blocks|key|15457|text|好吧，对于现在可能会发现这个的人来说，Android非常，非常，非常奇怪。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|15458|解决方案:转到build.gradle文件，在依赖项中找到appcompat文件，类似于compile+'com.android.support:appcompat-v7:XX.X.X'|offset|length|style|CODE|15459|将其更改为compile+'com.android.support:appcompat-v7:XX.X.%2B'|15460|然后单击编辑器右上角的Sync+Now。如果代码中的其他内容都是正确的，那么所有的红色行都应该删除。|15461|然后返回到您的build.gradle，并将其更改为无论是什么，然后单击Sync+Now|15462|这一切都应该是好的，因为你有最新版本的一切或接近它。|15463|entityMap^0|0|7|C|T|9|18|1D|0|5|1D|0|B|8|0|7|C|10|8|0|0^^$0|@$1|2|3|4|5|6|7|R|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|S|8|@$D|T|E|U|F|G]|$D|V|E|W|F|G]|$D|X|E|Y|F|G]]|9|@]|A|$]]|$1|H|3|I|5|6|7|Z|8|@$D|10|E|11|F|G]]|9|@]|A|$]]|$1|J|3|K|5|6|7|12|8|@$D|13|E|14|F|G]]|9|@]|A|$]]|$1|L|3|M|5|6|7|15|8|@$D|16|E|17|F|G]|$D|18|E|19|F|G]]|9|@]|A|$]]|$1|N|3|O|5|6|7|1A|8|@]|9|@]|A|$]]|$1|P|3|-4|5|6|7|1B|8|@]|9|@]|A|$]]]|Q|$]]

Okay, to anybody who might stumble upon this now, Android Studio is very, very, very weird. 

Solution : Go to your <code>build.gradle</code> file and in your dependencies find the <code>appcompat</code> one, something like <code>compile 'com.android.support:appcompat-v7:XX.X.X'</code>

Change it to <code>compile 'com.android.support:appcompat-v7:XX.X.+'</code> 

Then click on <code>Sync Now</code> on the top right of the editor. All the red squiggly lines should go if everything else in your code is correct. 

Then go back to your <code>build.gradle</code> and change it back to whatever it was and click <code>Sync Now</code>

It should all be fine given that you have the latest version of everything or close to it.

blocks|key|36283|text|让我们一步一步地进行:首先使用|type|unstyled|depth|inlineStyleRanges|entityRanges|data|36284|建设->清洁|blockquote|36285|如果这没什么用，那就用你的第二个武器|36286|File>Invalidate缓存/重新启动。|36287|但是真正的问题是当以上所有选项都不起作用时，所以使用您的终极武器关闭项目，转到项目位置目录和删除|offset|length|style|BOLD|36288|.idea|36289|您现在可以再次打开项目。这对我来说是有效的，当您使用VCS时，问题经常出现。而且这个问题与android组件类无关，也与库相关，两者都显示出相同的错误。|36290|entityMap^0|0|0|0|0|1A|2|0|0|0^^$0|@$1|2|3|4|5|6|7|U|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|V|8|@]|9|@]|A|$]]|$1|E|3|F|5|6|7|W|8|@]|9|@]|A|$]]|$1|G|3|H|5|D|7|X|8|@]|9|@]|A|$]]|$1|I|3|J|5|6|7|Y|8|@$K|Z|L|10|M|N]]|9|@]|A|$]]|$1|O|3|P|5|D|7|11|8|@]|9|@]|A|$]]|$1|Q|3|R|5|6|7|12|8|@]|9|@]|A|$]]|$1|S|3|-4|5|6|7|13|8|@]|9|@]|A|$]]]|T|$]]

Lets get going step by step:
first clean project by using 

<blockquote>
 Build->Clean
</blockquote>

if this doesn't helps then use your second weapon

<blockquote>
 File>Invalidate Caches/Restart...
</blockquote>

But the real problem begins when all the above options doesn't works
so use your ultimate weapon close project and go to project location directory and delete 

<blockquote>
 .idea
</blockquote>

You can open your project now again. This worked for me and problem comes most often when you are working with VCS. And this problem is not associated with android component classes but also to the libraries, both shows the same error.

blocks|key|2041530|text|我也有同样的问题，但是当我选择Invalidate+cache/restart的时候，它对我来说很好。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|2041531|首先，尝试清除所有构建文件。|2041532|建设->清洁|blockquote|2041533|如果不起作用，请尝试以下选项|2041534|文件->使缓存失效/重新启动|2041535|entityMap^0|F|O|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|Q|8|@$9|R|A|S|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|T|8|@]|D|@]|E|$]]|$1|H|3|I|5|J|7|U|8|@]|D|@]|E|$]]|$1|K|3|L|5|6|7|V|8|@]|D|@]|E|$]]|$1|M|3|N|5|J|7|W|8|@]|D|@]|E|$]]|$1|O|3|-4|5|6|7|X|8|@]|D|@]|E|$]]]|P|$]]

I was having the same issue, but when I did <code>Invalidate cache/restart</code> option it worked fine for me.

First try to clear all your build files

<blockquote>
 Build->Clean
</blockquote>

if it doesn't work try this option

<blockquote>
 File -> Invalidate cache/Restart
</blockquote>

blocks|key|15960|text|如果方法如|type|unstyled|depth|inlineStyleRanges|entityRanges|data|15961|清洁工程|15962|清理Android缓存|15963|清洁级缓存|15964|重新安装IDE|15965|文件->使缓存/重新启动无效|15966|不要工作。使用此方法解决问题。|15967|退出Android|15968|在项目中导航到.idea文件夹|15969|删除.idea文件夹|15970|重新启动Android+Studio。|15971|就是这样。|15972|entityMap^0|0|0|0|0|0|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|Z|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|10|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|11|8|@]|9|@]|A|$]]|$1|F|3|G|5|6|7|12|8|@]|9|@]|A|$]]|$1|H|3|I|5|6|7|13|8|@]|9|@]|A|$]]|$1|J|3|K|5|6|7|14|8|@]|9|@]|A|$]]|$1|L|3|M|5|6|7|15|8|@]|9|@]|A|$]]|$1|N|3|O|5|6|7|16|8|@]|9|@]|A|$]]|$1|P|3|Q|5|6|7|17|8|@]|9|@]|A|$]]|$1|R|3|S|5|6|7|18|8|@]|9|@]|A|$]]|$1|T|3|U|5|6|7|19|8|@]|9|@]|A|$]]|$1|V|3|W|5|6|7|1A|8|@]|9|@]|A|$]]|$1|X|3|-4|5|6|7|1B|8|@]|9|@]|A|$]]]|Y|$]]

If methods like

Clean project

Clean Android Studio cache

Clean Gradle cache

Re-installed IDE

File-> Invalidate cache/restart

do not work. use this method to resolve your issue.

Exit Android Studio

Navigate in your project to the .idea folder

Remove .idea folder

Restart Android Studio.

Thats it.

blocks|key|36635|text|该问题可能是导入引用，请尝试更改它。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|36636|发自：|36637|import+android.support.v7.app.AppCompatActivity;|code-block|syntax|javascript|36638|至：|36639|import+androidx.appcompat.app.AppCompatActivity;|36640|entityMap^0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|O|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|P|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|Q|8|@]|9|@]|A|$G|H]]|$1|I|3|J|5|6|7|R|8|@]|9|@]|A|$]]|$1|K|3|L|5|F|7|S|8|@]|9|@]|A|$G|H]]|$1|M|3|-4|5|6|7|T|8|@]|9|@]|A|$]]]|N|$]]

The issue could be import reference, try changing it.

From:

<pre><code>import android.support.v7.app.AppCompatActivity;
</code></pre>

To:

<pre><code>import androidx.appcompat.app.AppCompatActivity;
</code></pre>

blocks|key|2041599|text|对我来说已经足够继续下去了：-+Tools+>+ANDROID+>用GRADLE文件同步项目|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2041600|​|2041601|📷|atomic|offset|length|2041602|2041603|entityMap|0|IMAGE|mutability|IMMUTABLE|imageUrl|https://developer.qcloudimg.com/http-save/yehe-900000/10459f627f2a6a0983bf820404c78f08.png|imageAlt^0|0|0|0|1|0|0|0^^$0|@$1|2|3|4|5|6|7|S|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|T|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|U|8|@]|9|@$G|V|H|W|1|X]]|A|$]]|$1|I|3|C|5|6|7|Y|8|@]|9|@]|A|$]]|$1|J|3|-4|5|6|7|Z|8|@]|9|@]|A|$]]]|K|$L|$5|M|N|O|A|$P|Q|R|-4]]]]

For me was enough to go on: 
 - Tools > ANDROID > SYNC PROJECT WITH GRADLE FILES

<a href="https://i.stack.imgur.com/Yex6S.png" rel="noreferrer"><img src="https://i.stack.imgur.com/Yex6S.png" alt="The first voice"></a>

blocks|key|35608|text|就在几分钟前，我还遇到了同样的问题，并通过做一个Build+>+Rebuild+Project修复了它。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|35609|希望能帮上忙。|35610|entityMap^0|O|N|0|0^^$0|@$1|2|3|4|5|6|7|J|8|@$9|K|A|L|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|M|8|@]|D|@]|E|$]]|$1|H|3|-4|5|6|7|N|8|@]|D|@]|E|$]]]|I|$]]

I was having this exact same issue just a couple minutes ago and fixed it by doing a <code>Build &gt; Rebuild Project</code>.

Hope it helps.

blocks|key|35612|text|只有删除整个.gradle文件夹才能帮助我。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|35613|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

Only removing whole .gradle folder helped me.

blocks|key|2041764|text|奇怪的是，这就是对我有用的东西：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2041765|将implementation+'com.android.support:appcompat-v7:26%2B'更改为implementation+'com.android.support:appcompat-v7:26.0.1'|ordered-list-item|offset|length|style|CODE|2041766|转到文件→使缓存失效/重新启动|2041767|等待Gradle同步在重新启动时完成。一旦完成，系统将提示您添加Google支持存储库。单击相应的链接，让Gradle解析依赖项。|2041768|错误现在应该消失了|2041769|FWIW，在我的例子中，问题发生在将我的Android安装移到一台新机器并升级到AS+3.1之后。|2041770|entityMap^0|0|1|1H|1L|1K|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|S|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|T|8|@$E|U|F|V|G|H]|$E|W|F|X|G|H]]|9|@]|A|$]]|$1|I|3|J|5|D|7|Y|8|@]|9|@]|A|$]]|$1|K|3|L|5|D|7|Z|8|@]|9|@]|A|$]]|$1|M|3|N|5|D|7|10|8|@]|9|@]|A|$]]|$1|O|3|P|5|6|7|11|8|@]|9|@]|A|$]]|$1|Q|3|-4|5|6|7|12|8|@]|9|@]|A|$]]]|R|$]]

This is what worked for me, strangely enough:

<ol>
<li>Change <code>implementation 'com.android.support:appcompat-v7:26+'</code> to <code>implementation 'com.android.support:appcompat-v7:26.0.1'</code></li>
<li>Go to File → Invalidate Caches/Restart</li>
<li>Wait for the Gradle sync to finish on restarting. Once it does you will be prompted to add the Google maven support repository. Click on the corresponding link, and let Gradle resolve the dependencies.</li>
<li>The errors should be gone now</li>
</ol>

FWIW, in my case the issue occured after moving my Android Studio installation to a new machine and upgrading to AS 3.1.

blocks|key|2041906|text|删除".idea“文件夹对我有用。正如上面维卡斯·库马尔所提到的|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|2041907|然后我使用这修复"SDK位置错误“。|2041908|entityMap|0|LINK|mutability|MUTABLE|url|https://stackoverflow.com/a/45080228/2350490|1|https://stackoverflow.com/a/45865789/2350490^0|L|7|0|0|5|1|1|0^^$0|@$1|2|3|4|5|6|7|P|8|@]|9|@$A|Q|B|R|1|S]]|C|$]]|$1|D|3|E|5|6|7|T|8|@]|9|@$A|U|B|V|1|W]]|C|$]]|$1|F|3|-4|5|6|7|X|8|@]|9|@]|C|$]]]|G|$H|$5|I|J|K|C|$L|M]]|N|$5|I|J|K|C|$L|O]]]]

Deleting the ".idea" folder worked for me. As mentioned above by <a href="https://stackoverflow.com/a/45080228/2350490">Vikas Kumar</a>

Then I using <a href="https://stackoverflow.com/a/45865789/2350490">this</a> to fix the "SDK location error"

blocks|key|15917|text|你的错误就像|type|unstyled|depth|inlineStyleRanges|entityRanges|data|15918|​|15919|📷|atomic|offset|length|15920|15921|1.在同一问题发生后，你首先要清理项目。|15922|2.检查库最低编译"com.android.support:appcompat-v7:22.1.0“|15923|检查库不使用两次或更多次。|ordered-list-item|15924|15925|15926|删除库依赖项的多次使用|15927|将项目清理和重建为got解决方案|15928|entityMap|0|IMAGE|mutability|IMMUTABLE|imageUrl|https://developer.qcloudimg.com/http-save/yehe-900000/08a6e8538baabc04ad64015ded89735b.png|imageAlt|1|https://developer.qcloudimg.com/http-save/yehe-900000/d88b14ceaeac07f97a75323e70e6ab2a.png^0|0|0|0|1|0|0|0|0|0|0|0|1|1|0|0|0|0^^$0|@$1|2|3|4|5|6|7|17|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|18|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|19|8|@]|9|@$G|1A|H|1B|1|1C]]|A|$]]|$1|I|3|C|5|6|7|1D|8|@]|9|@]|A|$]]|$1|J|3|K|5|6|7|1E|8|@]|9|@]|A|$]]|$1|L|3|M|5|6|7|1F|8|@]|9|@]|A|$]]|$1|N|3|O|5|P|7|1G|8|@]|9|@]|A|$]]|$1|Q|3|E|5|F|7|1H|8|@]|9|@$G|1I|H|1J|1|1K]]|A|$]]|$1|R|3|C|5|6|7|1L|8|@]|9|@]|A|$]]|$1|S|3|T|5|P|7|1M|8|@]|9|@]|A|$]]|$1|U|3|V|5|P|7|1N|8|@]|9|@]|A|$]]|$1|W|3|-4|5|6|7|1O|8|@]|9|@]|A|$]]]|X|$Y|$5|Z|10|11|A|$12|13|14|-4]]|15|$5|Z|10|11|A|$12|16|14|-4]]]]

You got error like

<a href="https://i.stack.imgur.com/ADV4O.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/ADV4O.png" alt="enter image description here"></a>

1.First you have to clean project 
after that same issue got then

2.check library minimum
compile "com.android.support:appcompat-v7:22.1.0"

<ol start="3">
<li>Check library does not use two or more times
<a href="https://i.stack.imgur.com/PLH6b.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/PLH6b.png" alt="enter image description here"></a></li>
<li>Remove multiple time usage of library dependency </li>
<li>Clean and Rebuild Project to a got solution</li>
</ol>

blocks|key|2042082|text|我被这个问题困住了，我花了一整天时间来解决this.Finally+--我用以下步骤解决了这个问题|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2042083|关闭你的安卓工作室|ordered-list-item|2042084|转到您的项目文件夹并查找.idea文件夹|2042085|查找库文件夹并重命名它|2042086|然后启动你的Android工作室，然后再打开那个项目。|2042087|就是这样。|2042088|entityMap^0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|O|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|P|8|@]|9|@]|A|$]]|$1|E|3|F|5|D|7|Q|8|@]|9|@]|A|$]]|$1|G|3|H|5|D|7|R|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|S|8|@]|9|@]|A|$]]|$1|K|3|L|5|6|7|T|8|@]|9|@]|A|$]]|$1|M|3|-4|5|6|7|U|8|@]|9|@]|A|$]]]|N|$]]

I was stuck with this problem and its take me whole day to solve this.Finally i solved this with the following steps

<ol>
<li>Close your android studio</li>
<li>Go to you project folder and look for .idea folder</li>
<li>Find libraries folder and rename it</li>
<li>Then start your Android studio and open that project again.</li>
</ol>

Thats it.

blocks|key|15583|text|我也有过同样的问题，下面这个问题对我很有帮助|type|unstyled|depth|inlineStyleRanges|entityRanges|data|15584|File+->+Invalidate+cache/Restart|code-block|syntax|javascript|15585|正在发生某种缓存问题。如果出现这样的问题，请尝试这样做，因为大多数情况下它都会起作用:D|15586|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|K|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|L|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|M|8|@]|9|@]|A|$]]|$1|I|3|-4|5|6|7|N|8|@]|9|@]|A|$]]]|J|$]]

I had the same issue and the following worked for me

<pre><code>File -&gt; Invalidate cache/Restart
</code></pre>

There is some kind of caching issue that is happening. If such kind of issue appears try this since most of the time it will work :D

blocks|key|2041678|text|在使用ubuntu16.04上的android时，我遇到了这个问题，转到File>invalid缓存/还原>失效并重新启动，这对我来说很好|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2041679|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

i have this problem while using the android studio on ubuntu 16.04 go to File>invalid cache/Restore>Invalidate and Restart that works fine for me

blocks|key|15649|text|转到您的build.gradle文件中，在您的依赖项中找到appcompat，类似于编译'com.android.support:appcompat-v7:25.X.X‘，将它更改为编译'com.android.support:appcompat-v7:25.3.%2B’，然后单击编辑器右上方的Sync。然后返回到您的build.gradle，并将其更改回它是什么，并单击同步，现在，这对我有效。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|15650|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

Go to your build.gradle file and in your dependencies find the appcompat one, something like compile 'com.android.support:appcompat-v7:25.X.X'
Change it to compile 'com.android.support:appcompat-v7:25.3.+'
Then click on Sync Now on the top right of the editor.
Then go back to your build.gradle and change it back to whatever it was and click Sync Now
This worked for me.

blocks|key|35717|text|这对我来说很有用，在macOS上：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|35718|转到Android+->+Preferences+->+Preferences&+Behavior+->+System+Settings+->+Android+->+SDK平台，下载Android平台XX和Android的源代码。其中XX在你的等级上是compileSdkVersion的值(模块:app)|35719|在Windows上，路由是:文件->设置.->外观与行为->系统设置->+Android+->+SDK平台|35720|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|H|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|I|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|J|8|@]|9|@]|A|$]]|$1|F|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|G|$]]

This working for me, on macOS:

go to, 
Android Studio -> Preferences -> Appearance &amp; Behavior -> System Settings 
-> Android SDK -> SDK Platforms
And download Android SDK Platform XX and Sources for Android XX. Where XX es the value of compileSdkVersion on your gradle (Module:app)

on Windows the route is:
File -> Settings... -> Appearance &amp; Behavior -> System Settings 
-> Android SDK -> SDK Platforms

blocks|key|36376|text|如果所有尝试都失败(无效缓存、清理和重建、重新启动、.)、关闭Android+(可能只是关闭项目)、运行任何文件管理器、转到项目文件夹、查找..idea/库文件夹并删除其内容(+xml文件列表)。在新的开始或开始项目后，问题应该得到解决。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|36377|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

If all attempts failed (invalidate cache, clean and rebuild, restart,...), close Android Studio (maybe just close the project), run any file manager, go to project folder, find .idea/libraries folder and delete its content (list of xml files). After new start AS or opening project the problem should be resolved.

blocks|key|15867|text|转到build.gradle(模块应用程序)|type|unstyled|depth|inlineStyleRanges|entityRanges|data|15868|如果您将鼠标悬停到com.android.support:appcompat上，弹出窗口会说新版本可用，只需在现有版本号中替换为新版本号，现在按同步键即可。|15869|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|F|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|G|8|@]|9|@]|A|$]]|$1|D|3|-4|5|6|7|H|8|@]|9|@]|A|$]]]|E|$]]

head over to build.gradle(module app)

if you hover your mouse over to com.android.support:appcompat
a popup will say new version is available
just replace in the existing version number with the new version number 
press sync now 
and you are good to go
p.s nothing else worked for me on 2 pc's

blocks|key|15871|text|在Android+3.1.2中，在重命名应用程序包名称后也会出现同样的问题。以下几点对我有用：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|15872|删除".idea“文件夹，然后删除File+->+Invalidate+Caches/Restart。|offset|length|style|CODE|15873|entityMap^0|0|H|X|0^^$0|@$1|2|3|4|5|6|7|J|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|K|8|@$D|L|E|M|F|G]]|9|@]|A|$]]|$1|H|3|-4|5|6|7|N|8|@]|9|@]|A|$]]]|I|$]]

In Android Studio 3.1.2, I have the same issue After renaming application package name. The following worked for me:

Deleting the ".idea" folder, Then <code>File -&gt; Invalidate Caches/Restart</code>.

blocks|key|35799|text|我注意到AndroidStudio3.1并没有无缝地引入支持库。这是如何触发它。在依赖项下的模块的implementation文件中，手动将支持库版本更新到27.x.x更新版本(您可以通过鼠标悬停在implementation旁边的依赖项下获得新版本信息)。然后重新同步这个项目。从这链接中查找可视化解决方案(图像)|type|unstyled|depth|inlineStyleRanges|offset|length|style|BOLD|entityRanges|data|35800|entityMap|0|LINK|mutability|MUTABLE|url|https://i.stack.imgur.com/ET46B.png^0|1D|E|2R|E|3X|1|0|0^^$0|@$1|2|3|4|5|6|7|N|8|@$9|O|A|P|B|C]|$9|Q|A|R|B|C]]|D|@$9|S|A|T|1|U]]|E|$]]|$1|F|3|-4|5|6|7|V|8|@]|D|@]|E|$]]]|G|$H|$5|I|J|K|E|$L|M]]]]

I've noticed that Android Studio 3.1 doesn't pull in the support libraries seamlessly. Here's how to trigger it. Update the support library version to 27.x.x
newer version (you can get the new version info by mouse hover over the version number next to implementation under dependencies ) manually in the module's <code>build.gradle</code> file under dependencies. Then re-sync the project. Find the Visual Solution(Image) from <a href="https://i.stack.imgur.com/ET46B.png" rel="nofollow noreferrer">this</a> Link

blocks|key|2042047|text|这通常发生在您打开任何旧项目时。打开您的Build.gradle+(app模块)文件，将依赖项更新到最新版本，您可以在最近创建的任何项目中找到该版本，或者创建一个新项目并复制依赖项。同步，然后清理项目。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2042048|例:旧版本->编译'com.android.support:appcompat-v7:24‘更新到最新版本-->实现'com.android.support:appcompat-v7:28.1.1’|2042049|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|F|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|G|8|@]|9|@]|A|$]]|$1|D|3|-4|5|6|7|H|8|@]|9|@]|A|$]]]|E|$]]

This usually happens when you open any old project. Open your Build.gradle (app module) file and update the dependencies to the latest version which you can find in any recently created project or create a new project and copy the dependencies. Sync and then clean your project.

EX: Older versions--> compile 'com.android.support:appcompat-v7:24' 
Update to latest version--> implementation 'com.android.support:appcompat-v7:28.1.1'

blocks|key|35843|text|如果通过gradle文件/“失效缓存”的方法不起作用，请使用以下方式：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|35844|在项目中导航到.idea文件夹|ordered-list-item|35845|只需重命名库文件夹|35846|重新启动Android+Studio。它将重新创建库文件夹并再次工作。|35847|entityMap^0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|K|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|L|8|@]|9|@]|A|$]]|$1|E|3|F|5|D|7|M|8|@]|9|@]|A|$]]|$1|G|3|H|5|D|7|N|8|@]|9|@]|A|$]]|$1|I|3|-4|5|6|7|O|8|@]|9|@]|A|$]]]|J|$]]

If the methods via gradle file / "Invalidate caches" do not work, use this way:

<ol>
<li>Navigate in your project to the .idea folder</li>
<li>Just Rename the libraries folder</li>
<li>Restart Android Studio. It will recreate the libraries folder and work again.</li>
</ol>

blocks|key|16035|text|伙计们，我知道这听起来很愚蠢，但我只改变了我的项目，好像我有2个项目在我的系统中，所以我打开了另一个项目，当它完全准备好了，我点击打开打开我的实际项目，它工作了，所以只要尝试切换项目，并重新打开它，这应该会解决你的问题。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|16036|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

Guys I know this is gonna sound stupid but I only switched my project like I had 2 projects in my system so I opened the other project when it was completely ready I clicked on open to open my actual project and it worked so just try to switch projects and re open it, this should sort out your issue.

blocks|key|16132|text|简单|type|unstyled|depth|inlineStyleRanges|entityRanges|data|16133|开放应用程序级build.gradle|unordered-list-item|offset|length|style|CODE|16134|删除应用程序-v7依赖项&同步。|BOLD|16135|再次添加依赖项&同步。|16136|错误不再存在！:)|16137|在此之前|16138|​|16139|📷|atomic|16140|16141|之后|16142|16143|16144|16145|entityMap|0|IMAGE|mutability|IMMUTABLE|imageUrl|https://developer.qcloudimg.com/http-save/yehe-900000/e967838900e4508576839bbc74119306.png|imageAlt|1|https://developer.qcloudimg.com/http-save/yehe-900000/32a90347a0f93c0f96266bf82d774d7f.png^0|0|7|C|0|2|7|0|0|0|0|0|0|1|0|0|0|0|0|0|1|1|0|0^^$0|@$1|2|3|4|5|6|7|1D|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|1E|8|@$E|1F|F|1G|G|H]]|9|@]|A|$]]|$1|I|3|J|5|D|7|1H|8|@$E|1I|F|1J|G|K]]|9|@]|A|$]]|$1|L|3|M|5|D|7|1K|8|@]|9|@]|A|$]]|$1|N|3|O|5|6|7|1L|8|@]|9|@]|A|$]]|$1|P|3|Q|5|6|7|1M|8|@]|9|@]|A|$]]|$1|R|3|S|5|6|7|1N|8|@]|9|@]|A|$]]|$1|T|3|U|5|V|7|1O|8|@]|9|@$E|1P|F|1Q|1|1R]]|A|$]]|$1|W|3|S|5|6|7|1S|8|@]|9|@]|A|$]]|$1|X|3|Y|5|6|7|1T|8|@]|9|@]|A|$]]|$1|Z|3|S|5|6|7|1U|8|@]|9|@]|A|$]]|$1|10|3|U|5|V|7|1V|8|@]|9|@$E|1W|F|1X|1|1Y]]|A|$]]|$1|11|3|S|5|6|7|1Z|8|@]|9|@]|A|$]]|$1|12|3|-4|5|6|7|20|8|@]|9|@]|A|$]]]|13|$14|$5|15|16|17|A|$18|19|1A|-4]]|1B|$5|15|16|17|A|$18|1C|1A|-4]]]]

<h2>Easy</h2>
<ul>
<li>Open app level <code>build.gradle</code></li>
<li>Remove appcompact-v7 dependency &amp; Sync.</li>
<li>Add dependency again &amp; Sync.</li>
</ul>
Error is no more! :)
<h3>Before</h3>
<a href="https://i.stack.imgur.com/195Yi.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/195Yi.png" alt="before" /></a>
<h3>After</h3>
<a href="https://i.stack.imgur.com/8cJ3Y.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/8cJ3Y.png" alt="after" /></a>

blocks|key|36477|text|关闭您的项目使用文件->关闭项目，然后打开，为我工作的所有问题。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|36478|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

Close your project using FILE -> CLOSE PROJECT and then Open, worked for me fixed all the issues.

blocks|key|36480|text|我尝试删除.idea文件夹，清理生成，验证缓存/重新启动，更改构建工具vers.他们都不为我工作。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|36481|我通过选择红色带下划线的代码，然后按Alt%2BEnter并选择Import+选项来解决这个错误。它将导入更改为**androidx.core。*。如果您对所有带红色下划线的代码执行此操作，您的错误应该已经解决。|offset|length|style|BOLD|36482|entityMap^0|0|I|9|U|7|0^^$0|@$1|2|3|4|5|6|7|J|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|K|8|@$D|L|E|M|F|G]|$D|N|E|O|F|G]]|9|@]|A|$]]|$1|H|3|-4|5|6|7|P|8|@]|9|@]|A|$]]]|I|$]]

I tried deleting the .idea folder, Clean Build, Validate Caches / Restart, changing Build Tools vers...
 None of them worked for me.

I solved this error with selecting the red underlined codes then pressing Alt+Enter and selecting the Import Class option.
 It changes the imports to **androidx.core.*******.
 If you do this to all your red underlined codes your errors should have resolved.

blocks|key|15510|text|我已经将appcompat库从25.0.1版本更新为25.1.0|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|15511|在格蕾中：|15512|from:+com.android.support:appcompat-v7:25.0.1

to:+com.android.support:appcompat-v7:25.1.0|code-block|syntax|javascript|15513|现在一切都很顺利。|15514|entityMap^0|4|9|F|6|Q|6|0|0|0|0^^$0|@$1|2|3|4|5|6|7|Q|8|@$9|R|A|S|B|C]|$9|T|A|U|B|C]|$9|V|A|W|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|X|8|@]|D|@]|E|$]]|$1|H|3|I|5|J|7|Y|8|@]|D|@]|E|$K|L]]|$1|M|3|N|5|6|7|Z|8|@]|D|@]|E|$]]|$1|O|3|-4|5|6|7|10|8|@]|D|@]|E|$]]]|P|$]]

I've updated <code>appcompat</code> library from version <code>25.0.1</code> to <code>25.1.0</code>

In gradle:

<pre><code>from: com.android.support:appcompat-v7:25.0.1

to: com.android.support:appcompat-v7:25.1.0
</code></pre>

Everything works well now.

blocks|key|2041722|text|当您的所有依赖项和SDK工具都是在线的，并且仍然存在同步错误时。将此代码添加到您的build.Gradle中并同步：|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|2041723|allprojects+{
++++repositories+{
++++++++jcenter()
++++++++maven+{
++++++++++++url+"https://maven.google.com"
++++++++}
++++}
}|code-block|syntax|javascript|2041724|还想指出一点，在此之前，我测试了这里提到的所有技巧。但只有这样才适用于我的Android+(2.3)+-2月17日发行版。|2041725|entityMap^0|15|C|0|0|0^^$0|@$1|2|3|4|5|6|7|O|8|@$9|P|A|Q|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|R|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|S|8|@]|D|@]|E|$]]|$1|M|3|-4|5|6|7|T|8|@]|D|@]|E|$]]]|N|$]]

When all your dependencies and SDK tools are in-line and it still is giving sync error. Add this code in your <code>build.Gradle</code> and sync it: 

<pre><code>allprojects {
 repositories {
 jcenter()
 maven {
 url "https://maven.google.com"
 }
 }
}
</code></pre>

Also like to point, before doing this I tested all tricks mentioned here. But only this worked for my Android (2.3) - Feb'17 release.

blocks|key|35825|text|随着AndroidStudio3.1.3的新版本，您必须将这段代码放入build.gradle中。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|35826|implementation+'com.android.support:design:23.4.0'|code-block|syntax|javascript|35827|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

With the new Release of android studio 3.1.3, you must put this code inside build.gradle.

<pre><code>implementation 'com.android.support:design:23.4.0'
</code></pre>

blocks|key|2042230|text|打开build.gradle文件，并在您的依赖项和|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2042231|用|2042232|实现'androidx.appcompat:appcompat:1.0.0-beta01‘|2042233|这对我有用..。|2042234|entityMap^0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|J|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|K|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|L|8|@]|9|@]|A|$]]|$1|F|3|G|5|6|7|M|8|@]|9|@]|A|$]]|$1|H|3|-4|5|6|7|N|8|@]|9|@]|A|$]]]|I|$]]

Open build.gradle file and in your dependencies and 

Change file with 

implementation 'androidx.appcompat:appcompat:1.0.0-beta01'

This worked for me..

blocks|key|2042246|text|我也遇到了同样的问题，它通过安装构建工具库来解决。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2042247|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

I had the same issue and it resolved by installing build-tools library.

blocks|key|36555|text|最近我遇到了这个问题，我在我的应用程序的开发中使用了java代码和Kotlin。我真的花了很多时间去寻找问题的原因。我试过了这条评论中的所有内容，但问题仍然没有解决，程序还不能运行。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|36556|因此，我遇到的问题是这个项目的Kotlin版本与android框架中的版本不匹配。|blockquote|36557|当我将其等号时，问题就消失了，程序再次运行。希望这能有所帮助。|36558|​|36559|📷|atomic|offset|length|36560|36561|entityMap|0|IMAGE|mutability|IMMUTABLE|imageUrl|https://developer.qcloudimg.com/http-save/yehe-900000/7709ed6478b9f0bec95c67e4c3a3d70b.png|imageAlt^0|0|0|0|0|0|1|0|0|0^^$0|@$1|2|3|4|5|6|7|X|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|Y|8|@]|9|@]|A|$]]|$1|E|3|F|5|6|7|Z|8|@]|9|@]|A|$]]|$1|G|3|H|5|6|7|10|8|@]|9|@]|A|$]]|$1|I|3|J|5|K|7|11|8|@]|9|@$L|12|M|13|1|14]]|A|$]]|$1|N|3|H|5|6|7|15|8|@]|9|@]|A|$]]|$1|O|3|-4|5|6|7|16|8|@]|9|@]|A|$]]]|P|$Q|$5|R|S|T|A|$U|V|W|-4]]]]

recently I encountered this problem,
I used java code and Kotlin in the development of my application. I really spent a lot of time searching for the cause of the problem. I tried everything in this comment but the problem still doesn't go away, and the program can't be run yet.

<blockquote>
 so the problem I have is about the Kotlin version of the project that
 doesn't match the version that is in the android studio framework.
</blockquote>

when I equate it, the problem disappears and the program runs again.
Hope this helps.

<a href="https://i.stack.imgur.com/wKGuz.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/wKGuz.png" alt="enter image description here"></a>

blocks|key|16303|text|首先尝试这两种解决方案:+1.+文件>>使缓存失效/重新启动和|type|unstyled|depth|inlineStyleRanges|offset|length|style|BOLD|entityRanges|data|16304|支持:appcompat-v7:xx.x.%2B到实现'com.android.support:appcompat-v7:XX.X.%2B'|ordered-list-item|16305|如果以上两种解决方案都不起作用，那就试试吧|16306|i)+退出Android+.|16307|转到您的项目目录.|16308|在项目目录.中找到.idea文件夹|16309|(四)删除.idea文件夹.|16310|(+v)+启动你的安卓工作室.|16311|entityMap^0|G|E|0|0|0|3|B|0|0|9|0|0|6|0|3|B|0|5|A|0^^$0|@$1|2|3|4|5|6|7|W|8|@$9|X|A|Y|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|Z|8|@]|D|@]|E|$]]|$1|I|3|J|5|6|7|10|8|@]|D|@]|E|$]]|$1|K|3|L|5|6|7|11|8|@$9|12|A|13|B|C]]|D|@]|E|$]]|$1|M|3|N|5|6|7|14|8|@$9|15|A|16|B|C]]|D|@]|E|$]]|$1|O|3|P|5|6|7|17|8|@$9|18|A|19|B|C]]|D|@]|E|$]]|$1|Q|3|R|5|6|7|1A|8|@$9|1B|A|1C|B|C]]|D|@]|E|$]]|$1|S|3|T|5|6|7|1D|8|@$9|1E|A|1F|B|C]]|D|@]|E|$]]|$1|U|3|-4|5|6|7|1G|8|@]|D|@]|E|$]]]|V|$]]

Try first these both solution:
 1. File >> Invalidate Caches / Restart and

<ol start="2">
<li>Change implementation 'com.android.support:appcompat-v7:XX.X.+' to implementation 'com.android.support:appcompat-v7:XX.X.+'</li>
</ol>

If above both solution doesn't work ,try this it surely work, it work for me even

i) Exit the Android studio.

ii) Go to your project directory.

iii) Find the .idea folder in your project directory.

iv) Delete .idea folder.

v) Start your android studio.

blocks|key|2042338|text|我也有同样的问题，但当重建项目时，它对我很好。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2042339|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

I was having the same issue, but when re-built project it worked fine for me.

blocks|key|2041797|text|在IDE中导入其他项目时，有时会发生这种情况。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2041798|在这种情况下，尝试构建您的项目2至3次，它会自动得到解决。|2041799|为了重新构建，您需要遵循以下路径：|2041800|建设->重建项目|2041801|entityMap^0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|J|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|K|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|L|8|@]|9|@]|A|$]]|$1|F|3|G|5|6|7|M|8|@]|9|@]|A|$]]|$1|H|3|-4|5|6|7|N|8|@]|9|@]|A|$]]]|I|$]]

This sometimes happens when you import some other projects in your IDE. 
In that case try Building your project 2 to 3 times it automatically gets resolved. 

For Rebuild you need to follow the following path:

Build-> Rebuild Project

After renaming application package name, Android Studio displayed this error <code>can't resolve symbol AppCompatActivity</code> 

Things I tried: 

<ul>
<li>Clean project </li>
<li>Clean Android Studio cache</li>
<li>Clean Gradle cache</li>
<li>Re-installed IDE</li>
<li>File-> Invalidate cache/restart </li>
</ul>

But nothing really helped. The project build successfully, but I can not use syntax highlighting. 

I used <code>com.android.support:appcompat-v7:23.1.1</code> 

Screenshots

<a href="https://i.stack.imgur.com/GIfQw.png" rel="noreferrer"><img src="https://i.stack.imgur.com/GIfQw.png" alt="BaseActivity"></a>

<a href="https://i.stack.imgur.com/7s9uk.png" rel="noreferrer"><img src="https://i.stack.imgur.com/7s9uk.png" alt="Build gradle file"></a>

can't resolve AppCompatActivity

翻译质量差，导致语言生硬或混乱。

没有提供实际的解决方法或示例。

解答不清晰，无法理解或解决问题。

页面排版不美观，阅读体验差。

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重命名应用程序包名称后，Android显示此错误can't resolve symbol AppCompatActivity我尝试过的：清洁工程清理Android缓存清洁级缓存重新安装IDE文件->使缓存/重新启动无效但没有什么真正的帮助。该项目构建成功，但我不能使用语法高亮显示。我用了com.android.support:appcompat-v7:23.1.1截图​​​​

问无法解决AppCompatActivity
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