<?xml version="1.0" encoding="utf-8"?> <manifest xmlns:android="http://schemas.android.com/apk/res/android"  package="online.geekgalaxy.layoutlearn">  <application  android:allowBackup="true"  android:icon="@mipmap/ic_launcher"  android:label="@string/app_name"  android:roundIcon="@mipmap/ic_launcher_round"  android:supportsRtl="true"  android:theme="@style/AppTheme">  <activity android:name=".MainActivity">   <intent-filter>   <action android:name="android.intent.action.MAIN" />   <category android:name="android.intent.category.LAUNCHER" />   </intent-filter>  </activity>  <activity android:name=".login">   <intent-filter>   <action android:name="android.intent.action.MAIN" />   </intent-filter>  </activity>  <activity android:name=".register">   <intent-filter>   <action android:name="android.intent.action.MAIN" />   </intent-filter>  </activity>  </application> </manifest> 

package online.geekgalaxy.layoutlearn; import android.content.DialogInterface; import android.content.Intent; import android.support.v7.app.AlertDialog; import android.support.v7.app.AppCompatActivity; import android.os.Bundle; import android.view.View; import android.widget.Button; import android.widget.EditText; public class MainActivity extends AppCompatActivity {  @Override  protected void onCreate(Bundle savedInstanceState) {  super.onCreate(savedInstanceState);  setContentView(R.layout.activity_main);  //login button  final Button login = (Button) findViewById(R.id.button);  final String user = "admin";  final String pass = "hello";   login.setOnClickListener(new View.OnClickListener() {   public void onClick(View view) {   String username = "";   EditText editText1 = (EditText)findViewById(R.id.editText);   username = editText1.getText().toString();   String password = "";   EditText editText2 = (EditText)findViewById(R.id.editText2);   password = editText2.getText().toString();   if (username.equals(user) & password.equals(pass)) {    Intent intent = new Intent(MainActivity.this, login.class);    startActivity(intent);   }   else {    new AlertDialog.Builder(MainActivity.this).setTitle("Error!").setMessage("Wrong username or password.")     .setNegativeButton("OK",null)     .show();   }   }  });  //register button  final Button register = (Button) findViewById(R.id.button2);  register.setOnClickListener(new View.OnClickListener() {   public void onClick(View view) {   //提示框確定是否跳轉   new AlertDialog.Builder(MainActivity.this).setTitle("Jump").setMessage("Ready to jump?")    .setPositiveButton("Yes", new DialogInterface.OnClickListener() {     public void onClick(DialogInterface dialog, int which) {     Intent intent = new Intent(MainActivity.this, register.class);     startActivity(intent);     }})    .setNegativeButton("No",null)    .show();   }  });  } } 

package online.geekgalaxy.layoutlearn; import android.app.Activity; import android.os.Bundle; /**  * Created by jailman on 2017/9/18.  */ public class login extends Activity {  @Override  protected void onCreate(Bundle savedInstanceState) {  super.onCreate(savedInstanceState);  setContentView(R.layout.login);  } } 

package online.geekgalaxy.layoutlearn; import android.app.Activity; import android.os.Bundle; /**  * Created by jailman on 2017/9/18.  */ public class register extends Activity{  @Override  protected void onCreate(Bundle savedInstanceState) {  super.onCreate(savedInstanceState);  setContentView(R.layout.register);  } } 

<?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:layout_width="match_parent"  android:layout_height="match_parent"  android:layout_centerVertical="true"  tools:context="online.geekgalaxy.layoutlearn.MainActivity">  <Button  android:id="@+id/button"  android:layout_width="wrap_content"  android:layout_height="wrap_content"  android:layout_marginRight="68dp"  android:text="@string/login"  app:layout_constraintRight_toLeftOf="@+id/button2"  tools:layout_constraintTop_creator="1"  android:layout_marginEnd="68dp"  android:layout_marginTop="26dp"  app:layout_constraintTop_toBottomOf="@+id/editText2" />  <Button  android:id="@+id/button2"  android:layout_width="wrap_content"  android:layout_height="wrap_content"  android:text="@string/register"  tools:layout_constraintTop_creator="1"  tools:layout_constraintRight_creator="1"  android:layout_marginEnd="68dp"  app:layout_constraintRight_toRightOf="parent"  android:layout_marginTop="26dp"  app:layout_constraintTop_toBottomOf="@+id/editText2" />  <EditText  android:id="@+id/editText"  android:layout_width="240dp"  android:layout_height="45dp"  android:layout_marginBottom="35dp"  android:layout_marginEnd="68dp"  android:layout_marginLeft="8dp"  android:layout_marginRight="8dp"  android:layout_marginStart="68dp"  android:ems="10"  android:hint="@string/username"  android:inputType="textPersonName"  app:layout_constraintBottom_toTopOf="@+id/editText2"  app:layout_constraintHorizontal_bias="0.516"  app:layout_constraintLeft_toLeftOf="@+id/editText2"  app:layout_constraintRight_toRightOf="@+id/editText2"  tools:layout_constraintLeft_creator="1"  tools:layout_constraintRight_creator="1"  tools:layout_editor_absoluteX="-15dp"  tools:layout_editor_absoluteY="152dp" />  <EditText  android:id="@+id/editText2"  android:layout_width="240dp"  android:layout_height="45dp"  android:layout_marginEnd="69dp"  android:layout_marginLeft="0dp"  android:layout_marginRight="0dp"  android:layout_marginStart="69dp"  android:ems="10"  android:hint="@string/password"  android:inputType="textPassword"  app:layout_constraintBottom_toBottomOf="parent"  app:layout_constraintLeft_toLeftOf="@+id/button"  app:layout_constraintRight_toRightOf="@+id/button2"  app:layout_constraintTop_toTopOf="parent"  tools:layout_constraintBottom_creator="1"  tools:layout_constraintLeft_creator="1"  tools:layout_constraintRight_creator="1"  tools:layout_constraintTop_creator="1" />  <TextView  android:id="@+id/textView2"  android:layout_width="250dp"  android:layout_height="65dp"  android:layout_marginBottom="50dp"  android:layout_marginLeft="8dp"  android:layout_marginRight="8dp"  android:autoText="false"  android:text="Welcome"  android:textAlignment="center"  android:textSize="50sp"  android:textStyle="bold"  app:layout_constraintBottom_toTopOf="@+id/editText"  app:layout_constraintHorizontal_bias="0.509"  app:layout_constraintLeft_toLeftOf="@+id/editText"  app:layout_constraintRight_toRightOf="@+id/editText" /> </android.support.constraint.ConstraintLayout> 

<?xml version="1.0" encoding="utf-8"?> <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"  android:layout_width="match_parent"  android:layout_height="match_parent"  android:weightSum="1">  <TextView  android:id="@+id/textView3"  android:layout_width="0dp"  android:layout_height="118dp"  android:layout_marginTop="200dp"  android:layout_weight="1"  android:text="@string/great_you_ve_login"  android:textAlignment="center"  android:textSize="24sp"  android:textStyle="bold" /> </LinearLayout> 

<?xml version="1.0" encoding="utf-8"?> <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"  android:layout_width="match_parent"  android:layout_height="match_parent">  <LinearLayout  android:layout_width="match_parent"  android:layout_height="match_parent"  android:gravity="center_vertical|center_horizontal"  android:orientation="vertical">  <EditText   android:id="@+id/editText5"   android:layout_width="270dp"   android:layout_height="wrap_content"   android:ems="10"   android:hint="Username"   android:inputType="textPersonName" />  <EditText   android:id="@+id/editText6"   android:layout_width="270dp"   android:layout_height="wrap_content"   android:ems="10"   android:hint="Email"   android:inputType="textPersonName" />  <EditText   android:id="@+id/editText7"   android:layout_width="270dp"   android:layout_height="wrap_content"   android:ems="10"   android:hint="Password"   android:inputType="textPassword" />  <EditText   android:id="@+id/editText8"   android:layout_width="270dp"   android:layout_height="wrap_content"   android:ems="10"   android:hint="Confirm password"   android:inputType="textPassword" />  <Button   android:id="@+id/button3"   android:layout_width="270dp"   android:layout_height="wrap_content"   android:text="Submit" />  </LinearLayout> </LinearLayout> 

<resources>  <string name="app_name">LayoutLearn</string>  <string name="login">Login</string>  <string name="register">Register</string>  <string name="username">Username</string>  <string name="password">Password</string>  <string name="great_you_ve_login">Great, you/'ve logged in!</string> </resources> 

apply plugin: 'com.android.application' android {  compileSdkVersion 25  buildToolsVersion "25.0.3"  defaultConfig {  applicationId "online.geekgalaxy.layoutlearn"  minSdkVersion 19  targetSdkVersion 25  versionCode 1  versionName "1.0"  testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"  }  buildTypes {  release {   minifyEnabled false   proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'  }  } } dependencies {  compile fileTree(dir: 'libs', include: ['*.jar'])  androidTestCompile('com.android.support.test.espresso:espresso-core:2.2.2', {  exclude group: 'com.android.support', module: 'support-annotations'  })  compile 'com.android.support:appcompat-v7:25.3.1'  compile 'com.android.support.constraint:constraint-layout:1.0.2'  testCompile 'junit:junit:4.12' }