0 Put this page and the chat side by side

You will have this page on the left and the course chat on the right. Ask the chat whenever you are stuck.

1. Click this button: Open the course chat
   It opens the chat in a new tab and then tries to bring you back here. Most browsers still switch to the new tab anyway. That is normal: just click back on this tab.
2. Now put the two tabs side by side:
   Chrome: right-click this tab at the top of the window → Add tab to new split view → then pick the Grok tab.
   Edge: click the three dots (…) next to the address bar → Split screen → then select the Grok tab.

3. Type your questions into the chat on the right. This page has no ask box.

1 Open BlueJ and the enhanceForChoices project

1. Download enhanceForChoices.zip (the Original lab code). Do not unzip it yourself.
2. Open BlueJ from the Windows Start menu. (Can’t see it? Type BlueJ in the search bar.) The first time, it is an empty grey window.
3. Click Project → Open ZIP/JAR… → find enhanceForChoices.zip → Open.
4. BlueJ unzips it into a folder called enhanceForChoices, next to the zip, and opens it.
5. Look at the yellow boxes. Drag them around if you like; your layout may look different from the slide.

Examples have normal names (HelloWorld, HelloPlaces, HomeTown, EstateAgent). Exercises start with Ex_. Model answers start with Sol_.

2 Open HelloWorld and compile it

1. Double-click the yellow HelloWorld box. An editor window opens with coloured Java code.
2. Read the comments in it. For now, focus on one line: System.out.println("Hello World");
3. Click Compile at the top of the editor window (or close it, right-click the box → Compile).
4. Look at the box in the main window. What changed?

public class HelloWorld {
    public void run() {
        System.out.println("Hello World");
    }

    public static void main(String[] args) {
        new HelloWorld().run();
    }
}

class HelloWorld:
    def run(self):
        print("Hello World")

HelloWorld().run()

• public class HelloWorld starts the class. The name matches the file name.
• Curly brackets { } show where the class and each method start and end. Python uses a colon and indentation instead. Every { needs a matching }. Click next to a bracket in BlueJ and its partner lights up.
• Semicolon ; ends the instruction. Python uses the end of the line. Forget it and BlueJ says ';' expected.
• System.out.println("Hello World") is Python’s print("Hello World").
• main is where Java starts. Here it just runs run().

3 Run HelloWorld and find the terminal

1. Right-click the HelloWorld box. Two new options are at the top of the menu.
2. Click void main(String[] args). A small box appears; click OK.
3. A new window opens. Read what it says.
4. Now try the second way: right-click HelloWorld → new HelloWorld() → OK. A red box appears at the bottom left. Right-click the red box → void run().

The first way (void main) is the standard way to run a Java program. main is always the starting point.

4 Make a mistake on purpose, then duplicate a class

1. Open HelloWorld. Delete the semicolon at the end of the println line.
2. Click Compile. Read the message at the bottom of the editor.
3. Put the semicolon back. Compile again.
4. Now close the editor. Right-click HelloWorld → Duplicate… → name it HelloExperiment (start with a capital letter) → OK.
5. Open HelloExperiment, change "Hello World" to "Hello Charlie", compile and run it.

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

print("Hello Charlie")

Errors are not failures. They are a normal part of programming. Duplicating lets you experiment while keeping the original safe.

5 Run HelloBrighton: four instructions in order

1. Open HelloBrighton and read it. Count the println lines.
2. Compile it, then run it (right-click → void main → OK).
3. Put your “lazy hat” on: what is repeated?

System.out.println("Hello World");
System.out.println("Hello Europe");
System.out.println("Hello UK");
System.out.println("Hello Brighton");

print("Hello World")
print("Hello Europe")
print("Hello UK")
print("Hello Brighton")

This is sequence: Java runs the instructions in the order you give them, top to bottom.

Hello World
Hello Europe
Hello UK
Hello Brighton

6 HelloPlaces: an array and an enhanced for loop

1. Open HelloPlaces. Compile and run it. Compare the output with HelloBrighton.
2. Duplicate it as HelloPlaces2. Add two more places to the array, for example "Hove" and "Lewes" (commas between them, quotes round each).
3. Change "Hello " to "Bonjour ". Compile and run.

String[] placeList = {"World", "Europe", "UK", "Brighton"};

for (String place: placeList)
{
    System.out.println("Hello " + place);
}

placeList = ["World", "Europe", "UK", "Brighton"]

for place in placeList:
    print("Hello " + place)

Enhanced for loop. for (String place : placeList) { ... } means “for each String in placeList, copy it into place and run the body”. It stops after the last item. Read the colon : as Python’s in. The body goes in curly brackets.

"Hello " + place joins two Strings, just like in Python. placeList and place are variables: named boxes for data.

Bonjour World
Bonjour Europe
Bonjour UK
Bonjour Brighton
Bonjour Hove
Bonjour Lewes

7 HomeTown: your first if

1. Open HomeTown. Find the new variable home and the if inside the loop.
2. Before you run it, predict: which place gets an extra message?
3. Compile and run it.

String home = "Brighton";
for (String place: placeList)
{
    System.out.println("Hello " + place);
    if (place.equals(home)) {
        System.out.print("there's no place like home, ");
        System.out.println("my favourite place is " + place);
    }
}

home = "Brighton"
for place in placeList:
    print("Hello " + place)
    if place == home:
        print("there's no place like home, ", end="")
        print("my favourite place is " + place)

• if. The test goes in round brackets. The block in curly brackets runs only when the test is true. Python writes if test: and indents.
• .equals versus ==. To compare text in Java, write place.equals(home). Do not write place == home. == checks whether two things are the very same object in memory, so it can say false even when the letters match. In Python == is fine for text; in Java use == for numbers and .equals for Strings.
• print versus println. System.out.print prints and stays on the same line (Python print(..., end="")). System.out.println prints and then moves to a new line (a normal Python print). Here they build one line from two pieces.

Hello London
Hello Paris
Hello Brighton
there's no place like home, my favourite place is Brighton
Hello New York

8 Exercise: Ex_HomeTown part 1 (add an else)

1. Open Ex_HomeTown and read the instructions at the top. The code is a copy of HomeTown.
2. After the closing } of the if, add else { ... }.
3. Inside the else, print I've never been there with System.out.println.
4. Compile and run.

if (place.equals(home)) {
    ...
} else {
    System.out.println("I've never been there");
}

if place == home:
    ...
else:
    print("I've never been there")

else is the “otherwise” part: its block runs when the test is false. No test and no colon after else in Java.

if (place.equals(home)) {
    System.out.print("there's no place like home, ");
    System.out.println("my favourite place is " + place);
} else {
    System.out.println("I've never been there");
}

Hello London
I've never been there
Hello Paris
I've never been there
Hello Brighton
there's no place like home, my favourite place is Brighton
Hello New York
I've never been there

9 Exercise: Ex_HomeTown part 2 (else if for London)

1. Still in Ex_HomeTown: give an opinion on London as well.
2. Between the if block and the else, insert else if (place.equals("London")) { ... }.
3. Inside it, print your own opinion of London. Any message is fine.
4. Compile and run.

if (place.equals(home)) {
    ...
} else if (place.equals("London")) {
    System.out.println("London is big and busy");
} else {
    ...
}

if place == home:
    ...
elif place == "London":
    print("London is big and busy")
else:
    ...

if / else if / else. Java tests each condition in order and runs the first block whose test is true, then skips the rest. If none is true, the else runs. Java else if = Python elif.

if (place.equals(home)) {
    System.out.print("there's no place like home, ");
    System.out.println("my favourite place is " + place);
} else if (place.equals("London")) {
    System.out.println("London is big and busy, but I like visiting");
} else {
    System.out.println("I've never been there");
}

Hello London
London is big and busy, but I like visiting
Hello Paris
I've never been there
Hello Brighton
there's no place like home, my favourite place is Brighton
Hello New York
I've never been there

10 EstateAgent: a chain of ifs

1. Open EstateAgent. The array is now rooms: {"hall", "kitchen", "lounge", "kitchen", "bedroom", "hall"}.
2. Find the chain of if / else if / else. How many tests are there?
3. Find where it uses print and where it uses println.
4. Compile and run.

if (room.equals("kitchen")) {
    System.out.print("Can you smell the coffee? ");
} else if (room.equals("bedroom")) {
    System.out.print("Plenty of room for all your clothes.");
} else {
    System.out.print("What is this room used for?");
}

if room == "kitchen":
    print("Can you smell the coffee? ", end="")
elif room == "bedroom":
    print("Plenty of room for all your clothes.", end="")
else:
    print("What is this room used for?", end="")

Each comment uses print, so it joins the “Here we are in the …” text on the same line. The empty System.out.println(); at the end of the loop body finishes the line (Python print()).

The code works, but every test checks the same variable. The next step does it more neatly with a switch.

Here we are in the hall. Nice and big windows.
Here we are in the kitchen. Can you smell the coffee? 
Here we are in the lounge. Notice the original fireplace.
Here we are in the kitchen. Can you smell the coffee? 
Here we are in the bedroom. Plenty of room for all your clothes.
Here we are in the hall. Nice and big windows.

11 Exercise: Ex_EstateAgent_Switch

1. Open Ex_EstateAgent_Switch. It is almost empty: just a comment showing the shape of a switch.
2. Copy everything inside EstateAgent’s class (the run() and main() methods) into the Ex class, between its curly brackets.
3. In main, change new EstateAgent() to new Ex_EstateAgent_Switch().
4. Replace the whole if / else if / else chain with a switch (room): one case per room, each ending in break;, and default: for anything else.
5. Compile and run. The output should be the same as EstateAgent’s.

switch (room) {
    case "kitchen":
        System.out.print("Can you smell the coffee? ");
        break;
    case "bedroom":
        System.out.print("Plenty of room for all your clothes.");
        break;
    default:
        System.out.print("What is this room used for?");
}

match room:                 # Python 3.10+
    case "kitchen":
        print("Can you smell the coffee? ", end="")
    case "bedroom":
        print("Plenty of room for all your clothes.", end="")
    case _:
        print("What is this room used for?", end="")

switch / case / break. switch (room) looks at the value of room and jumps to the case with the same value. A case label ends with a colon :, and there are no curly brackets round each case. break; means “I’m done, leave the switch”. Without it, Java falls through into the next case. default: is the “anything else” case, like a final else; it needs no break because it is last.

public class Ex_EstateAgent_Switch{

    public void run() {

        String[] rooms = {"hall", "kitchen", "lounge", "kitchen", "bedroom", "hall"};

        // Loop: once for each room, stopping after the last "hall".
        for (String room: rooms){
            System.out.print("Here we are ");
            System.out.print("in the " + room + ". ");

            // SWITCH: Java looks at the value of room and jumps to the case with the same value.
            // Each case ends with break; which means "I'm done, leave the switch now".
            // Without break, Java would carry on into the next case (this is called fall-through).
            // default: runs when no case matches, like a final else.
            // Notice: a case label ends with a colon : and there are no { } around each case.
            // Python 3.10+: match room:  case "kitchen": ...  case _: ...
            switch(room){
                case "kitchen":
                    System.out.print("Can you smell the coffee? ");
                    break;
                case "bedroom":
                    System.out.print("Plenty of room for all your clothes.");
                    break;
                case "lounge":
                    System.out.print("Notice the original fireplace.");
                    break;
                case "hall":
                    System.out.print("Nice and big windows.");
                    break;
                default:
                    // The last case needs no break: there is nothing after it to fall into.
                    System.out.print("What is this room used for?");
            }

            System.out.println(); // finish printing the line

        } // end of loop body

    }

    public static void main(String[] args){
        new Ex_EstateAgent_Switch().run();
    }
}

12 Check your work against the Sol_ classes

1. Only now, open Sol_HomeTown. Compare it with your Ex_HomeTown. Run it.
2. Open Sol_EstateAgent_Switch. Compare it with yours. Run it.
3. Note one thing that is different from yours.

13 Optional: StarRating, a switch without break optional

1. Open StarRating and read the comment at the top.
2. Predict the output for ratings 1, 3 and 5. Then compile and run.
3. Challenge from the lecture: duplicate it as StarRating2 and rewrite the switch with a break after every case, keeping the same output. (The lecturer’s version is in the comment at the bottom of StarRating.java.)

switch (rating){
    case 5: System.out.print("★");
    case 4: System.out.print("★");
    case 3: System.out.print("★");
    case 2: System.out.print("★");
    case 1: System.out.print("★");
            break;
    default:
        System.out.print("Invalid rating");
}

stars = {5: "★★★★★", 4: "★★★★", 3: "★★★",
         2: "★★", 1: "★"}
print(stars.get(rating, "Invalid rating"), end="")
# Python's match cannot fall through,
# so there is no direct copy of this trick.

Here the case values are numbers (case 5:) because rating is an int (a whole number). There is no break until case 1, so Java jumps in at the matching case and falls through every case below it.

Rating 1: ★
Rating 3: ★★★
Rating 5: ★★★★★

14 Optional: Ex_IfLoop_NurseryRhyme optional

1. Open Ex_IfLoop_NurseryRhyme. It prints “10 Green Bottles” one println at a time (and skips the middle).
2. Read the hints at the bottom. First, comment out the old code: put // at the start of each println line, so you can still see it.
3. Make an array of the number words: "ten", "nine", ... "one", "no".
4. Loop over it with an enhanced for loop. Use if statements for the first and last verses, which are slightly different, and for “bottle” versus “bottles”.

if (! bottleAmount.equals("ten")) {
    System.out.println("there'll be " + line);
}

if bottleAmount != "ten":
    print("there'll be " + line)

! means not. ! bottleAmount.equals("ten") is true for every word except “ten”. Python writes != or not.

for (String bottleAmount: bottleAmountList) {
    String bottlesOrBottle = "bottles";
    if (bottleAmount.equals("one")) {
        bottlesOrBottle = "bottle";
    }
    String line = bottleAmount + " green " + bottlesOrBottle + ", hanging on the wall";
    if (! bottleAmount.equals("ten")) {
        System.out.println("there'll be " + line);
    }
    if (! bottleAmount.equals("no")) {
        System.out.println(line);
        System.out.println("and if one green bottle accidentally fall,");
    }
}

15 Optional: Star Ratings with two arrays (challenging) optional

For experienced students. Don’t worry if this is too much in Week 1.

1. Open Ex_StartRating_Challenging (the file name has a typo: “Start” for “Star”). Read the task at the top.
2. There are two arrays: hotels (names) and ratings (numbers). Print each hotel with its stars, like Grand Plaza (5 stars): ★★★★★.
3. Reuse the star switch from StarRating (no breaks, fall-through).
4. Compare with Sol_StarRating. (The comment in the Ex file says the solution is called HotelRating, but in the project it is Sol_StarRating.)

for (int i = 0; i < hotels.length; i++) {
    System.out.print(hotels[i] + " (" + ratings[i] + " stars): ");
    // ... the star switch on ratings[i] ...
    System.out.println();
}

for i in range(len(hotels)):
    print(hotels[i] + " (" + str(ratings[i]) + " stars): ", end="")
    # ... the stars ...
    print()

To walk through two arrays together you need a counter, i. The model answer uses a normal for loop, which is not taught until later (the roadmap lists for loops for Week 3). hotels[i] gets item number i, counting from 0, like a Python list.

Grand Plaza (5 stars): ★★★★★
Seaside Inn (3 stars): ★★★
Budget Stay (1 stars): ★
Royal Palace (4 stars): ★★★★