[Q] Runtime.getRuntime().exec dosn't work - Android Q&A, Help & Troubleshooting

Hi i'm trying to make an app with a button that free cache of the phone.. This is the code:
Code:
package com.mkyong.android;
import android.annotation.SuppressLint;
import android.app.Activity;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.Toast;
import java.io.IOException;
import com.example.toast.R;
public class MainActivity extends Activity {
private Button button;
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.tab1);
button = (Button) findViewById(R.id.button1);
button.setOnClickListener(new OnClickListener() {
@SuppressLint("SdCardPath")
@Override
public void onClick(View arg0) {
final Runtime runtime = Runtime.getRuntime();
try {
runtime.exec("su -c 'echo 3 > /proc/sys/vm/drop_caches'");
Toast.makeText(MainActivity.this, "Script lanciato con `successo, memoria svuotata.", Toast.LENGTH_LONG).show();`
}
catch (IOException e) {
e.printStackTrace();
}
}
});
}
}
Well i've got no errors but nothing happen.. When i press the button the free ram memory rest the same. (to see the free memory i open the terminal emulator and i write the "free" command"). Someone can help me? What's wrong?