Android : items are not displayed in spinner

Spinner is not displaying items. Items are not shown when spinner is clicked. Following is the code in mainactivity.java. Items are not displayed even when the spinner is double clicked.

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


if (savedInstanceState == null) {
getSupportFragmentManager().beginTransaction()
.add(R.id.container, new PlaceholderFragment())
.commit();
    } 

    Spinner s1 = (Spinner) findViewById(R.id.spinner1);
    ArrayAdapter<String> adapter = new ArrayAdapter<String>(this,
            android.R.layout.simple_spinner_item, presidents);

    if(s1 != null){
        s1.setAdapter(adapter);
        s1.setOnItemSelectedListener(new OnItemSelectedListener()
        {
            public void onItemSelected(AdapterView<?> arg0,
                    View arg1, int arg2, long arg3)
            {
                int index = arg0.getSelectedItemPosition();
                Toast.makeText(getBaseContext(), presidents[index], Toast.LENGTH_SHORT).show();

            }
            public void onNothingSelected(AdapterView<?> arg0){

            }
        });
    }


}