:String,    age:Int)</span>    {</span>                   init{           <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//TODO</span>        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//在init中做构造方法要做的事情，比如初始化数据等 </span>    }}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li></ul>

—–类继承—– 
任何类在默认情况下都是继承自 Any （就如同Java中所有类都是Object的子类），当然同java一样我们可以继承其它类。 
但是在kotlin中所有的类默认都是不可继承的（相当于java中类有个性符final），因此只能继承那些明确声明为open或者为abstract 的类：

<code class="hljs javascript has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//定义一个open类Bird</span>open <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">class</span> Bird(name: <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">String</span>)<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//定义一个类Duck继承自Bird</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">class</span> Duck(name: <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">String</span>, surname: <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">String</span>) : Bird(name)<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//如果只有单个构造器时，需要在父类继承下来的构造器中指定需要的参数。来替换Java中super调用。</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li></ul>

—-方法（函数）介绍——

函数，也是就我们说的方法在Kotlin中可以使用 fun 关键字定义:

<code class="hljs fsharp has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//定义onCreate方法,没有指定返回值时，它就会返回Unit，与Java中的   void类似</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">fun</span> onCreate(savedInstanceState:    Bundle?)    {      <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//TODO</span>}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li></ul>

Unit 是一个真正的对象。你当然也可以指定任何其它的返回类型：

<code class="hljs vbscript has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">fun add(x:<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">Int</span>,y:<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">Int</span>):<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">Int</span>{            return x+y     }</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li></ul>

如果返回的结果可以使用一个表达式计算出来，你可以不使用括号而是使用等 号：

<code class="hljs vbscript has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">fun add(x:  <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">Int</span>,y:  <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">Int</span>)    :   <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">Int</span> =   x   +   y</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li></ul>

小提示：分号也不是必须的。当然 你也可以使用分号，分号不是必须的。

—-函数参数—— 
Kotlin中的参数与Java中有些不同。上面我们写的一些函数就可以发现：在（）中先写参数的名字再写参数的数据类型：

<code class="hljs vbscript has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">fun add(x:<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">Int</span>,y:<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">Int</span>):<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">Int</span>{      return x+y  }</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li></ul>

下面看下给参数指定一个默认值使得它们变得可选，这是非常有帮助的。这里有一 个例子，在Activity中创建了一个函数用来toast一段信息：

<code class="hljs avrasm has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">//我们指定length的值为Toast<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.LENGTH</span>_SHORTfun toast(message:String,length:Int =Toast<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.LENGTH</span>_SHORT){                                           Toast<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.makeText</span>(this,message,length)<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.show</span>() //就等同于我们java的代码Toast<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.makeText</span>(this,message,Toast<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.LENGTH</span>_SHORT)<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.show</span>()<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">;</span>}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li></ul>

调用的时候可以传入第二个值或者不传，这样可以避免你需要的重载函数：

<code class="hljs scss has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-function" style="box-sizing: border-box;">toast(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"Hello"</span>)</span> <span class="hljs-function" style="box-sizing: border-box;">toast(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"Hello"</span>,   Toast.LENGTH_LONG)</span>;</code>