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Intro to Java

Data Types

  • Variables
  • Variable names
  • Data types
    • int
    • double
    • boolean
    • String
  • Declaration and initialization
  • Print with different data types

Material:

Variables

Look at this statement:

int salary = 2200;

This statement does two things:

  1. It declares a variable of type int and name salary.
  2. It assigns the value 2200 to salary.

In Java, each variable, and not only, must have a type.

Statement like a = 10 is not accepted. int a = 10; is the correct form to declare a.

Variables are called so because we can change their values. In our example, we can change the value of salary, like this:

salary = 2500;

In Java the = symbol is used for assignment, unlike in maths where it means equality. In programming both sides of the equal sign do not need to have the same value. The value right of the = sign is assigned (copied) to what it is on the left, our variable.

If salary had a different value before (2200 in our example), that previous value got lost after a new value is assigned to it.

Statements like salary = salary + 100; are correct, in Java. Not in math.

Variable names

You’re free to name a variable as you prefer. Please try to give it a name that carries a clear meaning.

Rules:

  • Variable names should start with a letter, an underscore, or the $ character.
  • Variable names cannot have spaces.
  • longer names should have the format dateOfBirth: each word has a capital letter except the first.
  • No special characters.
  • No Java reserved words: class, public, static, if, while, for, return, …

Data types

Each variable must have a data type.

Data types determine, what type of values we can put in a variable and what actions can be done with them. A variable declaration is always starting with stating the type of the variable.

int numerOfLegs = 2;
double pi = 3.1415; 
boolean isAlive = true;
String firstName = "Peter";
char c = 'K';

Non-fractional Numbers

You can use the following types to represent integer values:

  • int for non-fractional numbers from -2000000000 to +2000000000. (approx)
  • long for non-fractional numbers much higher than what int can represent.
  • short for non-fractional numbers from -65000 to +65000. (approx)
  • byte for non-fractional numbers from -128 to 127.

HINT: always use int for integer numbers, unless there’s a valid reason not to.

Decimal Numbers

You can use the following types to represent decimal values:

  • float for fractional numbers (smaller precision).
  • double for fractional numbers (higher, double precision).

HINT: always use double for fractional numbers, and always use dot . to separate the decimal part from the fractional part:

Example: 3.1415, not 3,1415.

HINT: always use int for numbers, unless you know the concept you’re trying to represent with a variable is fractional, then use double.

Examples:

  • int people = 21;, not 21.76 people? no …
  • int month = 3;, 3 means March, what would 3.80 mean?
  • int uBahnLine = 6;, like U6, that’s no U6.25.
  • double cost = 4.99;, in euro.
  • double temperature = 21.6;, in °C.
  • double height = 1.79;, in meters.

Boolean

This is a very important type. It can have only two values, true or false. It’s used to store the result of logical calculation. It’s very important when programming a computer.

The name of the type is boolean.

Examples:

  • boolean isRaining = true;
  • boolean needUmbrella = true;
  • boolean isWeekend = false;
  • boolean isStudent = true;
  • boolean speaksEnglish = true;
  • boolean speaksGerman = true; or false, but not ein bisschen.

Text

For text, use String.

Examples:

  • String firstName = "Martha";
  • String midName = ""; yes, this is valid, just some empty text.
  • String surname = "Smith";
  • String fullName = "Martha Smith";

These are all variables, so we can of course change their value as we did with salary above:

fullName = "Claire Smith";

The type char is used to represent single characters:

  • char initial = 'M';

NOTE: String is written uppercase and text is enclosed in double quotes ". char is written lowercase, and characters are enclosed in single quotes '.

Printing experiments

Instead of just printing strings directly, we can also print the values of a variable.

What will the following code print?

class Main {
    public static void main(String[] args) {
        String name = "Jeff" + " Jonson";
        System.out.println(name);
        name = "Jennifer";
        System.out.println(name);
        name = name + " Lopez";
        System.out.println(name);
    }
}

You can print more than strings. You can print any value of any data type.

What will the following code print?

class Main {
    public static void main(String[] args) {
        int age = 31;
        System.out.println(age);
        int salary = 2000;
        System.out.println(salary);
        salary = salary + 200;
        System.out.println(salary);
        salary = salary * 2;
        System.out.println(salary);
    }
}

You could even combine strings and other data types.

What will be printed on the screen by the following code?

class Main {
    public static void main(String[] args) {
        int age = 31;
        System.out.println("Age: " + age);
        int salary = 2000;
        System.out.println("Salary: " + salary);
        salary = salary + 200;
        System.out.println("New salary: " + salary);
    }
}

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