What is programming? How programs execute

Programs are precise instructions

A program is a list of instructions that a computer follows exactly. Computers are fast but literal: they do precisely what the code says, in order, without guessing what you meant. Programming is the skill of describing a task so clearly that a machine can carry it out.

Every program, from a calculator to a banking app, does three things:

1. Takes input — a key press, a file, a form submission or data from another system.
2. Processes it — calculates, compares, sorts or transforms the data.
3. Produces output — text on screen, a saved file, a web page or a message to another system.

From source code to a running program

You write instructions in a programming language such as Java, Python or JavaScript. This human-readable text is called source code. A computer's processor cannot run Java source directly, so it is translated first.

Java uses two steps:

1. The compiler (javac) checks your code for mistakes and translates it into bytecode, stored in a .class file.
2. The Java Virtual Machine (JVM) reads the bytecode and runs it on your computer. Because every operating system has its own JVM, the same bytecode runs on Windows, macOS and Linux. This is Java's "write once, run anywhere" idea.

Your first Java program

public class Hello {
    public static void main(String[] args) {
        System.out.println("Hello, DevCrest!");
        System.out.println("Programs run from top to bottom.");
    }
}

Let's walk through it:

• public class Hello declares a class named Hello. In Java, all code lives inside a class, and the file must be named Hello.java.
• public static void main(String[] args) is the entry point. When you run the program, the JVM starts here.
• System.out.println(...) prints a line of text to the console.
• Each statement ends with a semicolon ;. Curly braces { } group statements into blocks.

Running it yourself

Install a Java Development Kit (JDK 21 or newer), save the code as Hello.java, then run these commands in a terminal:

javac Hello.java   # compile: creates Hello.class
java Hello         # run the bytecode on the JVM

Since Java 11 you can also run a single source file directly with java Hello.java, which compiles and runs in one step.

Execution order

Statements inside main run sequentially, top to bottom. Later lessons show how conditions and loops change this order. Reading code in the order the computer executes it is the most important habit a new programmer can build.