Java Android App Development Course, Variables and Functions

In Android app development, Java is one of the most widely used programming languages.
In this course, we will take a closer look at the basic concepts of Java, specifically variables and functions.
It is essential for beginners to understand the fundamentals of variables and functions,
as it will greatly aid in implementing more complex logic later.

1. Variables

To understand what a variable is, one must first grasp the basics of computer programming.
A variable is, simply put, a named space that stores data. Through this, we can
easily access and manipulate data. In Java, various types of variables can be used.

1.1 Types of Variables

When declaring a variable in Java, you need to specify the type of the variable.
This includes primitive data types and reference data types.

1.1.1 Primitive Data Types

  • boolean: Stores true or false. Example: boolean isActive = true;
  • char: A single character. Example: char grade = 'A';
  • int: An integer. Example: int age = 25;
  • double: A double-precision floating-point number. Example: double price = 19.99;

1.1.2 Reference Data Types

Reference data types are variables that refer to objects. For example, there are instances of classes or arrays.
The following is an example of a String data type. 

String name = "John Doe";

1.2 Variable Declaration and Initialization

To use a variable, you must first declare and initialize it. When declaring a variable,
it is written as follows, including the type and variable name. 

int number; // Variable declaration
number = 10; // Variable initialization

1.3 Scope of Variables

The scope of a variable refers to the area in which the variable can be accessed. In Java, the scope of a variable
varies depending on where it is declared, and typically includes the following types of scopes.

  • Global Variable: A variable declared within a class can be accessed by all methods of the class.
  • Local Variable: A variable declared within a method can only be used inside that method.

2. Functions

A function is a collection of code that performs a specific task. In Java, functions are usually referred to as
methods and are included as part of a class. A method performs a specific task when called and can
return a value when needed.

2.1 Method Declaration

Methods are declared in the following format. 

ReturnType methodName(ParameterList) {
    // Method content
}

2.2 Method Parameters and Return Values

Methods can receive data from outside via parameters and return results via return values.
For example, a method that adds two integers is as follows. 

public int add(int a, int b) {
    return a + b;
}

2.3 Method Overloading

In Java, you can define multiple methods with the same name but with different parameter types or counts.
This is called method overloading. For example, you can use it as follows. 

public int multiply(int a, int b) {
    return a * b;
}

public double multiply(double a, double b) {
    return a * b;
}

3. Example: Using Variables and Functions in an Android App

Now, let’s write a simple example demonstrating how to declare variables and use methods in an Android app.
Below is a simple code for an app that takes two numbers from the user and displays the sum.

3.1 MainActivity.java

package com.example.myapp;

import android.os.Bundle;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;
import androidx.appcompat.app.AppCompatActivity;

public class MainActivity extends AppCompatActivity {
    private EditText number1;
    private EditText number2;
    private TextView result;
    private Button addButton;

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

        number1 = findViewById(R.id.editTextNumber1);
        number2 = findViewById(R.id.editTextNumber2);
        result = findViewById(R.id.textViewResult);
        addButton = findViewById(R.id.buttonAdd);

        addButton.setOnClickListener(v -> {
            int num1 = Integer.parseInt(number1.getText().toString());
            int num2 = Integer.parseInt(number2.getText().toString());
            int sum = add(num1, num2);
            result.setText("Result: " + sum);
        });
    }

    private int add(int a, int b) {
        return a + b;
    }
}

3.2 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" >

    <EditText
        android:id="@+id/editTextNumber1"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:hint="First Number"
        android:inputType="number" />

    <EditText
        android:id="@+id/editTextNumber2"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:hint="Second Number"
        android:inputType="number" />

    <Button
        android:id="@+id/buttonAdd"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Add" />

    <TextView
        android:id="@+id/textViewResult"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Result: " />

</LinearLayout>

4. Conclusion

In this course, we thoroughly examined Java’s variables and functions.
We learned to store data using variables and perform specific tasks using functions,
as well as how to modularize code. This foundational knowledge will greatly assist you in
developing complex apps in the future. I hope this course helps you on your Android app development journey.