U3

一个项目里面可以有多个Activity

AndroidManifest.xml
<intent-filter>
    <action android:name="android.intent.action.MAIN" />

    <category android:name="android.intent.category.LAUNCHER" />
</intent-filter>

在谁的里面就运行谁

在两个Activity 里切换

package com.example.chapter31;

import android.content.Intent;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.widget.Button;

public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Log.i("Activity1","onCreate");
        Button  button =(Button)findViewById(R.id.button);
        button.setOnClickListener (new View.OnClickListener(){
            @Override
            public void onClick (View view){
                Intent intent = new Intent (MainActivity.this,SecondActivity.class);
                startActivity(intent);
            }});
    }
    @Override
    protected void onStart() {
        super.onStart();
        Log.i("Activity1","onStart()");
    }
    @Override
    protected void onRestart() {
        super.onRestart();
        Log.i("Activity1","onRestart()");
    }
    @Override
    protected void onResume() {
        super.onResume();
        Log.i("Activity1","onResume()");
    }
    @Override
    protected void onPause() {
        super.onPause();
        Log.i("Activity1","onPause()");
    }
    @Override
    protected void onStop() {
        super.onStop();
        Log.i("Activity1","onStop()");
    }
    @Override
    protected void onDestroy() {
        super.onDestroy();
        Log.i("Activity1","onDestroy()");
    }
}

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout 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"
    tools:context=".MainActivity">
<Button
    android:id = "@+id/button"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_weight = ""
    android:text = "跳转到第二个Activity"  />

</LinearLayout>

package com.example.chapter31;
import android.app.Activity;
import android.content.Intent;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.widget.Button;

public class SecondActivity extends Activity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.layout1);
        Log.i("Activity1","onCreate");
        Button button = (Button)findViewById(R.id.button);
        button.setOnClickListener (new View.OnClickListener(){
            @Override
            public void onClick (View view){
                Intent intent = new Intent (SecondActivity.this,MainActivity.class);
从2到1
                startActivity(intent);
            }});
    }
    @Override
    protected void onStart() {
        super.onStart();
        Log.i("Activity2","onStart()");
    }
    @Override
    protected void onRestart() {
        super.onRestart();
        Log.i("Activity2","onRestart()");
    }
    @Override
    protected void onResume() {
        super.onResume();
        Log.i("Activity2","onResume()");
    }
    @Override
    protected void onPause() {
        super.onPause();
        Log.i("Activity2","onPause()");
    }
    @Override
    protected void onStop() {
        super.onStop();
        Log.i("Activity2","onStop()");
    }
    @Override
    protected void onDestroy() {
        super.onDestroy();
        Log.i("Activity2","onDestroy()");
    }
}

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout 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"
    tools:context=".SecondActivity">
    <Button
        android:id = "@+id/button"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_weight = ""
        android:text = "跳转到Activity1"  />

</LinearLayout>

别忘了在AndroidManifest 里面注册