The first solution

New version installation SDK The file starts with two XML file ,activity_main.xml and fragment_main.xml,
Those who are not used to it can be dealt with in this way :
1. Delete fragment_main.xml The whole document
2. Yes activity_main.xml, Delete the contents . And then switch to Graphy Layout, Put one in LinearLayout Can
3. Yes MainActivity.java, You can delete part of the content , And then MainActivity extends ActionBarActivity Change it to MainActivity extends Activity :
It's changed to read as follows :
package com.zmcorp.weightcalculator;

import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.view.ViewGroup;
import android.os.Build;

MainActivity extends Activity {

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

}

@Override
public boolean onCreateOptionsMenu(Menu menu) {

// Inflate the menu; this adds items to the action bar if it is present.
getMenuInflater().inflate(R.menu.main, menu);
return true;
}

@Override
public boolean onOptionsItemSelected(MenuItem item) {
// Handle action bar item clicks here. The action bar will
// automatically handle clicks on the Home/Up button, so long
// as you specify a parent activity in AndroidManifest.xml.
int id = item.getItemId();
if (id == R.id.action_settings) {
return true;
}
return super.onOptionsItemSelected(item);
}

}

}

The second method ::

myTextview,myButton Is in fragment_main.xml Medium , In the absence of introduction fragment_main.xml Before , You are through findViewById I can't find ..

public static class PlaceholderFragment extends Fragment {

public PlaceholderFragment() {
}

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
View rootView = inflater.inflate(R.layout.fragment_main, container,
false);
TextView textview = (TextView)rootView.findViewById(R.id.view);
System.out.println(textview);
testview.setText("Hello Android!");

return rootView;
}
}

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