package com.example.testdemo;
public interface BackHandledInterface {
  public abstract void setSelectedFragment(BackHandledFragment selectedFragment);
}

package com.example.testdemo;
import android.os.Bundle;
import android.support.v4.app.Fragment;
public abstract class BackHandledFragment extends Fragment {
  protected BackHandledInterface mBackHandledInterface;
  /**
   * 所有继承BackHandledFragment的子类都将在这个方法中实现物理Back键按下后的逻辑
   */
  protected abstract boolean onBackPressed();
  @Override
  public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    if (!(getActivity() instanceof BackHandledInterface)) {
      throw new ClassCastException(
          "Hosting Activity must implement BackHandledInterface");
    } else {
      this.mBackHandledInterface = (BackHandledInterface) getActivity();
    }
  }
  @Override
  public void onStart() {
    super.onStart();
    // 告诉FragmentActivity，当前Fragment在栈顶
    mBackHandledInterface.setSelectedFragment(this);
  }
}

package com.example.testdemo;
import android.os.Bundle;
import android.support.v4.app.FragmentActivity;
import android.support.v4.app.FragmentManager;
import android.support.v4.app.FragmentTransaction;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
public class MainActivity extends FragmentActivity implements
    BackHandledInterface {
  private static MainActivity mInstance;
  private BackHandledFragment mBackHandedFragment;
  private Button btnSecond;
  @Override
  public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    btnSecond = (Button) findViewById(R.id.btnSecond);
    btnSecond.setOnClickListener(new OnClickListener() {
      @Override
      public void onClick(View v) {
        FirstFragment first = new FirstFragment();
        loadFragment(first);
        btnSecond.setVisibility(View.GONE);
      }
    });
  }
  public static MainActivity getInstance() {
    if (mInstance == null) {
      mInstance = new MainActivity();
    }
    return mInstance;
  }
  public void loadFragment(BackHandledFragment fragment) {
    BackHandledFragment second = fragment;
    FragmentManager fm = getSupportFragmentManager();
    FragmentTransaction ft = fm.beginTransaction();
    ft.replace(R.id.firstFragment, second, "other");
    ft.addToBackStack("tag");
    ft.commit();
  }
  @Override
  public void setSelectedFragment(BackHandledFragment selectedFragment) {
    this.mBackHandedFragment = selectedFragment;
  }
  @Override
  public void onBackPressed() {
    if (mBackHandedFragment == null || !mBackHandedFragment.onBackPressed()) {
      if (getSupportFragmentManager().getBackStackEntryCount() == 0) {
        super.onBackPressed();
      } else {
        if (getSupportFragmentManager().getBackStackEntryCount() == 1) {
          btnSecond.setVisibility(View.VISIBLE);
        }
        getSupportFragmentManager().popBackStack();
      }
    }
  }
}

package com.example.testdemo;
import android.os.Bundle;
import android.support.annotation.Nullable;
import android.support.v4.app.FragmentManager;
import android.support.v4.app.FragmentTransaction;
import android.view.LayoutInflater;
import android.view.View;
import android.view.View.OnClickListener;
import android.view.ViewGroup;
import android.widget.Button;
public class FirstFragment extends BackHandledFragment {
  private View myView;
  private Button btnSecond;
  @Override
  public View onCreateView(LayoutInflater inflater,
      @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
    myView = inflater.inflate(R.layout.fragment_first, null);
    initView();
    return myView;
  }
  private void initView() {
    btnSecond = (Button) myView.findViewById(R.id.btnSecond);
    btnSecond.setOnClickListener(new OnClickListener() {
      @Override
      public void onClick(View v) {
        SecondFragment second = new SecondFragment();
        FragmentManager fm = getFragmentManager();
        FragmentTransaction ft = fm.beginTransaction();
        ft.replace(R.id.firstFragment, second);
        ft.addToBackStack("tag");
        ft.commit();
      }
    });
  }
  @Override
  protected boolean onBackPressed() {
    return false;
  }
}

package com.example.testdemo;
import android.os.Bundle;
import android.support.annotation.Nullable;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
public class SecondFragment extends BackHandledFragment {
  private View mView;
  @Override
  public View onCreateView(LayoutInflater inflater,
      @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
    mView = inflater.inflate(R.layout.fragment_second, null);
    return mView;
  }
  @Override
  protected boolean onBackPressed() {
    return false;
  }
}

<RelativeLayout 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" >
  <TextView
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_centerInParent="true"
    android:text="FragmentActivity 父界面"
    android:textSize="26sp" />
  <Button
    android:id="@+id/btnSecond"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_alignParentBottom="true"
    android:text="跳转到FirstFragment" />
  <FrameLayout
    android:id="@+id/firstFragment"
    android:layout_width="match_parent"
    android:layout_height="match_parent" >
  </FrameLayout>
</RelativeLayout>

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
  android:layout_width="match_parent"
  android:layout_height="match_parent"
  android:background="#e5e5e5"
  android:orientation="vertical" >
  <TextView
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_centerInParent="true"
    android:text="FirstFragment"
    android:textColor="#000000"
    android:textSize="26sp" />
  <Button
    android:id="@+id/btnSecond"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_alignParentBottom="true"
    android:text="打开SecondFragment" />
</RelativeLayout>

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
  android:layout_width="match_parent"
  android:layout_height="match_parent"
  android:background="#e5e5e5"
  android:orientation="vertical" >
  <TextView
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_centerInParent="true"
    android:text="SecondFragment"
    android:textColor="#000000"
    android:textSize="26sp" />
</RelativeLayout>