improved app

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    android:padding="16dp">

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginBottom="8dp"
        android:text="Quantity"
        android:textAllCaps="true"/>

    <LinearLayout
        android:orientation="horizontal"
        android:layout_marginBottom="8dp"
        android:layout_marginTop="8dp"
        android:layout_width="match_parent"
        android:layout_height="wrap_content">

        <Button
            android:gravity="center"
            android:layout_width="48dp"
            android:layout_height="48dp"
            android:onClick="subtract"
            android:text="-"
            />

        <TextView
            android:id="@+id/current_guess"
            android:layout_marginEnd="8dp"
            android:layout_marginStart="8dp"
            android:layout_gravity="center_vertical"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:textColor="@android:color/black"
            android:textSize="16sp"/>

        <Button
            android:gravity="center"
            android:layout_width="48dp"
            android:layout_height="48dp"
            android:onClick="add"
            android:text="+"/>
    </LinearLayout>

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginBottom="8dp"
        android:layout_marginTop="8dp"
        android:text="Price"
        android:textAllCaps="true"/>

    <TextView
        android:id="@+id/priceTextView"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginBottom="8dp"
        android:layout_marginTop="8dp"
        android:text="$0"
        android:textColor="@android:color/black"
        android:textSize="16sp"/>

</LinearLayout>

MainActivity.java

package com.example.yitzi.mathapp;

import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.view.View;
import android.widget.TextView;

import java.text.NumberFormat;

public class MainActivity extends AppCompatActivity {
    int numberOfCoffees = 0;

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

    public void subtract(View view) {
        if (numberOfCoffees > 0) {
            numberOfCoffees = numberOfCoffees - 1;
            display(numberOfCoffees);
            displayPrice(numberOfCoffees * 5);
        }
    }

    public void add(View view){
        numberOfCoffees = numberOfCoffees + 1;
        display(numberOfCoffees);
        displayPrice(numberOfCoffees * 5);
    }

    /**
     * This method displays the given quantity value on the screen.
     */
    private void display(int number) {
        TextView quantityTextView = findViewById(R.id.current_guess);
        quantityTextView.setText("" + number);
    }

    private void displayPrice(int number) {
        TextView priceTextView = findViewById(R.id.priceTextView);
        priceTextView.setText(NumberFormat.getCurrencyInstance().format(number));
    }
}