Learning Java: Understanding the Structure and Rules of Writing Java Syntax

Each programming language has different structure and rules for writing syntax.

Java is a programming language developed from the C language and certainly will follow the writing style of C.

When you first see a Java program, maybe you will ask questions.

What is this? What is that?

Example:

Consider the following program:

package com.petanikode.program;

class Program {
    public static void main(String args[]){
        System.out.println("Hello World");
    }
}

Many things we don't know yet.

What is it package?

What is it class?

... and why is it written like that?

Therefore, we need to learn the basic syntax and structure of Java programs.

Let's start…

Basic Structure of Java Programs

The structure of the Java program is generally divided into 4 parts:

1. Package Declaration
2. Import Library
3. Class Section
4. Main Method

Let's look at an example:

package com.petanikode.program; //<- 1. deklarasi package

import java.io.File; //<- 2. Impor library

class Program { //<- 3. Bagian class

    public static void main(String args[]){ //<- 4. Method main
        System.out.println("Hello World");
    }

}

Let's discuss them one by one…

1. Declaration of Package

Package is a folder that contains a set of Java programs.

Package declarations are usually done when creating a large program or application.

Example package declaration:

package com.petanikode.program;

Usually the package name follows the domain name from a vendor who issued the program.

In the example above, it com.petanikodeis the domain name of the code farmer.

The rule: the domain name is reversed, then followed by the name of the program.

What if we don't declare a package ?

It's okay and the program will still work.

But later during production, for example when creating an Android application .

We must declare the package .

2. Import Section

In this section, we import the library needed for the program.

Library is a set of classes and functions that we can use in creating programs.

Examples of library imports:

import java.util.Scanner;

In this example, we import the class Scanner from the package java.util.

3. Class Section

Java is a programming language that uses the OOP (Object Oriented Programming) paradigm .

Each program must be wrapped in a class so that later it can be made into an object.

If you don't understand what OOP is?

Simply understand the class as the program name declaration.

class NamaProgram {
    public static void main(String args[]){
        System.out.println("Hello World");
    }
}

This is a class block.

The class block opens with curly brackets { then closes or ends with }.

In the class block, we can fill it with methods or functions and also variables.

In the example above, there is a method main().

4. Play Method

Method main()or function main()is a program block that will be executed first.

This is the program entry point.

The method main()we must make. If not, the program will not be executed.

Example method main().

public static void main(String args[]){
    System.out.println("Hello World");
}

Writing must be like this ...

Method main()has parameters args[]. This parameter will then store a value from the arguments in the command line .

Please read: What are the Functions of String [] args in Java Programming?

Then in the method main(), there is a statement or function:

System.out.println("Hello World");

This is a function to display text to the monitor screen.

Statement and Expression in Java

Statements and expressions are the smallest part of the program. Every statement and expression in Java must end with a semicolon ( ;).

Example statement and expression:

System.out.println("Hello World");
System.out.println("Apa kabar?");
var x = 3;
var y = 8;
var z = x + y;

Statements and expressions will be instructions that will be done by the computer.

In the example above, we tell the computer to display text "Hello World", and "Apa kabar?".

Then we tell him to calculate the value x + y.

Java Program Blocks

The program block is a collection of statements and expressions wrapped in one.

The program block is always opened with curly braces {and closed with }.

Example block program:

// blok program main
public static void main(String args[]){
    System.out.println("Hello World");
    System.out.println("Hello Kode");

    // blok program if
    if( true ){
        System.out.println('True');
    }

    // blok program for
    for ( int i = 0; i<10; i++){
        System.out.println("Perulangan ke"+i);
    }
}

The bottom line: if you find parentheses {and }, then that is a program block.

The program block can also contain other program blocks (nested) .

In the example above, the program main()block contains an ifand for block .

We will study this block at:

• Branching in Java ;
• Repetition in Java .

Writing Comments on Java

Comments are part of a program that will not be executed by the computer.

Comments are usually used for:

• Give information to the program code;
• Disabling certain functions;
• Making documentation;
• etc.

Writing comments in java, same as in C language. Namely using:

1. Double slash ( //) for single line comments;
2. Star slash ( /*...*/) for comments that are more than one line.

Example:

public static void main(String args[]){
    // ini adalah komentar satu baris
    System.out.println("Hello World");
    
    // komentar akan diabaikan oleh komputer
    // berikut ini fungsi yang di-non-aktifkan dengan komentar 
    // System.out.println("Hello World");

    /*
    Ini adalah penulisan komentar
    yang lebih dari
    satu baris
    */
}

String and Character Writing

String is a collection of characters. We often know it with text.

Example string: "Hello world"

The rules for writing strings in Java, must be enclosed in double quotes as in the example above.

If flanked by single quotes, it will become a character.

Example: 'Hello world'.

So please distinguish:

• Double quotation marks ( "...") for making strings;
• While single quotes ( '...') to make characters.

Case Sensitive

Java is Case Sensitive , meaning uppercase or capital and lowercase letters are distinguished.

Example:

String nama = "Petani Kode";
String Nama = "petanikode";
String NAMA = "Petanikode.com";

System.out.println(nama);
System.out.println(Nama);
System.out.println(NAMA);

The three variables are three different variables, although they are both named nama.

Many beginners are often wrong about this. Because it can't tell which variables use uppercase letters and which ones use lowercase letters.

If we create a variable like this:

String jenisKelamin = "Laki-laki";

Then we have to call it like this:

System.out.println(jenisKelamin);

Not like this:

System.out.println(jeniskelamin);

Note, letters Kare uppercase letters.

Style of Case Writing

The case style used by Java is: camelCase , PascalCase , and ALL UPPER .

The camelCase writing style is used for variable names, object names, and method names.

Example:

String namaSaya = "Dian";

Then for PascalCase it is used for writing class names.

Example:

class HelloWOrld {
    //...
}

Notice the class name, we use capital letters at the beginning, and capital letters in letters Wto separate the two syllables.

Whereas the front letter camelCase uses lowercase letters, and the next syllable prefix uses uppercase letters.

// ini camelCase
belajarJava

// ini PascalCase
BelajarJava

Then, writing ALL UPPER or as desired capital is used to create constant names.

Example:

public final String DB_NAME = "petanikode";

To write two syllables or more, ALL UPPER is separated by the bottom line or underscore ( _).

May I write carelessly?

For example for the class name using ALL UPPER ?

It's okay, the program will not error. But the program code that you write will look dirty and out of the grid that has been set.

Sumber: https://www.petanikode.com/java-sintaks/