Lecture 3 · 100 min

Variables and Data Types

How does a program remember, change and use values?

Prepared by: Eng. Badereddin Mohammed Elhalimi · Eng. Mohammed Zuhair Kouhail

Teacher view

By the end of this lecture you will be able to

Explain what a variable is using the box example
Declare a variable, assign it a value and use it
Choose the right data type for each value
Use operators to calculate and update values
Name variables using Java rules and fix naming errors

In this lecture

Every app remembers values that change while it runs
A variable is a named box that holds one value
Three steps to work with any variable
Five data types cover most of our first programs
Java has eight primitive types, each with a fixed size
First program: store two numbers and add them
The value changes, the name stays: track score line by line
Variable names follow rules set by the compiler
The compiler rejects a value of the wrong type
Swap challenge: a third variable saves the value
The + sign joins strings into one new string
Five arithmetic operators
++ and -- change a value by one, before or after use
+= and -= update a variable in one short step
Final exercise: a program that prints a student card
Review questions

Part one: variables and their types

Every app remembers values that change while it runs

Mobile game

Your score goes up with every level you finish and stays at the top of the screen.

Calculator

The M+ button saves a number so you can use it after several operations.

Online store

The cart total changes every time you add or remove a product.

A variable is a named box that holds one value

firstNumber2int

secondNumber3int

A program needs to keep values while it runs.
A number alone is not a variable; a variable is a name that holds it.
A box holds only one value at a time.
Variables are not only for numbers.

Part	Meaning
`int`	Data type: a whole number
`firstNumber`	Variable name
`2`	The value stored in it

Three steps to work with any variable

Declare

Reserve a box and give it a type and a name. It has no value yet.

int age;

Assign

Put a value in the box using =. Read it as "takes the value".

age = 19;

Use

Write the variable name, and the program reads the value stored in it.

println(age);

Try the three steps

Run the program, then press "Debug line by line" and watch the box appear and fill. Change the age and run it again.

public class Age {
    public static void main(String[] args) {
        int age;
        age = 19;
        System.out.println(age);
    }
}

Output

The name goes on the left

Fix the line so the program works and prints 19.

public class Age {
    public static void main(String[] args) {
        int age;
        19 = age;
        System.out.println(age);
    }
}

Compiler messageunexpected type

Show the fix

public class Age {
    public static void main(String[] args) {
        int age;
        age = 19;
        System.out.println(age);
    }
}

Output

Why? The left side of = must be a box that receives a value, and the number 19 is not a box.

Five data types cover most of our first programs

Type	Holds	Example	How to spot it
`int`	A whole number	`19`	No decimal point
`double`	A decimal number	`3.75`	Has a decimal point
`char`	One character	`'A'`	Inside ' '
`String`	Text	`"Sara"`	Inside " "
`boolean`	True or false	`true`	Only two values

All five types in one program

public class Types {
    public static void main(String[] args) {
        int age = 19;
        double gpa = 3.75;
        char grade = 'A';
        String name = "Sara";
        boolean passed = true;

        System.out.println(name);
        System.out.println(age);
        System.out.println(gpa);
        System.out.println(grade);
        System.out.println(passed);
    }
}

Output

Sara
19
3.75
A
true

Which type fits the number of students in a class, such as 32?

Which type fits the price of a book, such as 12.5?

Why? A price has a decimal part, so double fits it.

Which type fits "Has the student paid the fees?"

Why? The answer is only yes or no, meaning true or false.

Java has eight primitive types, each with a fixed size

This course uses int, double, char and boolean; the others store the same kinds of values in more or less memory.

Type	Size	Range or examples
`boolean`	Logical values	`true, false`
`byte`	8 bits	`-128 … 127`
`char`	16 bits	One Unicode character: `'a'`
`short`	16 bits	`-32,768 … 32,767`
`int`	32 bits	About 2.1 billion, positive or negative
`long`	64 bits	`10L`
`float`	32 bits	About 7 significant digits: `3.14f`
`double`	64 bits	About 16 significant digits: `3.14`

What happens when the box is full?

Every type has a limit. Add 1 to the largest int, then change the type to long and add L to the number.

public class Limits {
    public static void main(String[] args) {
        int biggest = 2147483647;
        System.out.println(biggest);
        System.out.println(biggest + 1);
    }
}

Output

2147483647
-2147483648

Part two: let's code

First program: store two numbers and add them

Write the lines inside main
Predict the output before running
Run itIn the lab press F6; here use the Run button or Ctrl+Enter.
Change the two values and run again

package firstprogram;

public class FirstProgram {
    public static void main(String[] args) {
        int firstNumber = 2;
        int secondNumber = 3;
        int sum = firstNumber + secondNumber;
        System.out.println(sum);
    }
}

Output

Quotation marks separate a variable name from text. What does this print?

public class Names {
    public static void main(String[] args) {
        int sum = 5;
        System.out.println(sum);
        System.out.println("sum");
        System.out.println("Sum = " + sum);
    }
}

Show the output

Output

5
sum
Sum = 5

Why? Without quotation marks the program prints the value; inside them it prints the word as written. The + joins text and a variable.

The value changes, the name stays: track score line by line

What is the final value of `score`?

public class Score {
    public static void main(String[] args) {
        int score = 10;
        score = score + 5;
        score = score * 2;
        System.out.println(score);
    }
}

Show the output

Output

Why? Line 2 is not a math equation: we calculate the right side first (10 + 5), then put the result in the box and the old value is erased. Then 15 × 2 = 30.

Variable names follow rules set by the compiler

Name	Verdict	Why
`2ndNumber`	Not allowed	Starts with a digit
`my age`	Not allowed	Has a space
`int`	Not allowed	A reserved word in Java
`total%`	Not allowed	The symbol `%` is not allowed in names
`secondNumber`	Allowed and clear	Starts with a letter
`myAge`	Allowed and clear	Two words joined together
`totalPrice`	Allowed and clear	Describes what it stores
`Age`	Allowed, but avoid it	A different name from `age`, but it breaks the camelCase convention
`total$`	Allowed, but avoid it	`$` and `_` are allowed, but programmers rarely use them in names

Fix the variable name

public class Naming {
    public static void main(String[] args) {
        int 2ndNumber = 7;
        System.out.println(2ndNumber);
    }
}

Compiler messagenot a statement

Show the fix

public class Naming {
    public static void main(String[] args) {
        int secondNumber = 7;
        System.out.println(secondNumber);
    }
}

Output

Why? A name cannot start with a digit. The compiler reads 2 as a number and cannot make sense of the rest, which is why the message looks unrelated to the real cause.

The compiler rejects a value of the wrong type

Planned test

We put the age inside quotation marks. Notice the red underline in the editor, then fix the line.

public class Age {
    public static void main(String[] args) {
        int age = "19";
        System.out.println(age);
    }
}

Compiler messageincompatible types: String cannot be converted to int

Show the fix

public class Age {
    public static void main(String[] args) {
        int age = 19;
        System.out.println(age);
    }
}

Output

Why? The type decides what the box accepts: a box made for numbers does not accept text.

Another common error

We used a variable before declaring it.

public class Age {
    public static void main(String[] args) {
        System.out.println(age);
        int age = 19;
    }
}

Compiler messagecannot find symbol

Show the fix

public class Age {
    public static void main(String[] args) {
        int age = 19;
        System.out.println(age);
    }
}

Output

Why? The compiler reads top to bottom, so it does not know the name age before the line that declares it.

Swap challenge: a third variable saves the value

Trace the swap

Press "Debug line by line" and watch the three boxes. Then remove the int temp = a; line and see what happens.

public class Swap {
    public static void main(String[] args) {
        int a = 5;
        int b = 9;

        int temp = a;
        a = b;
        b = temp;

        System.out.println(a);
        System.out.println(b);
    }
}

Output

9
5

Part three: operators in action

The + sign joins strings into one new string

Joining text with + is called concatenation. Java reads + from left to right, and that is where the trap lies.

Predict: text and numbers

public class Concat {
    public static void main(String[] args) {
        System.out.println("Total: " + 2 + 3);
        System.out.println(2 + 3 + " total");
        System.out.println("Total: " + (2 + 3));
    }
}

Show the output

Output

Total: 23
5 total
Total: 5

Why? In line 1, "Total: " + 2 becomes text, so 3 is stuck onto it. In line 2, 2 + 3 is added first because both are numbers. The parentheses in line 3 force the addition before the join.

Pair activity: print your full name

Declare two String variables, firstName and lastName, holding Sara and Ali, then print the full name with a space between them, using concatenation.

public class FullName {
    public static void main(String[] args) {
        // Declare firstName and lastName here

    }
}

Expected output

Sara Ali

Hint 1

You need a piece of text between them holding one space: " ".

Solution

public class FullName {
    public static void main(String[] args) {
        String firstName = "Sara";
        String lastName = "Ali";
        System.out.println(firstName + " " + lastName);
    }
}

Five arithmetic operators

Operator	Name	`A = 10`, `B = 20`	Note
`+`	Addition	`30`	Adds both sides
`-`	Subtraction	`-10`	Right from left
`*`	Multiplication	`200`	Multiplies both sides
`/`	Division	`0`	Dividing whole numbers drops the decimal
`%`	Modulo	`10`	Gives the remainder

Change the values and predict

Change A to 17 and B to 5, and predict all five results before running.

public class Arithmetic {
    public static void main(String[] args) {
        int A = 10;
        int B = 20;
        System.out.println(A + B);
        System.out.println(A - B);
        System.out.println(A * B);
        System.out.println(A / B);
        System.out.println(A % B);
    }
}

Output

Turn seconds into minutes and seconds

The variable total holds 125 seconds. Work out the minutes and the remaining seconds using / and %, then print them as in the expected output.

public class Duration {
    public static void main(String[] args) {
        int total = 125;
        // Work out the minutes and the seconds here

    }
}

Expected output

Minutes: 2
Seconds: 5

Hint 1

A minute has 60 seconds, and dividing whole numbers drops the decimal: total / 60.

Hint 2

The remaining seconds are the remainder: total % 60.

Solution

public class Duration {
    public static void main(String[] args) {
        int total = 125;
        int minutes = total / 60;
        int seconds = total % 60;
        System.out.println("Minutes: " + minutes);
        System.out.println("Seconds: " + seconds);
    }
}

`++` and `--` change a value by one, before or after use

Prefix: ++x

Adds one first, then uses the new value. Result: y = 35, x = 35.

int x = 34;
int y = ++x;

Postfix: x++

Uses the old value first, then adds one. Result: y = 34, x = 35.

int x = 34;
int y = x++;

Vote: what does each line print?

public class Vote {
    public static void main(String[] args) {
        int x = 14;
        System.out.println(x++);
        System.out.println(x);
    }
}

Show the output

Output

14
15

Why? x++ gives the old value 14 to print, then x becomes 15.

`+=` and `-=` update a variable in one short step

The rule: Java applies the operator to the old value, and the result replaces it. *= and /= work the same way.

Start with `num1 = 4` and `num2 = 8`, and write `num2` on paper after each line.

public class Compound {
    public static void main(String[] args) {
        int num1 = 4;
        int num2 = 8;
        num2 += num1;
        System.out.println(num2);
        num2 -= num1;
        System.out.println(num2);
        num2 *= 3;
        System.out.println(num2);
    }
}

Show the output

Output

12
8
24

Why? num2 += num1 means num2 = num2 + num1, which is 12. Subtracting 4 gives 8, and multiplying by 3 gives 24.

Final exercise: a program that prints a student card

Student card

Declare three variables: String name holding Ahmed, double gpa holding 3.75 and int courses holding 5. Then print each on its own line, as in the expected output.

public class StudentCard {
    public static void main(String[] args) {
        // Declare the three variables, then print them

    }
}

Expected output

Name: Ahmed
GPA: 3.75
Courses: 5

Hint 1

Join text and a variable with +, like: "Name: " + name.

Hint 2

You need three println statements.

Solution

public class StudentCard {
    public static void main(String[] args) {
        String name = "Ahmed";
        double gpa = 3.75;
        int courses = 5;
        System.out.println("Name: " + name);
        System.out.println("GPA: " + gpa);
        System.out.println("Courses: " + courses);
    }
}

Review questions

What is the difference between declaring and assigning?

Why? int age; declares, age = 19; assigns, and they can be combined: int age = 19;.

Which type fits a temperature like 36.6?

Why does the compiler reject the name `2ndNumber`?

What is `x` after these two lines?

public class Review {
    public static void main(String[] args) {
        int x = 4;
        x = x * 3;
        System.out.println(x);
    }
}

Show the output

Output

Why? The right side uses the old value: 4 × 3 = 12.

Every app remembers values that change while it runs

A variable is a named box that holds one value

Three steps to work with any variable

Try the three steps

The name goes on the left

Five data types cover most of our first programs

All five types in one program

Which type fits the number of students in a class, such as 32?

Which type fits the price of a book, such as 12.5?

Which type fits "Has the student paid the fees?"

Java has eight primitive types, each with a fixed size

What happens when the box is full?

First program: store two numbers and add them

Quotation marks separate a variable name from text. What does this print?

The value changes, the name stays: track score line by line

What is the final value of score?

Variable names follow rules set by the compiler

Fix the variable name

The compiler rejects a value of the wrong type

Planned test

Another common error

Swap challenge: a third variable saves the value

Trace the swap

The + sign joins strings into one new string

Predict: text and numbers

Pair activity: print your full name

Five arithmetic operators

Change the values and predict

Turn seconds into minutes and seconds

++ and -- change a value by one, before or after use

Vote: what does each line print?

+= and -= update a variable in one short step

Start with num1 = 4 and num2 = 8, and write num2 on paper after each line.

Final exercise: a program that prints a student card

Student card

Review questions

What is the difference between declaring and assigning?

Which type fits a temperature like 36.6?

Why does the compiler reject the name 2ndNumber?

What is x after these two lines?

What is the final value of `score`?

`++` and `--` change a value by one, before or after use

`+=` and `-=` update a variable in one short step

Start with `num1 = 4` and `num2 = 8`, and write `num2` on paper after each line.

Why does the compiler reject the name `2ndNumber`?

What is `x` after these two lines?