错误：android studio中的java.lang.ClassNotFoundException

布局文件：

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/activity_main"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context="org.androidtown.sampledialog.MainActivity">

    <TextView
        android:id="@+id/textView"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginTop="80dp"
        android:text="버튼을 누르면 대화상자가 뜹니다."
        android:textSize="24dp"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <Button
        android:id="@+id/button"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginTop="50dp"
        android:text="띄우기"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/textView" />
</android.support.constraint.ConstraintLayout>

主要活动：

package org.androidtown.sampledialog;

import android.content.DialogInterface;
import android.os.Bundle;
import android.support.v7.app.AlertDialog;
import android.support.v7.app.AppCompatActivity;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;

public class MainActivity extends AppCompatActivity {
    TextView textView;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        textView = (TextView) findViewById(R.id.textView);

        Button button = (Button) findViewById(R.id.button);
        button.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                showMessage();
            }
        });
    }

    private void showMessage() {
        AlertDialog.Builder builder = new AlertDialog.Builder(this);
        builder.setTitle("안내");
        builder.setMessage("종료하시겠습니까?");
        builder.setIcon(android.R.drawable.ic_dialog_alert);

        builder.setPositiveButton("예", new DialogInterface.OnClickListener() {
            public void onClick(DialogInterface dialog, int whichButton) {
                String message = "예 버튼이 눌렀습니다.";
                textView.setText(message);
            }
        });

        builder.setNeutralButton("취소",new DialogInterface.OnClickListener() {
            public void onClick(DialogInterface dialog, int whichButton) {
                String message = "취소 버튼이 눌렸습니다.";
                textView.setText(message);
            }
        });

        builder.setNegativeButton("아니오", new DialogInterface.OnClickListener() {
            public void onClick(DialogInterface dialog, int whichButton) {
                String message = "아니오 버튼이 눌렸습니다.";
                textView.setText(message);
            }
        });

        AlertDialog dialog = builder.create();
        dialog.show();
    }

}

我无法构建我的第一个应用程序 . 我应该怎么做？这是我的logcat，xml和java代码 .

引起：java.lang.ClassNotFoundException：未在路径上找到类“android.support.v7.app.AppCompatActivity”：DexPathList [[zip file“/data/app/org.androidtown.sampledialog-1/base . apk“，zip文件”/data/app/org.androidtown.sampledialog-1/split_lib_slice_0_apk.apk“，zip文件”/data/app/org.androidtown.sampledialog-1/split_lib_slice_1_apk.apk“，zip文件”/ data /app/org.androidtown.sampledialog-1/split_lib_slice_2_apk.apk“，zip文件”/data/app/org.androidtown.sampledialog-1/split_lib_slice_3_apk.apk“，zip文件”/data/app/org.androidtown.sampledialog -1 / split_lib_slice_4_apk.apk“，zip文件”/data/app/org.androidtown.sampledialog-1/split_lib_slice_5_apk.apk“，zip文件”/data/app/org.androidtown.sampledialog-1/split_lib_slice_6_apk.apk“， zip文件“/data/app/org.androidtown.sampledialog-1/split_lib_slice_7_apk.apk”，zip文件“/data/app/org.androidtown.sampledialog-1/split_lib_slice_8_apk.apk”，zip文件“/ data / app / org.androidtown.sampledialog-1 / split_lib_slice_9_apk.apk“]，nativeLibraryDirec tories = [/ data / app / org.androidtown.sampledialog-1 / lib / x86，/ system / lib，/ vendor / lib]]在java.lang上的dalvik.system.BaseDexClassLoader.findClass（BaseDexClassLoader.java:56） .ClassLoader.loadClass（ClassLoader.java:380）at java.lang.ClassLoader.loadClass（ClassLoader.java:312）... 15更多