MainActivity.java

package com.example.testapp.app;

import android.support.v7.app.ActionBarActivity;
import android.os.Bundle;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.widget.AdapterView;
import android.widget.AdapterView.OnItemSelectedListener;
import android.widget.Spinner;
import android.widget.ArrayAdapter;
import android.widget.TextView;
import android.widget.EditText;
import android.text.TextWatcher;
import android.text.Editable;

public class MainActivity extends ActionBarActivity {
    private TextView textResult;
    private EditText inputValue;
    private Spinner spinnerCategories, spinnerValue1, spinnerValue2;
    private Unit[] spinnerTemperatureUnits = {new Unit("Celsius", 0), new Unit("Kelvin", 0), new Unit("Fahrenheit", 0)};
    private Unit[] spinnerLengthUnits = {new Unit("Centimetres", 1), new Unit("Millimetres", 0.1f), new Unit("Metres", 100), new Unit("Kilometres", 100000), new Unit("Miles", 160934), new Unit("Feet", 30.48f), new Unit("Inches", 2.54f)};
    private Unit[] spinnerVolumeUnits = {new Unit("Litres", 1), new Unit("Cubic Meters", 0), new Unit("Cubic Kilometres", 0), new Unit("Gallons", 3.78541f)};
    private Unit[] spinnerWeightUnits = {new Unit("Kilos", 1), new Unit("Grams", 0.001f), new Unit("Tonnes", 1000), new Unit("Pounds", 0.453592f)};
    private int catNumber, fromNumber, toNumber;

    private void fillList(Spinner spinner, Unit[] list) {
        String[] names = new String[list.length];
        for (int i = 0; i < list.length; i++) names[i] = list[i].getName();
        spinner.setAdapter(new ArrayAdapter<String>(this,android.R.layout.simple_spinner_item, names));
    }

    private void updateCategory() {
        catNumber = spinnerCategories.getSelectedItemPosition();
        switch (catNumber) {
            case 0: // Temperature
                fillList(spinnerValue1, spinnerTemperatureUnits);
                fillList(spinnerValue2, spinnerTemperatureUnits);
                spinnerValue1.setSelection(0); // Celsius
                spinnerValue2.setSelection(2); // Fahrenheit
                break;
            case 1: // Length
                fillList(spinnerValue1, spinnerLengthUnits);
                fillList(spinnerValue2, spinnerLengthUnits);
                spinnerValue1.setSelection(2); // Metres
                spinnerValue2.setSelection(5); // Feet
                break;
            case 2: // Volume
                fillList(spinnerValue1, spinnerVolumeUnits);
                fillList(spinnerValue2, spinnerVolumeUnits);
                spinnerValue1.setSelection(0); // Litres
                spinnerValue2.setSelection(3); // Gallons
                break;
            case 3: // Weight
                fillList(spinnerValue1, spinnerWeightUnits);
                fillList(spinnerValue2, spinnerWeightUnits);
                spinnerValue1.setSelection(0); // Kilos
                spinnerValue2.setSelection(3); // Pounds
                break;
        }
        updateValues();
    }
    private void updateValues() {
        float result;
        fromNumber = spinnerValue1.getSelectedItemPosition();
        toNumber = spinnerValue2.getSelectedItemPosition();

        if (catNumber == 0) { // Temperature conversion

        } else {

        }
    }

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

        inputValue = (EditText) findViewById(R.id.inputValue);
        textResult = (TextView) findViewById(R.id.textResult);
        spinnerCategories = (Spinner) findViewById(R.id.spinnerCategories);
        spinnerValue1 = (Spinner) findViewById(R.id.spinnerValue1);
        spinnerValue2 = (Spinner) findViewById(R.id.spinnerValue2);

        TextWatcher textWatcher = new TextWatcher() {
            @Override
            public void afterTextChanged(Editable editable) {updateValues();}
        };
        inputValue.addTextChangedListener(textWatcher);

        spinnerCategories.setOnItemSelectedListener(new OnItemSelectedListener() {
            @Override
            public void onItemSelected(AdapterView<?> parent, View view, int position, long id) {updateCategory();}
            @Override
            public void onNothingSelected(AdapterView<?> arg0) {}
        });

        spinnerValue1.setOnItemSelectedListener(new OnItemSelectedListener() {
            @Override
            public void onItemSelected(AdapterView<?> parent, View view, int position, long id) {updateValues();}
            @Override
            public void onNothingSelected(AdapterView<?> arg0) {}
        });

        spinnerValue2.setOnItemSelectedListener(new OnItemSelectedListener() {
            @Override
            public void onItemSelected(AdapterView<?> parent, View view, int position, long id) {updateValues();}
            @Override
            public void onNothingSelected(AdapterView<?> arg0) {}
        });
    }

    @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);
    }

}

Unit.java
package com.example.testapp.app;

public class Unit {
    Unit(String name, float ratio) {
        Name = name;
        Ratio = ratio;
    }
    public String getName() {return Name;}
    public float getRatio() {return Ratio;}

    private String Name;
    private float Ratio;
}

activity_main.xml
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    android:paddingBottom="@dimen/activity_vertical_margin"
    tools:context="com.example.testapp.app.MainActivity"
    android:background="#ffc833">

    <Spinner
        android:id="@+id/spinnerCategories"
        android:entries="@array/spinnerCategoriesList"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentTop="true" />

    <Spinner
        android:id="@+id/spinnerValue1"
        android:layout_marginLeft="150dp"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_below="@+id/spinnerCategories"
        android:layout_alignParentLeft="true"
        android:layout_alignParentStart="true"/>

    <Spinner
        android:id="@+id/spinnerValue2"
        android:layout_marginLeft="150dp"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_below="@+id/spinnerValue1"
        android:layout_alignParentLeft="true"
        android:layout_alignParentStart="true"/>

    <EditText
        android:id="@+id/inputValue"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:inputType="numberDecimal"
        android:ems="10"
        android:layout_alignTop="@+id/spinnerValue1"
        android:layout_alignParentLeft="true"
        android:layout_alignParentStart="true" />

    <TextView
        android:id="@+id/textResult"
        android:text="Large Text"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:textAppearance="?android:attr/textAppearanceLarge"
        android:layout_below="@+id/spinnerValue2"
        android:layout_alignRight="@+id/inputValue"
        android:layout_alignEnd="@+id/inputValue"
        android:layout_marginRight="26dp"
        android:layout_marginTop="77dp" />

</RelativeLayout>

strings.xml
<?xml version="1.0" encoding="utf-8"?>
<resources>

    <string name="app_name">testapp</string>
    <string name="action_settings">Settings</string>

    <string-array name="spinnerCategoriesList">
        <item>Temperature</item>
        <item>Length</item>
        <item>Volume</item>
        <item>Weight</item>
    </string-array>
</resources>