(Android) How to change spinner content in runtime and update spinner options?

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Mobile

Android

@Override
public boolean onOptionsItemSelected(MenuItem item){
    //change the spinners item depending on menu choice
    switch(item.getItemId()){
    case R.id.lengthMenu:
        adapter = ArrayAdapter.createFromResource(this, R.array.lengthArray, android.R.layout.simple_spinner_item);
        adapter.notifyDataSetChanged();
        return true;
    case R.id.weightMenu:
        adapter = ArrayAdapter.createFromResource(this, R.array.weigthArray, android.R.layout.simple_spinner_item);
        adapter.notifyDataSetChanged();
        return true;
    case R.id.liquidsMenu:
        adapter = ArrayAdapter.createFromResource(this, R.array.liquidsArray, android.R.layout.simple_spinner_item);
        adapter.notifyDataSetChanged();
        return true;
    default:
        return true;
    }

The problem is that when i run this code it doesn't change the spinner values/content and it sticks with the adapter value given in onCreate(). Also 2 spinners are using this data and all data is in a string-array in strings.xml.

Here is onCreate if needed:

ArrayAdapter<CharSequence> adapter;

/** Called when the activity is first created. */
@Override
public void onCreate(Bundle icicle) {
    super.onCreate(icicle);
    setContentView(R.layout.main);

    answer = (TextView) findViewById(R.id.answer);
    unitSize = (EditText) findViewById(R.id.unitSize);

    adapter = ArrayAdapter.createFromResource(this, R.array.lengthArray, android.R.layout.simple_spinner_item);
    adapter.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item);
    firstUnit = (Spinner) findViewById(R.id.firstUnit);
    secondUnit = (Spinner) findViewById(R.id.secondUnit);
    firstUnit.setOnItemSelectedListener(this);
    secondUnit.setOnItemSelectedListener(this);
    firstUnit.setAdapter(adapter);
    secondUnit.setAdapter(adapter);
}