Fragment

fragment需要两个来进行跳转
第一个：

    <FrameLayout 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"
        tools:context="com.example.administrator.tiaozhuan.FirstFragment">

        <TextView
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            android:gravity="center"
            android:text="This is first fragment!"/>
    </FrameLayout>

</FrameLayout>
第二个：

    <FrameLayout 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"
        tools:context="com.example.administrator.tiaozhuan.SecondFragment">

        <TextView
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            android:gravity="center"
            android:text="This is second fragment!"/>
    </FrameLayout>

</FrameLayout>
下面是主界面java代码  这些都是看同学的修改的。。
public class MainActivity extends AppCompatActivity implements View.OnClickListener{
private Fragment firstFragment;
private Fragment secondFragment;
private boolean skip = true;
private boolean exit = false;

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

    blankFragment = new firstFragment();
    FragmentManager manager = getFragmentManager();
    FragmentTransaction transaction = manager.beginTransaction();
    transaction.add(R.id.main, blankFragment);
    transaction.commit();
}

public void onClick(View view) {
    switch (view.getId()) {
        case R.id.btn_main:
            jump();
            break;
    }
}

private void jump() {
    boolean exit = true;

    if (skip) {
        FragmentManager manager = getFragmentManager();
        FragmentTransaction transaction = manager.beginTransaction();
        if (secondFragment == null) {
             secondFragment = new SecondFragment();
            transaction.replace(R.id.main, secondFragment);
            transaction.commit();
            skip = false;
        } else {
            transaction.replace(R.id.main, secondFragment);
            transaction.commit();
            skip = false;
        }
    } else {
        Toast.makeText(this, "This is second fragment!", Toast.LENGTH_LONG).show();
    }
}

public boolean onKeyDown(int keyCode, KeyEvent event) {

    if (event.getKeyCode() == KeyEvent.KEYCODE_BACK && exit) {

        FragmentManager manager = getFragmentManager();
        FragmentTransaction transaction = manager.beginTransaction();
        skip = true;
        exit = false;
        transaction.replace(R.id.main, blankFragment);
        transaction.commit();
        return false;
    } else {
        finish();
    }
    return super.onKeyDown(keyCode, event);

}
}