JV010 Java for kids: the Robot Factory

Predict

The file says System.out.println("Beep!"); What will the robot say when we run it?

Lesson

A program is a list of instructions in a file. Java reads it from the top and does each line. Our rhythm is always the same: run it, look at what it printed, change one word, run it again.

Demo

The tutor opens java/jv010/demo/Robot.java, runs java Robot.java in the terminal and Pixel says Beep!. Then the tutor changes Beep! to Boop! and runs it again so both answers are on the screen.

On screen

Explorer → java/jv010/demo/Robot.java → Terminal → java Robot.java → change the word → run again.

Hands-on

Open java/jv010/s1/Robot.java (a copy the tutor made). Change what Pixel says to a robot word of your own, like Zap or Whirr, then run it. Add a second println so Pixel says two things.

Explain

Say in one sentence what the computer did with the file.

Check

Pixel says two things of your own — checked (T3): seat_run

#s1-1

Predict

Three println lines print [o o], then | - |, then \___/. How many lines appear on the screen?

Lesson

println means print this and go to the next line. If you print one row of a picture per line, the rows stack up and you get a drawing made of letters and symbols.

Demo

The tutor adds five println lines to the demo file, one row each, and a robot face with square eyes and an antenna appears in the terminal. Then the tutor swaps the eyes from o to O to show how easy it is to change.

Hands-on

In java/jv010/s1/Face.java print a robot face at least four rows tall: an antenna row, an eyes row with two letters, a mouth row, and a chin row. Make it look like your own robot, not the demo's.

Explain

Why did we need one println for each row?

Check

Face.java prints a face at least four rows tall — checked (T3): seat_run

#s1-2

Predict

Line 4 is missing its semicolon at the end. What does Java do?

Lesson

When something is wrong, Java shows a red message. It is not angry: it tells you the line number and what is missing. Go to that line, look closely, fix one thing, run again.

Demo

The tutor removes a semicolon in the demo file, runs it, and reads the red message slowly: file name, line number, the word expected. The tutor goes to that line, puts the semicolon back, and Pixel speaks again.

Hands-on

Open java/jv010/samples/s1/Broken.java and copy it to java/jv010/s1/Fixed.java. It has two mistakes: a missing semicolon and a missing quote mark. Read each red message, fix the line it names, until Pixel says Fixed me!.

Explain

What two things does the red message tell you?

Check

Fixed.java runs and says Fixed me! — checked (T3): seat_run

#s1-3

Predict

The badge needs 5 rows. How many println lines is that?

Lesson

A show is a small program you make all by yourself from what you learned: print rows to draw, change the words, and fix any red message by reading the line number it names.

Demo

The tutor shows a finished badge in the demo file: a top border, two face rows, a bottom border and the model word, then runs it once so the shape is clear, and closes the file so the learner builds their own.

Hands-on

In java/jv010/s1/Badge.java print Pixel's badge: exactly five rows. Rows 1 and 5 are borders made of + and - signs, rows 2 and 3 are the face, and row 4 says MODEL: PIXEL-1. Run it with no red messages.

Explain

Which row would you change to give the robot a new model name?

Check

Badge.java prints a five-row badge with MODEL: PIXEL-1 — checked (T3): seat_run

#s1-4

Predict

We write int battery = 90; then System.out.println(battery); What prints?

Lesson

A variable is a box with a label. int boxes hold whole numbers, String boxes hold words. You put something in with =, and when you print the label without quotes you get what is inside.

Demo

The tutor makes three boxes in java/jv010/demo/Settings.java — String robotName = "Pixel", int battery = 90, String colour = "silver" — prints each, then changes battery to 55 and runs again to show the box changed.

Hands-on

In java/jv010/s2/Boxes.java make three boxes: a String for the robot's name (make one up), an int battery, and a String colour. Print each one on its own line.

Explain

What is the difference between printing "battery" with quotes and battery without?

Check

Boxes.java declares a String, an int and prints three lines — checked (T3): seat_file seat_run

#s2-1

Predict

int battery = 90; System.out.println(battery - 10); What prints?

Lesson

Number boxes can do sums: + adds, - takes away, * multiplies. Java works out the sum and prints the answer. You can also put the answer in a new box, like int later = battery - 10;.

Demo

The tutor prints battery - 10, battery * 2 and battery + 5 from the demo file, then adds int afterHour = battery - 10; and prints a sentence about it, so the sum is kept in a box.

Hands-on

In java/jv010/s2/Sums.java make int battery = 70; and int legs = 2;. Print the battery after one hour (10 less), the battery doubled, and how many legs four robots have (legs times 4).

Explain

What does Java do before it prints battery - 10?

Check

Sums.java prints 60, 140 and 8 — checked (T3): seat_run

#s2-2

Predict

String name = "Pixel"; System.out.println("Hi " + name + "!"); What prints?

Lesson

Plus between words glues them into one longer word. You can glue a box and some text to make a sentence. A program can also be given words when it starts: args[0] is the first one typed after the file name.

Demo

The tutor prints "Hi " + robotName + "!" from the demo, then changes the file to read String robotName = args[0]; and runs java Settings.java Bolt, then java Settings.java Zippy, so the greeting changes each time.

Hands-on

In java/jv010/s2/Hello.java take the robot's name from args[0] and print two lines: "Hello, I am <name>." and "<name> is ready!". Run it with java Hello.java Pixel and then with a different robot name.

Explain

Where did the robot name come from this time?

Check

Hello.java greets the robot named on the command line — proven (T4): seat_run

#s2-3

Predict

args[1] is the word "80". To do sums with it we write int battery = Integer.parseInt(args[1]); Why?

Lesson

Everything typed after the file name is a word, even "80". Integer.parseInt turns a word of digits into a number box, and then the sums from 2.2 work. Put the pieces together and you have a card.

Demo

The tutor builds the demo card: name from args[0], battery from Integer.parseInt(args[1]), a border line, the name, the battery, and the battery after one hour, then runs it with Bolt 80 and with Zippy 45.

Hands-on

In java/jv010/s2/Card.java read the robot's name from args[0] and the battery from args[1]. Print four lines: a border of = signs, "Robot: <name>", "Battery: <number>", and "After one hour: <number minus 10>".

Explain

Why does the card work for any robot name and battery?

Check

Card.java prints the settings card for any robot — proven (T4): seat_run

#s2-4

Predict

for (int i = 0; i < 5; i++) { System.out.println("Beep"); } How many Beeps?

Lesson

A for loop repeats the lines inside its curly brackets. i starts at 0, the loop runs while i is less than the number, and i goes up by one each turn. Five turns means five Beeps from one println.

Demo

The tutor writes a loop in java/jv010/demo/Loops.java that prints Beep five times, runs it, then changes 5 to 12 and runs again: twelve Beeps, still one println.

Hands-on

In java/jv010/s3/Beeps.java write one loop that prints "Beep!" exactly seven times, then one println after the loop that says "Pixel is awake".

Explain

Why is there only one println but seven Beeps?

Check

Beeps.java prints seven Beep! lines then Pixel is awake — checked (T3): seat_file seat_run

#s3-1

Predict

for (int i = 3; i >= 1; i--) { System.out.println(i); } What prints?

Lesson

The counter i is a real box you can print. Start it high and use i-- to count down. The middle part says when to keep going; when it becomes false the loop stops.

Demo

The tutor prints the counter inside the loop from 1 to 5, then flips it into a countdown from 5 to 1 and adds println("Blast-off!") after the loop, running each version.

Hands-on

In java/jv010/s3/Countdown.java count down from 10 to 1, one number per line, then print "Blast-off!".

Explain

What makes the loop stop?

Check

Countdown.java prints 10 down to 1 then Blast-off! — checked (T3): seat_run

#s3-2

Predict

"*".repeat(3) gives what?

Lesson

A loop and repeat make patterns. "*".repeat(i) is i stars; print it inside a loop where i grows and each row is one star longer: a staircase.

Demo

The tutor prints "*".repeat(i) for i from 1 to 5 and a staircase appears, then changes * to # and the start to 5 going down so the staircase flips upside down.

Hands-on

In java/jv010/s3/Stairs.java print a staircase of 6 rows: row 1 has one #, row 6 has six. Then print one row of six = signs as the floor.

Explain

Why does each row get longer?

Check

Stairs.java prints a six-row staircase and a floor — checked (T3): seat_run

#s3-3

Predict

int n = Integer.parseInt(args[0]); and the loop runs i from 1 to n. If we type 4, how many robots?

Lesson

Put it together: a number from the command line, a loop that counts from 1 to that number, a line printed each turn with the counter in it, and one line after the loop that uses the number again.

Demo

The tutor builds the demo army: n from args[0], a loop printing "[o_o] Robot " + i, and after the loop n + " robots reported for duty", running it with 3 and then with 6.

Hands-on

In java/jv010/s3/Army.java read a number from args[0]. Print that many lines "[o_o] Robot 1", "[o_o] Robot 2" and so on, then "<n> robots reported for duty". Run it with 3, then with 5.

Explain

What would change if we typed 100?

Check

Army.java prints the robot army for any number — proven (T4): seat_run

#s3-4

Predict

int battery = 15; if (battery < 20) { println("Grumpy"); } else { println("Happy"); } What prints?

Lesson

if asks a yes-or-no question about a box, like battery < 20. If yes, the lines in the first curly brackets run. else holds the lines for no. Only one of the two ever runs.

Demo

The tutor writes the battery question in java/jv010/demo/Moods.java with Grumpy and Happy, runs it with battery = 15 and then 60, and shows that changing the number changes which words appear.

Hands-on

In java/jv010/s4/Low.java make int battery = Integer.parseInt(args[0]);. If the battery is under 30 print "Pixel is grumpy" and a frowning face row, else print "Pixel is happy" and a smiling face row.

Explain

Why did only one mood print?

Check

Low.java picks grumpy or happy from the battery — proven (T4): seat_run

#s4-1

Predict

if (b < 20) Sleepy; else if (b < 80) Fine; else Super. Battery 80 gives?

Lesson

else if adds a second question that is only asked when the first was no. You can chain as many as you need, and the last else catches everything that is left. Java checks from the top and stops at the first yes.

Demo

The tutor turns the demo into three moods with else if, runs it with 10, 50 and 95, and points at which question said yes each time, then tries 20 and 79 to show the edges.

Hands-on

In java/jv010/s4/Three.java read the battery from args[0] and print exactly one mood word: "Sleepy" under 20, "Fine" from 20 to 79, "Super" at 80 or more.

Explain

Why does 50 not print Super too?

Check

Three.java prints one of Sleepy, Fine, Super — proven (T4): seat_run

#s4-2

Predict

new Random(7).nextInt(6) + 1 is run twice in two separate programs. Are the two numbers the same?

Lesson

Random makes dice rolls. nextInt(6) gives 0 to 5, so add 1 for a real dice. If you give Random a seed number, the rolls come out the same every time, which is handy for checking your game.

Demo

The tutor makes Random dice = new Random(7); and rolls three times in a loop, runs it twice to show the same three numbers, then changes the seed to 8 and gets different ones.

Hands-on

In java/jv010/s4/Dice.java read a seed from args[0], make new Random(seed), roll the dice 5 times in a loop and print each roll as "Roll: <number>". Then print "Pixel rolled 5 times".

Explain

Why do we add 1 to nextInt(6)?

Check

Dice.java prints five rolls between 1 and 6 from the seed — checked (T3): seat_file seat_run

#s4-3

Predict

The mood machine prints a mood word and then a face. For battery 5, which face?

Lesson

A branch can hold more than one line. Put the mood word and its face rows inside the same curly brackets so the whole mood prints together. Three branches, three different faces.

Demo

The tutor shows the finished demo mood machine with a two-row face in each branch, runs it with 5, 50 and 99, then closes it so the learner builds their own faces.

Hands-on

In java/jv010/s4/Mood.java read the battery from args[0]. Print the mood word (Sleepy, Fine or Super, same rules as 4.2) and then two rows of a face for that mood. Sleepy eyes are "-", Fine eyes are "o", Super eyes are "O".

Explain

How did you make the face print with the right mood?

Check

Mood.java prints the mood and its face for any battery — proven (T4): seat_run

#s4-4

Predict

static void wave() prints "o/ hello". main calls wave(); wave(); How many hellos?

Lesson

A method is a named trick. static void wave() { ... } teaches it; wave(); does it. Teach once, call as often as you like. main is also a method: it is the one Java runs first.

Demo

The tutor adds static void wave() to java/jv010/demo/Tricks.java under main, printing an arm and a hello, calls it three times from main and runs it. Then the tutor changes the hello text once and all three change.

Hands-on

In java/jv010/s5/Wave.java write a method wave that prints two lines (an arm row and "Pixel waves!"), then call it three times from main.

Explain

Why did changing wave once change all three greetings?

Check

Wave.java defines wave() and calls it three times — checked (T3): seat_file seat_run

#s5-1

Predict

static void beep(int times) loops times and prints Beep. main calls beep(2); beep(3); How many Beeps?

Lesson

A trick can take an input in its round brackets: static void beep(int times). Inside, times is a box holding whatever the caller passed. Call beep(2) and beep(3) and the same trick does different amounts.

Demo

The tutor writes beep(int times) with a for loop inside, calls beep(1), beep(2), beep(3) from main, runs it, and points at how the number in the brackets lands in the times box.

Hands-on

In java/jv010/s5/Beep.java write beep(int times) that prints "Beep" that many times, then one line "---" after. From main call beep(2) and then beep(4).

Explain

What is times inside the method?

Check

Beep.java prints 2 Beeps, ---, 4 Beeps, --- — checked (T3): seat_file seat_run

#s5-2

Predict

static int twice(int n) { return n * 2; } println(twice(21)); What prints?

Lesson

A trick can answer back. static int twice(int n) says the answer is a whole number, and return n * 2; hands it to whoever called. The caller can print it or put it in a box.

Demo

The tutor writes twice(int n), prints twice(5) and twice(21), then puts twice(4) in a box and prints the box, showing the answer travels back to main.

Hands-on

In java/jv010/s5/Answer.java write static int twice(int n) and static int addTen(int n). Read a number from args[0] and print "Twice: <answer>" and "Plus ten: <answer>".

Explain

What does return do?

Check

Answer.java returns twice and plus ten for any number — proven (T4): seat_file seat_run

#s5-3

Predict

main calls wave(); then beep(n); then prints twice(n). If n is 3, which comes first on the screen?

Lesson

A trick book is a file with several methods and a main that calls them in order. main reads the number from the command line and passes it to the tricks that need it.

Demo

The tutor shows the finished demo book: wave(), beep(int), twice(int) and a main that calls them with the number from args[0], runs it with 2 and 3, and closes the file.

Hands-on

In java/jv010/s5/Book.java write wave() (prints "Pixel waves!"), beep(int times) (prints Beep that many times) and twice(int n) (returns n times 2). main reads n from args[0], calls wave, beep(n) and prints "Twice: <answer>".

Explain

Why keep each trick in its own method instead of writing everything in main?

Check

Book.java runs three tricks in order for any number — proven (T4): seat_file seat_run

#s5-4

Predict

A plan written as // comments inside main. What does Java do with those lines?

Lesson

Before building, write the plan. In main, write one // comment per scene: title, face, army, mood, countdown, blast-off. Each comment will become one method call. Java ignores comments, so nothing runs yet.

Demo

The tutor opens java/jv010/demo/Show.java with an empty main and writes six comment lines, one per scene, then runs it to show that a program of only comments prints nothing and is still fine.

Hands-on

Create java/jv010/s6/Show.java with a main that holds six // comment lines naming the scenes in order. Run it: it should print nothing and show no red message.

Explain

Why plan with comments before writing code?

Check

Show.java has six scene comments and runs clean — checked (T3): seat_file seat_run

#s6-1

Predict

The army scene is a method army(int n). Where does n come from when the show runs?

Lesson

Turn the first four comments into methods: title(String name), face(), army(int n) and mood(int n). Each one is a trick from an earlier session. main calls them in order with the name and number from args.

Demo

The tutor fills in title, face, army and mood in the demo, one method at a time, running after each so the show grows on screen, and keeps the comments above each call.

Hands-on

In java/jv010/s6/Show.java add methods title(String name), face(), army(int n) and mood(int n) from your earlier files, and call them from main with args[0] and Integer.parseInt(args[1]). Run with Pixel 3.

Explain

Which earlier step did each method come from?

Check

Show.java has the four scene methods and prints the first scenes — checked (T3): seat_file seat_run

#s6-2

Predict

Inside the countdown loop: if (i == 1) print "ONE!" else print i. For 3 the lines are?

Lesson

A loop and an if together: the countdown prints each number, but on the last one an if swaps the number for a surprise. == asks is exactly equal; one = would be putting into a box.

Demo

The tutor writes countdown(int from) in the demo with an if (i == 1) inside, runs it from 5, then adds the blast-off line after the loop with the robot name glued in.

Hands-on

In java/jv010/s6/Show.java add countdown(int from) that prints from down to 2 and then "ONE!", and in main after it print "Blast-off, <name>!". Run with Pixel 3.

Explain

What is the difference between = and ==?

Check

Show.java ends with the countdown and Blast-off — checked (T3): seat_file seat_run

#s6-3

Predict

The machine runs your show with Bolt 5 instead of Pixel 3. What must be true for it to pass?

Lesson

The finale: the whole show runs from the command line. Read your main from the top, run it with two different names and numbers, and fix anything that still says Pixel or 3 when it should not.

Demo

The tutor runs the demo show with Bolt 5 and Zippy 2, points at each scene following the new values, then closes it: the learner's show is their own.

Hands-on

Finish java/jv010/s6/Show.java: title with the name, the face, an army of n robots, the mood for n times 10 as a battery, the countdown from n, and "Blast-off, <name>!". Run with Pixel 3 and then with Bolt 5.

Explain

Which scene was the hardest, and what fixed it?

Check

The Robot Show runs for any robot name and number — proven (T4): seat_file seat_run

#s6-4