PHASE 0Beginner ~29 min· topic 1 of 8

Topic 0.1

What a Program Is

In one line

A program is a list of exact instructions that a computer follows one after another, very fast and without ever guessing what you meant. A programming language like Java lets humans write those instructions in words, and a tool translates them into the numbers the computer's chip understands.

Think of it like this

A recipe for a cheese sandwich. "Take two slices of bread. Spread butter on one slice. Put cheese on the butter. Close the sandwich." Now imagine giving that recipe to a robot that does exactly what the words say and nothing else. If you forget to write "close the sandwich", the robot hands you an open sandwich. If you swap two steps, it puts cheese on the plate and butter on the cheese. A program is a recipe for a computer, and the computer is that very fast, very literal robot.

Words you'll meet

New words in this topic, in plain English. Come back here whenever one feels fuzzy.

Program: A list of exact instructions that a computer follows, one after another.
Instruction: One single step in a program, like "print this line" or "add these two numbers".
CPU: Central processing unit: the chip inside a computer that carries out instructions, billions per second.
Memory (RAM): Fast, temporary storage where a running program keeps its values. It is wiped when the computer turns off.
Machine code: Instructions written as numbers that a CPU understands directly. Each kind of CPU has its own machine code.
Programming language: A precise, human-readable way to write instructions, such as Java, Python or C. A tool translates it for the machine.
Source code: The text of a program as a human writes it, saved in a file. Java source files end in .java.
Output: What a program produces, for example lines of text printed on the screen.
Bug: A mistake in a program that makes it fail or give the wrong result.

Step by step

01Instructions, in order

A program is a recipe. Each line is one step, and the computer does the steps in the order they are written. It does not skip ahead, it does not get bored, and it does not fill in missing steps.

Here is the sandwich recipe as a Java program. You don't need to understand every word yet (Topic 0.5 explains each one). For now, notice that each System.out.println(...) line means "print this text on the screen, then move to a new line", and that the lines run from top to bottom.

Main.javawhole filejava

public class Main {
    public static void main(String[] args) {
        System.out.println("Step 1: Take two slices of bread.");
        System.out.println("Step 2: Spread butter on one slice.");
        System.out.println("Step 3: Put cheese on the butter.");
        System.out.println("Step 4: Close the sandwich.");
    }
}

02What is inside a computer

Three parts matter for programmers. The CPU does the work, one tiny instruction at a time, billions of times per second. Memory (RAM) holds the values a running program is using right now, such as your score in a game. Storage (the disk or SSD) keeps files, including your program's file, even when the power is off.

When you run a program, its instructions are copied from storage into memory, and the CPU reads them from memory one by one. Values the program creates, like the number 12, also live in memory while the program runs. When the program ends, that memory is handed back.

What is inside a computerdiagram

Rendering diagram…

03The CPU only understands numbers

A CPU has no idea what "print" or "sandwich" means. It understands machine code: instructions encoded as numbers, like 01001000 10000011 11000000 00000101, which might mean "add 5 to a register" (a register is a tiny storage box inside the CPU).

Writing programs that way would be slow and painful, and the result would only run on one kind of chip. An Intel or AMD chip (called x86-64) and an Apple M-series or phone chip (called ARM) have different machine codes. A program written in one chip's numbers would not run on the other.

04Programming languages are a bridge

A programming language gives you words and rules that are easy for people to read but have exactly one meaning, so a tool can translate them without guessing. total = price + tax; is clear to you and to the translator.

There are two classic ways to translate. A compiler translates the whole program ahead of time into another form, then you run that result. An interpreter reads your program and carries it out line by line while it runs. Java uses both ideas: the javac compiler turns your code into bytecode (a portable instruction format), and then the Java Virtual Machine interprets that bytecode and compiles the busy parts into real machine code while the program runs. Topics 0.2 and 0.8 show this in detail.

Programming languages are a bridgediagram

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05Input, processing, output

Almost every program has the same shape. It starts with some input (data), does some processing (work on the data), and produces output (a result).

In the program below the input is two fixed numbers, the processing is a division, and the output is three printed lines. int cookies = 12; creates a named box called cookies and puts 12 in it (Phase 1 covers these boxes, called variables, in depth). The / sign divides. The + between text and a number glues them into one line of text.

Main.javawhole filejava

public class Main {
    public static void main(String[] args) {
        int cookies = 12;               // input
        int friends = 4;                // input
        int each = cookies / friends;   // processing
        System.out.println("Each friend gets: " + each);   // output
    }
}

terminal

$ java Main.java

── expected output ──

Each friend gets: 3

06Order matters

Because instructions run top to bottom, a value can change as the program goes. Below, money starts at 10, grows by 5, then shrinks by 3. Each print shows the value at that moment.

Swap the two middle lines and the final answer stays 12, but the printed line in between becomes different. In bigger programs, swapping lines can change the final answer too, for example printing a total before you finish adding to it.

Main.javawhole filejava

int money = 10;
money = money + 5;                         // a birthday gift
System.out.println("After gift: " + money);  // 15
money = money - 3;                         // buy a snack
System.out.println("After snack: " + money); // 12

07Bugs: the computer does what you wrote

If you meant to share 12 cookies among 4 friends but typed cookies / 3, Java happily prints 4. Nothing crashes. The program is wrong, but only you can tell, because only you know what you meant.

That is why programmers check their programs with examples whose answers they already know (12 cookies and 4 friends must give 3). Later in the course you'll write automated checks called unit tests (Topic 15.5). The other kind of bug, where the code breaks Java's rules, is caught by the compiler before the program even runs. Topic 0.7 teaches you to read those messages.

Try it yourself

1
Change the recipe
Open the first example and press Run. Then add a line System.out.println("Step 5: Cut it in half."); before the "Done!" line. Predict the output, then run it. Next, move Step 1 to the bottom and run again. The program runs exactly the order you wrote, even when it makes no sense.
2
Predict before you run
In the cookie example, change cookies to 20 and friends to 6. Write down what you expect for "Each friend gets" and "Left in the jar" before you press Run. (Answer: 3 and 2, because 6 × 3 = 18 and 20 − 18 = 2.)
3
Make the money example go negative
Change the snack price from 3 to 30. Predict the last two lines. Java does not stop you from spending money you don't have: it prints -15 and then -30. Checking for that is your job, with an if (Topic 2.1).

Code & diagrams

A recipe as a program New tab

Each println prints one line. The lines appear in exactly the order they are written.

Expected output

Step 1: Take two slices of bread.
Step 2: Spread butter on one slice.
Step 3: Put cheese on the butter.
Step 4: Close the sandwich.
Done! Enjoy your sandwich.

Input, processing, output New tab

The text after // is a comment: a note for humans that Java ignores. The % sign gives the remainder of a division.

Expected output

Cookies: 14
Friends: 4
Each friend gets: 3
Left in the jar: 2

Values change as the program runs New tab

Expected output

Start: 10
After gift: 15
After snack: 12
After doubling: 24

Break it on purpose

Errors are the best teachers. Make each change, read the error, guess what went wrong, then reveal the answer.

Break #1

Forget the quotes around text

In the recipe example, remove the quotes so a line reads System.out.println(Step 1: Take two slices of bread.);

terminal

$ javac Main.java

── what you'll see ──

Main.java:3: error: ')' or ',' expected