<?xml version="1.0" encoding="utf-8"?><LinearLayout xmlns:android="http://schemas.android.com/apk/res/android" xmlns:tools="http://schemas.android.com/tools" android:layout_width="match_parent" android:layout_height="match_parent" android:orientation="vertical" android:gravity="center" tools:context="com.example.wdh.fragmentwork.MainActivity"> <LinearLayout android:id="@+id/show" android:layout_width="match_parent" android:layout_height="261dp" android:orientation="vertical"> </LinearLayout> <LinearLayout android:layout_width="match_parent" android:layout_height="match_parent" android:orientation="vertical" android:gravity="center_horizontal"> <Button android:id="@+id/btnshow" android:layout_width="wrap_content" android:layout_height="wrap_content" android:gravity="center" android:onClick="onClick" android:text="@string/btnshow" /> </LinearLayout></LinearLayout>

<?xml version="1.0" encoding="utf-8"?><FrameLayoutxmlns:android="http://schemas.android.com/apk/res/android"xmlns:tools="http://schemas.android.com/tools"android:layout_width="match_parent"android:layout_height="match_parent"tools:context="com.example.wdh.fragmentwork.FirstFragment"><TextView android:layout_width="match_parent" android:layout_height="match_parent" android:gravity="center" style="@style/First"/></FrameLayout>

<?xml version="1.0" encoding="utf-8"?><FrameLayoutxmlns:android="http://schemas.android.com/apk/res/android"xmlns:tools="http://schemas.android.com/tools"android:layout_width="match_parent"android:layout_height="match_parent"tools:context="com.example.wdh.fragmentwork.ScondFragment"><TextView android:layout_width="match_parent" android:layout_height="match_parent" android:gravity="center" style="@style/Scond"/></FrameLayout>

package com.example.kimdemon.fragment;import android.app.FragmentManager;import android.app.FragmentTransaction;import android.os.Bundle;import android.support.v7.app.AppCompatActivity;import android.view.KeyEvent;import android.view.View;import android.widget.Toast;public class MainActivity extends AppCompatActivity implements View.OnClickListener{First Fragment1;Second Fragment2;private boolean qiehuan = true;private boolean tuichu = false;@Overrideprotected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main);FragmentManager manager = getFragmentManager();FragmentTransaction transaction = manager.beginTransaction();Fragment1 = new First(); transaction.add(R.id.yf_show,Fragment1);transaction.commit();}@Overridepublic void onClick(View view) { if(view.getId() == R.id.yf_btn1){ tuichu = true; if(qiehuan){ FragmentManager manager = getFragmentManager(); FragmentTransaction transaction = manager.beginTransaction(); if (Fragment2 == null){ Fragment2 = new fragment2(); transaction.replace(R.id.yf_show,Fragment2); transaction.commit(); qiehuan = false; } else{ transaction.replace(R.id.yf_show,Fragment2); transaction.commit(); qiehuan = false; } }else{ Toast.makeText(this,"This is second fragment",Toast.LENGTH_SHORT).show(); } }}public boolean onKeyDown(int keyCode, KeyEvent event) {if(event.getKeyCode()==KeyEvent.KEYCODE_BACK&&tuichu){FragmentManager manager = getFragmentManager(); FragmentTransaction transaction = manager.beginTransaction();qiehuan = true; tuichu = false;transaction.replace(R.id.yf_show,Fragment1); transaction.commit();return false; } else { finish(); } return super.onKeyDown(keyCode, event);}}