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 scannerWhile project

1. Download scannerWhile.zip (it is in the “Lecture and lab materials” folder). Do not unzip it.
2. Open BlueJ.
3. Click Project → Open ZIP/JAR... → choose scannerWhile.zip → Open.
4. You see orange boxes. Each box is one class.
5. Click Compile on the left. The striped boxes become plain.

Examples have normal names (SayHello, HelloEverybody, Santa). Exercises start with Ex_. Model answers start with Sol1_, Sol2_, Sol3_. Drag boxes around if you like; your layout may look different from the slide.

2 Read ScannerTemplate: the skeleton of every program

1. Double-click ScannerTemplate to open its code.
2. Read it top to bottom with the notes below.

import java.util.Scanner;

public class ScannerTemplate
{
    public void run()
    {
        Scanner myInput = new Scanner(System.in);
        // your code goes here
        myInput.close();
    }

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

# no import needed: input() is built in

class ScannerTemplate:
    def run(self):
        # your code goes here
        pass

ScannerTemplate().run()

• import java.util.Scanner; is like a Python import: it lets you use the Scanner class.
• Semicolon ; ends every Java instruction. Python uses the end of the line. Forget it and BlueJ says ';' expected.
• Curly brackets { } show where the class, a method, or a loop starts and ends. Python uses a colon and indentation. Every { needs a matching }.
• Scanner myInput = new Scanner(System.in); makes a Scanner called myInput that reads the keyboard (System.in).
• myInput.close(); closes it when you have finished.

3 Run SayHello and type your name

1. Right-click SayHello → click void main(String[] args) → click OK.
2. The terminal window opens with What is your name?.
3. Click in the bottom of the terminal, type Java, press Enter.
4. Run it again and type New York. Look carefully at the reply.

System.out.print("What is your name? ");
Scanner myInput = new Scanner(System.in);
String name = myInput.next();
System.out.println("Hello " + name + ", nice to meet you");
myInput.close();

name = input("What is your name? ")
print("Hello " + name + ", nice to meet you")

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). The prompt uses print so your answer appears right next to the question.

System.out.print("Hi ");
System.out.println("there");

print("Hi ", end="")
print("there")

String name makes a text variable. Java needs the type in front of a new variable; Python does not.

4 Run TripAdvisor and meet switch / case / break

1. Run TripAdvisor (right-click → void main → OK).
2. Type Brighton, Enter, then 3, Enter.
3. Run it again with 5, then again with 7. Predict each answer before you press Enter.
4. Open the code and find the switch.

switch (starNumber) {
    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("*");
    default:
        System.out.println("");
}

# Python's match never falls through,
# so the nearest is:
if starNumber in ["1","2","3","4","5"]:
    print("*" * int(starNumber), end="")
print("")

switch / case / break. switch (x) looks at x and jumps to the case with the same value. Each case ends with a colon :. break means “stop and leave the switch”. default is the “anything else” case, like a final else.

Here there is no break after the cases. So after jumping in, Java falls through and runs every line below. That is why 3 prints three stars. The cases have quotes ("3") because next() gives text, not a number.

5 Run HelloEverybody: your first while loop

1. Run HelloEverybody. Type Tom, then Harry, then bye.
2. Open the code and find the while line.
3. Find the line inside the loop that changes name.

String name = myInput.nextLine();
while (!name.equals("bye")) {
    System.out.println("Hello " + name + ", nice to meet you");
    System.out.print("What is your name (bye to exit)? ");
    name = myInput.nextLine();
}

name = input("What is your name (bye to exit)? ")
while name != "bye":
    print("Hello " + name + ", nice to meet you")
    name = input("What is your name (bye to exit)? ")

• while (test) { ... } repeats the body while the test is true. Test in round brackets, body in curly brackets.
• ! means not.
• .equals versus ==. To compare text in Java, write name.equals("bye"). Do not write name == "bye". == 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 always use .equals for Strings.
• name is the loop variable. The last line updates it. Without that line the loop never stops.

6 Run Santa: a loop that counts down

1. Run Santa. It needs no input.
2. Open the code. Change int x = 5; to int x = 3;. Compile and run.
3. Change x = x - 1; to x--;. Compile and run. Same result?
4. Put it back to 5 when you are done.

int x = 5;
while (x > 0)
{
    System.out.print("Ho! ");
    x = x - 1;     // or: x--;
}
System.out.println();

x = 5
while x > 0:
    print("Ho! ", end="")
    x = x - 1      # or: x -= 1
print()

int means a whole number. x = x - 1; is not maths: it means “take x, subtract 1, store it back in x”. x-- is a shortcut for that; x++ adds 1. Python has no ++ or --.

This loop is calculation-based, so it is predictable. HelloEverybody was input-based, so it depends on what you type.

7 Exercise: QueenBot (part 1, name and job)

The Queen asks each person their name and what they do, then comments on it.

1. Right-click HelloEverybody → Duplicate... → type QueenBot → OK.
2. Open QueenBot. Check the top says public class QueenBot and main says new QueenBot().run();. Fix any that still say HelloEverybody.
3. Under the line that reads name, add a new variable: String occupation = "";
4. Inside the while loop, after the hello: print the prompt what do you do? and read the job into occupation.
5. Print a comment such as occupation + " is such an interesting job!".
6. Keep the last two lines of the loop (ask for a name, update name). Compile, run, test with two people, then bye.

String occupation = "";
...
System.out.print("what do you do? ");
occupation = myInput.nextLine();
System.out.println(occupation + " is such an interesting job!");

occupation = ""
...
occupation = input("what do you do? ")
print(occupation + " is such an interesting job!")

The exercise file says myScanner.next(), but in HelloEverybody the Scanner is called myInput. Use myInput.

import java.util.Scanner;

public class QueenBot
{
    public void run() {
        System.out.print("What is your name (bye to exit)? ");
        Scanner myInput = new Scanner(System.in);
        String name = myInput.nextLine();
        String occupation = "";

        while (!name.equals("bye")) {
            System.out.print("Hello " + name + ", what do you do? ");
            occupation = myInput.nextLine();
            System.out.println(occupation + " is such an interesting job!");

            System.out.print("What is your name (bye to exit)? ");
            name = myInput.nextLine();
        }
        myInput.close();
    }

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

8 Exercise: QueenBot (part 2, a switch for jobs)

The exercise says you could also make the Queen say different things for different jobs.

1. In QueenBot, replace the single comment line with a switch (occupation).
2. Add case "queen": and case "student": with their own replies, each ending in break;.
3. Add default: with the general reply.
4. Compile and test with student, queen and chef.

switch (occupation) {
    case "student":
        System.out.println("I hope you are enjoying your studies!");
        break;
    default:
        System.out.println(occupation + " is such an interesting job!");
}

match occupation:
    case "student":
        print("I hope you are enjoying your studies!")
    case _:
        print(occupation + " is such an interesting job!")

This time you want break. Without it, a student would get the student reply and the default reply.

switch (occupation) {
    case "queen":
        System.out.println("Oh, God bless queen!");
        break;
    case "student":
        System.out.println("I hope you are enjoying your studies!");
        break;
    default:
        System.out.println(occupation + " is such an interesting job!");
}

9 Exercise: HouseTour (from EstateAgent)

Last week EstateAgent walked through a fixed list of rooms. Now the user chooses each room until they end the tour.

1. Open Ex_EstateAgent_To_HouseTour and read the old EstateAgent code in the comment block.
2. Right-click ScannerTemplate → Duplicate... → name it HouseTour. Fix the class name and the new ...().run() line if needed.
3. Inside run(), after the Scanner line: print Which room would you like to see (bye to exit)? and read String room = myInput.next();
4. Write while (!room.equals("bye")) { ... }.
5. Inside the loop, copy the room comments (the if / else if / else chain) from EstateAgent.
6. At the end of the loop body, ask again and update room. Compile and test.

Enhanced for (in the old code). for (String room : rooms) runs once for each item in the array rooms, putting it in room. You are replacing it with a while loop and the Scanner.

String[] rooms = {"hall", "kitchen"};
for (String room : rooms) {
    System.out.println(room);
}

rooms = ["hall", "kitchen"]
for room in rooms:
    print(room)

if / else if / else. Java tests each condition in order and runs the first one that is true. Python writes elif; Java writes else if. Each test goes in ( ), each block in { }.

if (room.equals("kitchen")) {
    System.out.print("Can you smell the coffee? ");
} else if (room.equals("hall")) {
    System.out.print("Nice and big windows.");
} else {
    System.out.print("What is this room used for?");
}

if room == "kitchen":
    print("Can you smell the coffee? ", end="")
elif room == "hall":
    print("Nice and big windows.", end="")
else:
    print("What is this room used for?", end="")

import java.util.Scanner;

public class HouseTour {
    public void run() {
        System.out.print("Which room would you like to see (bye to exit)? ");
        Scanner myInput = new Scanner(System.in);
        String room = myInput.next();

        while (!room.equals("bye")) {
            System.out.print("Here we are in the " + room + ". ");
            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 if (room.equals("lounge")) {
                System.out.print("Notice the original fireplace.");
            } else if (room.equals("hall")) {
                System.out.print("Nice and big windows.");
            } else {
                System.out.print("What is this room used for?");
            }
            System.out.println();
            System.out.print("Which room would you like to see (bye to exit)? ");
            room = myInput.next();
        }
        myInput.close();
    }

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

10 Exercise: HouseTour with a switch (optional part 3)

1. Change the if / else if / else chain in HouseTour to a switch (room).
2. One case per room, each ending in break;. Use default: for any other room.
3. Compile and test with the same rooms as before. The output should not change.

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

match room:
    case "kitchen":
        print("Can you smell the coffee? ", end="")
    case _:
        print("What is this room used for?", end="")

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 : System.out.print("What is this room used for?");
}

11 Extension: DrawSquare (do not worry if you cannot do it)

Draw a square of stars. If the user types 5, show 5 lines of 5 stars.

1. Open Ex_DrawSquare. The StarRating code from last week is copied in the comment block.
2. Click New Class..., name it DrawSquare, choose Class, OK. Open it, delete everything, and paste the StarRating code from the comment. Rename StarRating to DrawSquare in both places.
3. Read the size with int stars = myInput.nextInt(); (you will need the Scanner import and line from ScannerTemplate).
4. Make a counter int line = 0; and a loop while (line < stars) { ... line++; } around the switch.
5. Test with 3 and 5. Then try 7. What goes wrong?
6. Fix it: swap the switch for a second while loop inside the first one, with its own counter x.

int stars = myInput.nextInt();
int line = 0;
while (line < stars) {
    int x = 0;
    while (x < stars) {
        System.out.print("*");
        x++;
    }
    System.out.println();
    line++;
}

stars = int(input("How many stars? "))
line = 0
while line < stars:
    x = 0
    while x < stars:
        print("*", end="")
        x += 1
    print()
    line += 1

nextInt() reads a whole number, so you can compare it with <. A loop inside a loop needs different loop variables (line and x).

import java.util.Scanner;

public class DrawSquare
{
    public void run() {
        System.out.print("How many stars? ");
        Scanner myInput = new Scanner(System.in);
        int stars = myInput.nextInt();

        int line = 0;
        while (line < stars) {
            int x = 0;
            while (x < stars) {
                System.out.print("*");
                x++;
            }
            System.out.println();
            line++;
        }
        myInput.close();
    }

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

***
***
***

12 Check your work against the Sol_ classes

1. Only now, open Sol1_QueenBot and Sol2_QueenBot. Compare with yours.
2. Open Sol1_HouseTour_WhileIfElse and Sol2_HouseTour_WhileSwitch.
3. If you tried DrawSquare: open Sol1_DrawSquare_WhileSwitch and Sol2_DrawSquare_2Whiles.
4. Run each model answer once. Note one thing that is different from yours.

13 Optional: special replies for Tom and Harry optional

Lecture challenge (slide 23).

1. Duplicate HelloEverybody and call it SpecialNames.
2. Inside the loop, replace the hello line with an if / else if / else: Tom → “Do you also have a friend called Jerry?”, Harry → “How is Ron doing?”, anyone else → the normal greeting.
3. Test with Tom, Harry, Ann, bye.

while (!name.equals("bye")) {
    if (name.equals("Tom")) {
        System.out.println("Do you also have a friend called Jerry?");
    } else if (name.equals("Harry")) {
        System.out.println("How is Ron doing?");
    } else {
        System.out.println("Hello " + name + ", nice to meet you");
    }
    System.out.print("What is your name (bye to exit)? ");
    name = myInput.nextLine();
}

14 Optional: tell Santa how many Ho!s optional

Just-for-fun challenge (slide 27).

1. Duplicate ScannerTemplate as Santa1. Ask how many Ho!s, read it with nextInt(), then count it down in a while loop.
2. Then make Santa2: keep asking until the user types 0 or a negative number. Use two while loops.

System.out.print("How many Hos do you want Santa to say (0 exit)? ");
Scanner myInput = new Scanner(System.in);
int number = myInput.nextInt();

while (number > 0) {          // input-based
    while (number > 0) {      // calculation-based
        System.out.print("Ho! ");
        number = number - 1;
    }
    System.out.println("");
    System.out.print("How many Hos do you want Santa to say (0 exit)? ");
    number = myInput.nextInt();
}
myInput.close();

15 Optional: City Ratings (challenging, for experienced students) optional

1. Open Ex_CityRatings_Challenging and read the instructions in the comment at the top.
2. Use the enhanced for loop over cities. For each city ask “Have you visited ...? (yes/no)”.
3. If yes, ask for stars and print them (fall-through switch, or an if-else chain). If no, print “[city] is nice, you should visit someday!”.
4. Then compare with Sol1, Sol2 and Sol3_CityRatings. Each one explains at the top what it does differently.

for (String city : cities) {
    System.out.print("Have you visited " + city + "? (y/n): ");
    String yesOrNo = myInput.next();
    if (yesOrNo.equalsIgnoreCase("y")) {
        System.out.print("How many stars (1-5)? ");
        String stars = myInput.next();
        System.out.print(city + " (" + stars + " stars): ");
        switch (stars) {
            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.println();
    } else if (yesOrNo.equalsIgnoreCase("n")) {
        System.out.println(city + " is nice, you should visit someday!");
    }
}