Groovy与Java的差异

官网：

http://www.groovy-lang.org/differences.html

Groovy试图让Java开发人员尽可能自然。在设计Groovy时，我们尝试遵循最小惊喜的原则，特别是对于那些学习Groovy的开发人员来说，他们来自Java背景。

这里我们列出了Java和Groovy之间的所有主要区别。

1.默认导入

默认情况下导入所有这些包和类，即您不必使用显式import语句来使用它们：

• java.io. *

• 的java.lang。*

• java.math.BigDecimal中

• java.math.BigInteger中

• java.net。*

• java.util中。*

• groovy.lang。*

• groovy.util。*

2.多种方法

在Groovy中，将在运行时选择要调用的方法。这称为运行时调度或多方法。这意味着将根据运行时参数的类型选择方法。在Java中，这是相反的：在编译时根据声明的类型选择方法。

以下代码编写为Java代码，可以在Java和Groovy中编译，但它的行为会有所不同：

<span style="color:#343437"><span style="color:#333333"><code class="language-java"><span style="color:#000088">int</span><span style="color:#000000"> method</span><span style="color:#666600">(</span><span style="color:#660066">String</span><span style="color:#000000"> arg</span><span style="color:#666600">)</span> <span style="color:#666600">{</span>
    <span style="color:#000088">return</span> <span style="color:#006666">1</span><span style="color:#666600">;</span>
<span style="color:#666600">}</span>
<span style="color:#000088">int</span><span style="color:#000000"> method</span><span style="color:#666600">(</span><span style="color:#660066">Object</span><span style="color:#000000"> arg</span><span style="color:#666600">)</span> <span style="color:#666600">{</span>
    <span style="color:#000088">return</span> <span style="color:#006666">2</span><span style="color:#666600">;</span>
<span style="color:#666600">}</span>
<span style="color:#660066">Object</span><span style="color:#000000"> o </span><span style="color:#666600">=</span> <span style="color:#008800">"Object"</span><span style="color:#666600">;</span>
<span style="color:#000088">int</span><span style="color:#000000"> result </span><span style="color:#666600">=</span><span style="color:#000000"> method</span><span style="color:#666600">(</span><span style="color:#000000">o</span><span style="color:#666600">);</span></code></span></span>

在Java中，您将拥有：

<span style="color:#343437"><span style="color:#333333"><code class="language-java"><span style="color:#000000">assertEquals</span><span style="color:#666600">(</span><span style="color:#006666">2</span><span style="color:#666600">,</span><span style="color:#000000"> result</span><span style="color:#666600">);</span></code></span></span>

在Groovy中：

<span style="color:#343437"><span style="color:#333333"><code class="language-java"><span style="color:#000000">assertEquals</span><span style="color:#666600">(</span><span style="color:#006666">1</span><span style="color:#666600">,</span><span style="color:#000000"> result</span><span style="color:#666600">);</span></code></span></span>

这是因为Java将使用静态信息类型，该类型o被声明为Object，而Groovy将在运行时选择实际调用该方法。由于它是用a调用的String，因此调用该 String版本。

3.数组初始值设定项

在Groovy中，{ …​ }块被保留用于闭包。这意味着您无法使用以下语法创建数组文字：

<span style="color:#343437"><span style="color:#333333"><code class="language-groovy"><span style="color:#000088">int</span><span style="color:#666600">[]</span><span style="color:#000000"> array </span><span style="color:#666600">=</span> <span style="color:#666600">{</span> <span style="color:#006666">1</span><span style="color:#666600">,</span> <span style="color:#006666">2</span><span style="color:#666600">,</span> <span style="color:#006666">3</span><span style="color:#666600">}</span></code></span></span>

你实际上必须使用：

<span style="color:#343437"><span style="color:#333333"><code class="language-groovy"><span style="color:#000088">int</span><span style="color:#666600">[]</span><span style="color:#000000"> array </span><span style="color:#666600">=</span> <span style="color:#666600">[</span><span style="color:#006666">1</span><span style="color:#666600">,</span><span style="color:#006666">2</span><span style="color:#666600">,</span><span style="color:#006666">3</span><span style="color:#666600">]</span></code></span></span>

4.包装范围可见性

在Groovy中，省略字段上的修饰符不会导致像Java一样的包私有字段：

<span style="color:#343437"><span style="color:#333333"><code class="language-groovy"><span style="color:#000088">class</span> <span style="color:#660066">Person</span> <span style="color:#666600">{</span>
    <span style="color:#660066">String</span><span style="color:#000000"> name
</span><span style="color:#666600">}</span></code></span></span>

相反，它用于创建属性，即私有字段，关联的getter和关联的 setter。

可以通过以下方式对其进行批注来创建包私有字段@PackageScope：

<span style="color:#343437"><span style="color:#333333"><code class="language-groovy"><span style="color:#000088">class</span> <span style="color:#660066">Person</span> <span style="color:#666600">{</span>
    <span style="color:#006666">@PackageScope</span> <span style="color:#660066">String</span><span style="color:#000000"> name
</span><span style="color:#666600">}</span></code></span></span>

5. ARM块

Groovy不支持Java 7中的ARM（自动资源管理）块。相反，Groovy提供了依赖于闭包的各种方法，这些方法具有相同的效果，同时更具惯用性。例如：

<span style="color:#343437"><span style="color:#333333"><code class="language-java"><span style="color:#660066">Path</span><span style="color:#000000"> file </span><span style="color:#666600">=</span> <span style="color:#660066">Paths</span><span style="color:#666600">.</span><span style="color:#000000">get</span><span style="color:#666600">(</span><span style="color:#008800">"/path/to/file"</span><span style="color:#666600">);</span>
<span style="color:#660066">Charset</span><span style="color:#000000"> charset </span><span style="color:#666600">=</span> <span style="color:#660066">Charset</span><span style="color:#666600">.</span><span style="color:#000000">forName</span><span style="color:#666600">(</span><span style="color:#008800">"UTF-8"</span><span style="color:#666600">);</span>
<span style="color:#000088">try</span> <span style="color:#666600">(</span><span style="color:#660066">BufferedReader</span><span style="color:#000000"> reader </span><span style="color:#666600">=</span> <span style="color:#660066">Files</span><span style="color:#666600">.</span><span style="color:#000000">newBufferedReader</span><span style="color:#666600">(</span><span style="color:#000000">file</span><span style="color:#666600">,</span><span style="color:#000000"> charset</span><span style="color:#666600">))</span> <span style="color:#666600">{</span>
    <span style="color:#660066">String</span><span style="color:#000000"> line</span><span style="color:#666600">;</span>
    <span style="color:#000088">while</span> <span style="color:#666600">((</span><span style="color:#000000">line </span><span style="color:#666600">=</span><span style="color:#000000"> reader</span><span style="color:#666600">.</span><span style="color:#000000">readLine</span><span style="color:#666600">())</span> <span style="color:#666600">!=</span> <span style="color:#000088">null</span><span style="color:#666600">)</span> <span style="color:#666600">{</span>
        <span style="color:#660066">System</span><span style="color:#666600">.</span><span style="color:#000000">out</span><span style="color:#666600">.</span><span style="color:#000000">println</span><span style="color:#666600">(</span><span style="color:#000000">line</span><span style="color:#666600">);</span>
    <span style="color:#666600">}</span>

<span style="color:#666600">}</span> <span style="color:#000088">catch</span> <span style="color:#666600">(</span><span style="color:#660066">IOException</span><span style="color:#000000"> e</span><span style="color:#666600">)</span> <span style="color:#666600">{</span><span style="color:#000000">
    e</span><span style="color:#666600">.</span><span style="color:#000000">printStackTrace</span><span style="color:#666600">();</span>
<span style="color:#666600">}</span></code></span></span>

可以像这样写：

<span style="color:#343437"><span style="color:#333333"><code class="language-groovy"><span style="color:#000088">new</span> <span style="color:#660066">File</span><span style="color:#666600">(</span><span style="color:#008800">'/path/to/file'</span><span style="color:#666600">).</span><span style="color:#000000">eachLine</span><span style="color:#666600">(</span><span style="color:#008800">'UTF-8'</span><span style="color:#666600">)</span> <span style="color:#666600">{</span><span style="color:#000000">
   println it
</span><span style="color:#666600">}</span></code></span></span>

或者，如果你想要一个更接近Java的版本：

<span style="color:#343437"><span style="color:#333333"><code class="language-groovy"><span style="color:#000088">new</span> <span style="color:#660066">File</span><span style="color:#666600">(</span><span style="color:#008800">'/path/to/file'</span><span style="color:#666600">).</span><span style="color:#000000">withReader</span><span style="color:#666600">(</span><span style="color:#008800">'UTF-8'</span><span style="color:#666600">)</span> <span style="color:#666600">{</span><span style="color:#000000"> reader </span><span style="color:#666600">-></span><span style="color:#000000">
   reader</span><span style="color:#666600">.</span><span style="color:#000000">eachLine </span><span style="color:#666600">{</span><span style="color:#000000">
       println it
   </span><span style="color:#666600">}</span>
<span style="color:#666600">}</span></code></span></span>

6.内部课程

6.1。静态内部类

这是静态内部类的一个例子：

<span style="color:#343437"><span style="color:#333333"><code class="language-groovy"><span style="color:#000088">class</span><span style="color:#000000"> A </span><span style="color:#666600">{</span>
    <span style="color:#000088">static</span> <span style="color:#000088">class</span><span style="color:#000000"> B </span><span style="color:#666600">{}</span>
<span style="color:#666600">}</span>

<span style="color:#000088">new</span><span style="color:#000000"> A</span><span style="color:#666600">.</span><span style="color:#000000">B</span><span style="color:#666600">()</span></code></span></span>

静态内部类的使用是最受支持的。如果你绝对需要一个内部类，你应该把它变成一个静态类。

6.2。匿名内部类

<span style="color:#343437"><span style="color:#333333"><code class="language-groovy"><span style="color:#000088">import</span><span style="color:#000000"> java</span><span style="color:#666600">.</span><span style="color:#000000">util</span><span style="color:#666600">.</span><span style="color:#000000">concurrent</span><span style="color:#666600">.</span><span style="color:#660066">CountDownLatch</span>
<span style="color:#000088">import</span><span style="color:#000000"> java</span><span style="color:#666600">.</span><span style="color:#000000">util</span><span style="color:#666600">.</span><span style="color:#000000">concurrent</span><span style="color:#666600">.</span><span style="color:#660066">TimeUnit</span>

<span style="color:#660066">CountDownLatch</span><span style="color:#000000"> called </span><span style="color:#666600">=</span> <span style="color:#000088">new</span> <span style="color:#660066">CountDownLatch</span><span style="color:#666600">(</span><span style="color:#006666">1</span><span style="color:#666600">)</span>

<span style="color:#660066">Timer</span><span style="color:#000000"> timer </span><span style="color:#666600">=</span> <span style="color:#000088">new</span> <span style="color:#660066">Timer</span><span style="color:#666600">()</span><span style="color:#000000">
timer</span><span style="color:#666600">.</span><span style="color:#000000">schedule</span><span style="color:#666600">(</span><span style="color:#000088">new</span> <span style="color:#660066">TimerTask</span><span style="color:#666600">()</span> <span style="color:#666600">{</span>
    <span style="color:#000088">void</span><span style="color:#000000"> run</span><span style="color:#666600">()</span> <span style="color:#666600">{</span><span style="color:#000000">
        called</span><span style="color:#666600">.</span><span style="color:#000000">countDown</span><span style="color:#666600">()</span>
    <span style="color:#666600">}</span>
<span style="color:#666600">},</span> <span style="color:#006666">0</span><span style="color:#666600">)</span>

<span style="color:#000088">assert</span><span style="color:#000000"> called</span><span style="color:#666600">.</span><span style="color:#000000">await</span><span style="color:#666600">(</span><span style="color:#006666">10</span><span style="color:#666600">,</span> <span style="color:#660066">TimeUnit</span><span style="color:#666600">.</span><span style="color:#000000">SECONDS</span><span style="color:#666600">)</span></code></span></span>

6.3。创建非静态内部类的实例

在Java中，您可以这样做：

<span style="color:#343437"><span style="color:#333333"><code class="language-java"><span style="color:#000088">public</span> <span style="color:#000088">class</span><span style="color:#000000"> Y </span><span style="color:#666600">{</span>
    <span style="color:#000088">public</span> <span style="color:#000088">class</span><span style="color:#000000"> X </span><span style="color:#666600">{}</span>
    <span style="color:#000088">public</span><span style="color:#000000"> X foo</span><span style="color:#666600">()</span> <span style="color:#666600">{</span>
        <span style="color:#000088">return</span> <span style="color:#000088">new</span><span style="color:#000000"> X</span><span style="color:#666600">();</span>
    <span style="color:#666600">}</span>
    <span style="color:#000088">public</span> <span style="color:#000088">static</span><span style="color:#000000"> X createX</span><span style="color:#666600">(</span><span style="color:#000000">Y y</span><span style="color:#666600">)</span> <span style="color:#666600">{</span>
        <span style="color:#000088">return</span><span style="color:#000000"> y</span><span style="color:#666600">.</span><span style="color:#000088">new</span><span style="color:#000000"> X</span><span style="color:#666600">();</span>
    <span style="color:#666600">}</span>
<span style="color:#666600">}</span></code></span></span>

Groovy不支持y.new X()语法。相反，你必须写new X(y)，如下面的代码：

<span style="color:#343437"><span style="color:#333333"><code class="language-groovy"><span style="color:#000088">public</span> <span style="color:#000088">class</span><span style="color:#000000"> Y </span><span style="color:#666600">{</span>
    <span style="color:#000088">public</span> <span style="color:#000088">class</span><span style="color:#000000"> X </span><span style="color:#666600">{}</span>
    <span style="color:#000088">public</span><span style="color:#000000"> X foo</span><span style="color:#666600">()</span> <span style="color:#666600">{</span>
        <span style="color:#000088">return</span> <span style="color:#000088">new</span><span style="color:#000000"> X</span><span style="color:#666600">()</span>
    <span style="color:#666600">}</span>
    <span style="color:#000088">public</span> <span style="color:#000088">static</span><span style="color:#000000"> X createX</span><span style="color:#666600">(</span><span style="color:#000000">Y y</span><span style="color:#666600">)</span> <span style="color:#666600">{</span>
        <span style="color:#000088">return</span> <span style="color:#000088">new</span><span style="color:#000000"> X</span><span style="color:#666600">(</span><span style="color:#000000">y</span><span style="color:#666600">)</span>
    <span style="color:#666600">}</span>
<span style="color:#666600">}</span></code></span></span>

兰布达斯

Java 8支持lambda和方法引用：

<span style="color:#343437"><span style="color:#333333"><code class="language-groovy"><span style="color:#660066">Runnable</span><span style="color:#000000"> run </span><span style="color:#666600">=</span> <span style="color:#666600">()</span> <span style="color:#666600">-></span> <span style="color:#660066">System</span><span style="color:#666600">.</span><span style="color:#000088">out</span><span style="color:#666600">.</span><span style="color:#000000">println</span><span style="color:#666600">(</span><span style="color:#008800">"Run"</span><span style="color:#666600">);</span><span style="color:#000000">
list</span><span style="color:#666600">.</span><span style="color:#000000">forEach</span><span style="color:#666600">(</span><span style="color:#660066">System</span><span style="color:#666600">.</span><span style="color:#000088">out</span><span style="color:#666600">::</span><span style="color:#000000">println</span><span style="color:#666600">);</span></code></span></span>

Java 8 lambdas可以或多或少地被视为匿名内部类。Groovy不支持该语法，但有闭包：

<span style="color:#343437"><span style="color:#333333"><code class="language-groovy"><span style="color:#660066">Runnable</span><span style="color:#000000"> run </span><span style="color:#666600">=</span> <span style="color:#666600">{</span><span style="color:#000000"> println </span><span style="color:#008800">'run'</span> <span style="color:#666600">}</span><span style="color:#000000">
list</span><span style="color:#666600">.</span><span style="color:#000000">each </span><span style="color:#666600">{</span><span style="color:#000000"> println it </span><span style="color:#666600">}</span> <span style="color:#880000">// or list.each(this.&println)</span></code></span></span>

8. GStrings

由于双引号字符串文字被解释为GString值，如果String使用Groovy和Java编译器编译包含美元字符的文字的类，Groovy可能会因编译错误而失败或产生微妙的不同代码。

虽然通常情况下，Groovy会在两者之间自动转换GString，String如果API声明了参数的类型，请注意接受Object参数的Java API，然后检查实际类型。

9.字符串和字符文字

Groovy中单引号的文字用于String，双引号结果 String或GString取决于文字中是否有插值。

<span style="color:#343437"><span style="color:#333333"><code class="language-groovy"><span style="color:#000088">assert</span> <span style="color:#008800">'c'</span><span style="color:#666600">.</span><span style="color:#000000">getClass</span><span style="color:#666600">()==</span><span style="color:#660066">String</span>
<span style="color:#000088">assert</span> <span style="color:#008800">"c"</span><span style="color:#666600">.</span><span style="color:#000000">getClass</span><span style="color:#666600">()==</span><span style="color:#660066">String</span>
<span style="color:#000088">assert</span> <span style="color:#008800">"c${1}"</span><span style="color:#666600">.</span><span style="color:#000000">getClass</span><span style="color:#666600">()</span> <span style="color:#000088">in</span> <span style="color:#660066">GString</span></code></span></span>

常规将自动投单字符String到char指定给类型的变量时只char。当使用类型参数调用方法时，char我们需要显式转换或确保已提前转换值。

<span style="color:#343437"><span style="color:#333333"><code class="language-groovy"><span style="color:#000088">char</span><span style="color:#000000"> a</span><span style="color:#666600">=</span><span style="color:#008800">'a'</span>
<span style="color:#000088">assert</span> <span style="color:#660066">Character</span><span style="color:#666600">.</span><span style="color:#000000">digit</span><span style="color:#666600">(</span><span style="color:#000000">a</span><span style="color:#666600">,</span> <span style="color:#006666">16</span><span style="color:#666600">)==</span><span style="color:#006666">10</span> <span style="color:#666600">:</span> <span style="color:#008800">'But Groovy does boxing'</span>
<span style="color:#000088">assert</span> <span style="color:#660066">Character</span><span style="color:#666600">.</span><span style="color:#000000">digit</span><span style="color:#666600">((</span><span style="color:#000088">char</span><span style="color:#666600">)</span> <span style="color:#008800">'a'</span><span style="color:#666600">,</span> <span style="color:#006666">16</span><span style="color:#666600">)==</span><span style="color:#006666">10</span>

<span style="color:#000088">try</span> <span style="color:#666600">{</span>
  <span style="color:#000088">assert</span> <span style="color:#660066">Character</span><span style="color:#666600">.</span><span style="color:#000000">digit</span><span style="color:#666600">(</span><span style="color:#008800">'a'</span><span style="color:#666600">,</span> <span style="color:#006666">16</span><span style="color:#666600">)==</span><span style="color:#006666">10</span>
  <span style="color:#000088">assert</span> <span style="color:#000088">false</span><span style="color:#666600">:</span> <span style="color:#008800">'Need explicit cast'</span>
<span style="color:#666600">}</span> <span style="color:#000088">catch</span><span style="color:#666600">(</span><span style="color:#660066">MissingMethodException</span><span style="color:#000000"> e</span><span style="color:#666600">)</span> <span style="color:#666600">{</span>
<span style="color:#666600">}</span></code></span></span>

Groovy支持两种类型的转换，在转换的情况下，char在转换多字符串时会有细微差别。Groovy样式转换更宽松，将采用第一个字符，而C样式转换将失败，但异常。

<span style="color:#343437"><span style="color:#333333"><code class="language-groovy"><span style="color:#880000">// for single char strings, both are the same</span>
<span style="color:#000088">assert</span> <span style="color:#666600">((</span><span style="color:#000088">char</span><span style="color:#666600">)</span> <span style="color:#008800">"c"</span><span style="color:#666600">).</span><span style="color:#000088">class</span><span style="color:#666600">==</span><span style="color:#660066">Character</span>
<span style="color:#000088">assert</span> <span style="color:#666600">(</span><span style="color:#008800">"c"</span> <span style="color:#000088">as</span> <span style="color:#000088">char</span><span style="color:#666600">).</span><span style="color:#000088">class</span><span style="color:#666600">==</span><span style="color:#660066">Character</span>

<span style="color:#880000">// for multi char strings they are not</span>
<span style="color:#000088">try</span> <span style="color:#666600">{</span>
  <span style="color:#666600">((</span><span style="color:#000088">char</span><span style="color:#666600">)</span> <span style="color:#008800">'cx'</span><span style="color:#666600">)</span> <span style="color:#666600">==</span> <span style="color:#008800">'c'</span>
  <span style="color:#000088">assert</span> <span style="color:#000088">false</span><span style="color:#666600">:</span> <span style="color:#008800">'will fail - not castable'</span>
<span style="color:#666600">}</span> <span style="color:#000088">catch</span><span style="color:#666600">(</span><span style="color:#660066">GroovyCastException</span><span style="color:#000000"> e</span><span style="color:#666600">)</span> <span style="color:#666600">{</span>
<span style="color:#666600">}</span>
<span style="color:#000088">assert</span> <span style="color:#666600">(</span><span style="color:#008800">'cx'</span> <span style="color:#000088">as</span> <span style="color:#000088">char</span><span style="color:#666600">)</span> <span style="color:#666600">==</span> <span style="color:#008800">'c'</span>
<span style="color:#000088">assert</span> <span style="color:#008800">'cx'</span><span style="color:#666600">.</span><span style="color:#000000">asType</span><span style="color:#666600">(</span><span style="color:#000088">char</span><span style="color:#666600">)</span> <span style="color:#666600">==</span> <span style="color:#008800">'c'</span></code></span></span>

10.原始和包装

由于Groovy使用对象的一切，它autowraps到原语的引用。因此，它不遵循Java的扩展行为优先于拳击。这是一个使用的例子int

<span style="color:#343437"><span style="color:#333333"><code class="language-groovy"><span style="color:#000088">int</span><span style="color:#000000"> i
m</span><span style="color:#666600">(</span><span style="color:#000000">i</span><span style="color:#666600">)</span>

<span style="color:#000088">void</span><span style="color:#000000"> m</span><span style="color:#666600">(</span><span style="color:#000088">long</span><span style="color:#000000"> l</span><span style="color:#666600">)</span> <span style="color:#666600">{</span>           <span style="color:#000000">
  println </span><span style="color:#008800">"in m(long)"</span>
<span style="color:#666600">}</span>

<span style="color:#000088">void</span><span style="color:#000000"> m</span><span style="color:#666600">(</span><span style="color:#660066">Integer</span><span style="color:#000000"> i</span><span style="color:#666600">)</span> <span style="color:#666600">{</span>        <span style="color:#000000">
  println </span><span style="color:#008800">"in m(Integer)"</span>
<span style="color:#666600">}</span></code></span></span>

11.行为 ==

在Java中==意味着对象的基本类型或标识的相等性。在Groovy ==中a.compareTo(b)==0，如果是Comparable， 则转换为a.equals(b)。为了检查身份，有is。例如 a.is(b)。

12.转换

Java会自动扩展和缩小 转换次数。

* 'Y'表示Java可以进行的转换，'C'表示Java可以在存在显式转换时进行的转换，'T`表示Java可以进行的转换但是数据被截断，'N'表示Java可以转换'做。

Groovy对此有很大的扩展。

* 'Y'表示Groovy可以进行的转换，'D'表示Groovy在动态编译或显式转换时可以进行的转换，'T`表示Groovy可以进行转换，但数据被截断，'B'表示装箱/拆箱操作，'N'表示Groovy无法进行的转换。

转换为/ 时，截断使用Groovy Truth。从数字转换为字符会转换为。Groovy的构建体和 使用从一个转换时或者，否则的构造方法使用。其他转换的行为由其定义。booleanBooleanNumber.intvalue()charBigIntegerBigDecimalNumber.doubleValue()FloatDoubletoString()java.lang.Number

13.额外的关键字

Groovy中有一些关键字比Java更多。不要将它们用于变量名称等。

• as

• def

• in

• trait

最后

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