MainActivity.java

package com.gazzali.spinitmeow;

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.text.TextUtils;
import android.util.Log;
import android.view.View;
import android.widget.AdapterView;
import android.widget.ArrayAdapter;
import android.widget.Button;
import android.widget.EditText;
import android.widget.Spinner;
import android.widget.TextView;
import android.widget.Toast;

public class MainActivity extends AppCompatActivity implements AdapterView.OnItemSelectedListener {

    Spinner spinnerMainChoice;
    Spinner spinnerInputChoice;
    Spinner spinnerOutputChoice;

    EditText getInputValueID;
    double inputValue;

    TextView outputValue;

    Button buttonConvert;

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

        /* ------------ Main code Starts Here ----------------*/

        /* Main conversion Type choice with Spinner (Drop Down menu)*/
        spinnerMainChoice = findViewById(R.id.spinnerIDMainChoice);
        // [IMPORTANT] Set Spinner Click Listener
        spinnerMainChoice.setOnItemSelectedListener(this);
        // Create an ArrayAdapter using the string array and a default spinner layout
        ArrayAdapter<CharSequence> adapterMainChoice = ArrayAdapter.createFromResource(this,
                R.array.MainChoices_array, android.R.layout.simple_spinner_item);
        // Specify the layout to use when the list of choices appears
        adapterMainChoice.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item);
        // Apply the adapter to the spinner
        spinnerMainChoice.setAdapter(adapterMainChoice);

        /* Input Conversion type choice with Spinner */
        spinnerInputChoice = findViewById(R.id.spinnerIDInputChoice);
        spinnerInputChoice.setOnItemSelectedListener(this);

        /* Output Conversion type choice with Spinner */

        spinnerOutputChoice = findViewById(R.id.spinnerIDOutputChoice);
        spinnerOutputChoice.setOnItemSelectedListener(this);

        /* for input and output fields */
        getInputValueID = findViewById(R.id.editTextIDInputValue);
        String inputValueString = getInputValueID.getText().toString();
        if(!TextUtils.isEmpty(inputValueString))
        {
            try
            {
                inputValue = Double.parseDouble(inputValueString);
            }
            catch (Exception e1)
            {
                e1.printStackTrace();
            }
        }
        outputValue = findViewById(R.id.textViewIDOutputValue);

    }

    @Override
    public void onItemSelected(AdapterView<?> parent, View view, int pos, long id) {
        // An item was selected. retrieve the selected item
        String selectedMainChoice = parent.getItemAtPosition(pos).toString();
        Log.i("Selected", selectedMainChoice);
        //Toast.makeText(MainActivity.this, "Selected: " + selectedMainChoice, Toast.LENGTH_SHORT).show();

        setInputOutputSpinners(selectedMainChoice);

    }

    protected void setInputOutputSpinners(String selectedMainChoice) {
        switch (selectedMainChoice)
        {
            case "Length": {
                spinnerInputChoice.setAdapter(new ArrayAdapter<CharSequence>(this,
                        android.R.layout.simple_spinner_dropdown_item,
                        getResources().getStringArray(R.array.LengthChoices_array)));

                spinnerOutputChoice.setAdapter(new ArrayAdapter<CharSequence>(this,
                        android.R.layout.simple_spinner_dropdown_item,
                        getResources().getStringArray(R.array.LengthChoices_array)));
            }
            break;

            case "Temperature": {
                spinnerInputChoice.setAdapter(new ArrayAdapter<CharSequence>(this,
                        android.R.layout.simple_spinner_dropdown_item,
                        getResources().getStringArray(R.array.TemperatureChoices_array)));

                spinnerOutputChoice.setAdapter(new ArrayAdapter<CharSequence>(this,
                        android.R.layout.simple_spinner_dropdown_item,
                        getResources().getStringArray(R.array.TemperatureChoices_array)));
            }
            break;

            case "Weight": {
                spinnerInputChoice.setAdapter(new ArrayAdapter<CharSequence>(this,
                        android.R.layout.simple_spinner_dropdown_item,
                        getResources().getStringArray(R.array.WeightChoices_array)));

                spinnerOutputChoice.setAdapter(new ArrayAdapter<CharSequence>(this,
                        android.R.layout.simple_spinner_dropdown_item,
                        getResources().getStringArray(R.array.WeightChoices_array)));
            }
            break;
        }
    }

    @Override
    public void onNothingSelected(AdapterView<?> parent) {
        // Another interface callback
    }

}