The Building Blocks of Java Programming: An Introduction to Functions and Classes
Tags: java, functional, classes
Published: 2023-03-17 20:19:05

Are you new to Java programming and wondering where to start? Look no further than the anatomy of Java programs. At the core of every Java program are functions, which act as building blocks to perform specific tasks.

Let's take a closer look at how these functions work. In Java, we specify the return type of a function, such as a number or date time, or specify that it doesn't return anything with the `"void"` keyword. We then give the function a proper descriptive name and add parameters as needed.

For example, we might create a function for sending emails to people, with parameters such as the recipient's email address, subject line, and message content. These functions are essential for breaking down complex programming tasks into manageable chunks.

But functions don't exist on their own - they belong to classes, which act as containers for related functions. Think of classes as sections in a supermarket, with each section containing related products. Similarly, a class in Java contains related functions.

Access modifiers, such as `"public"` and `"private,"` determine if other classes and methods can access these classes and methods. Most of the time, we use the public access modifier, which we put in front of our class and method declarations.

At the heart of every Java program is the main class and main method, which serve as the entry point to our programs. The main method gets called whenever we execute a Java program, and the code inside this function gets executed. It's important to follow conventions for naming classes and methods in Java, using `PascalNamingConvention` for classes and `camel` naming convention for methods.

Are you ready to see these building blocks in action? Follow along with our Java tutorial to create a new Java project and explore the power of functions and classes for yourself.

Here is an example of a Java function that converts someone's weight from pounds to kilograms:

```java
public double convertWeight(double pounds) {
    double kilograms = pounds / 2.20462;
    return kilograms;
}
```

In this function, we specify the return type as "double" and give it the name "convertWeight." We then add a parameter for the weight in pounds. Inside the function, we perform the calculation to convert the weight to kilograms and return the result.

Now let's take a closer look at how to specify the return type of a function in Java. As mentioned earlier, some functions return a value, such as a number or a date time, while others don't return anything. Functions that don't return anything have a return type of "void", which is a reserved keyword in Java.

To illustrate how to specify the return type of a function, let's take the example of a function that calculates the area of a circle. The formula for calculating the area of a circle is:

Area = pi x radius^2

To create a function that calculates the area of a circle, we would define it like this:

```java
public double calculateArea(double radius) {
    double area = Math.PI * radius * radius;
    return area;
}
```

In this example, we are using the "public" access modifier to make the function accessible to other classes and methods in our program. We are also specifying the return type of the function as "double", since the area of a circle is a decimal value.

The function takes one parameter, which is the radius of the circle. Inside the function, we use the "Math.PI" constant to represent the value of pi, and we use the formula for calculating the area of a circle to calculate the area. Finally, we return the value of the area using the "return" keyword.

Now, let's look at an example of a function that doesn't return anything. Let's say we have a function that prints the lyrics to a song. We could define the function like this:

```java
public void printSongLyrics() {
    System.out.println("Verse 1: ...");
    System.out.println("Chorus: ...");
    System.out.println("Verse 2: ...");
    System.out.println("Chorus: ...");
}
```

In this example, we are using the "public" access modifier again, but this time we are specifying the return type of the function as "void", since the function doesn't return anything.

Inside the function, we are simply printing the lyrics to a song using the "System.out.println()" method. Since the function doesn't return anything, we don't need to use the "return" keyword.

Now that we've covered how to define functions in Java and how to specify their return types, let's move on to classes. Classes are containers for related functions, and are used to organize code in Java programs.

To illustrate how to define a class in Java, let's take the example of a class that represents a person. We could define the class like this:

```java
public class Person {
    private String name;
    private int age;
    
    public Person(String name, int age) {
        this.name = name;
        this.age = age;
    }
    
    public String getName() {
        return name;
    }
    
    public void setName(String name) {
        this.name = name;
    }
    
    public int getAge() {
        return age;
    }
    
    public void setAge(int age) {
        this.age = age;
    }
}
```

In this example, we are using the "public" access modifier to make the class accessible to other classes and methods in our program. We have also defined two private instance variables, "name" and "age", which can only be accessed within the class itself.

We have defined a constructor for the class, which takes two parameters: "name" and "age". Inside the constructor, we are using the "this" keyword to refer to the instance of the class that is being created, and we are setting the values of the "name" and "age" instance variables.

We have also defined four methods for the class: "getName()", "

The structure of a Java program can seem daunting at first, but by understanding the basics of functions, classes, and access modifiers, you can start building your own applications in no time.

Let's take a closer look at some code examples to help solidify these concepts. First, let's create a simple function that takes in two integers and returns their sum:

```java
public static int add(int num1, int num2) {
  return num1 + num2;
}
```

Here, we've declared the function as public, which means that it can be accessed by other classes and methods in the program. We've also specified the return type as int, which means that the function will return an integer value. We've given the function a descriptive name, "add", and we've specified the parameters that the function takes in: two integers named "num1" and "num2". Finally, we've written the code that the function will execute, which adds num1 and num2 together and returns the result.

Now, let's create a class that contains this function:

```java
public class Calculator {
  public static int add(int num1, int num2) {
    return num1 + num2;
  }
}
```

We've created a public class called "Calculator", which contains a single public static method called "add". Since this method is static, it can be called without creating an instance of the Calculator class. Inside the method, we simply call the add function that we defined earlier.

Finally, let's create a main class that calls the add function from our Calculator class:

```java
public class Main {
  public static void main(String[] args) {
    int sum = Calculator.add(5, 10);
    System.out.println(sum);
  }
}
```

Here, we've created a public class called "Main", which contains a public static method called "main". This method is the entry point to our program. Inside the method, we've called the add function from our Calculator class, passing in the integers 5 and 10 as arguments. We've stored the result of the function call in a variable called "sum", and then printed that variable to the console using the System.out.println function.

By creating and running this code, you can see the concepts of functions, classes, and access modifiers in action. As you become more familiar with Java programming, you can build more complex applications using these building blocks.

Finally, we can write the code for our main method, which is the entry point to our Java program:

```java
public static void main(String[] args) {
   // Code goes here
}
```

In this example, our main method is declared with the public access modifier, which means that other classes and methods can access it. The method is also declared with the static keyword, which means that it can be called without creating an instance of the class it belongs to. The method has a return type of void, which means it doesn't return any value.

We can now add our code inside the main method to make our program do something. For example, we can print a message to the console:

```java
public static void main(String[] args) {
   System.out.println("Hello, world!");
}
```

This code will print the message "Hello, world!" to the console when the program is executed.

In conclusion, understanding the anatomy of Java programs is essential for any Java developer. Functions are the building blocks of Java programs, and classes are containers for related functions. Access modifiers determine which classes and methods can access other classes and methods. The main class and main method are the entry point to a Java program. By following these basic principles, developers can create powerful and effective Java programs.