Data types

Types in Java

Java has several primitive data types, which are the basic types of data that can be directly operated on. They include:

Name	Description	Example
String	Represents textual data	"The Great Gatsby"
int	Represents integer numbers	2023, 42
double	Represents floating-point numbers	19.99, 3.14
boolean	Represents a logical entity having two values	true, false
char	Represents a single 16-bit Unicode character	'A', 'z'
long	Represents integer numbers with a larger range	9007199254740992L

Assigning types

Java is a statically typed language, meaning the type of a variable is checked before the code actually runs.

public class Main {
    public static void main(String[] args) {
        int pageCount = 210;
        String ISBN = "978-3-16-148410-0";
        boolean premiumMember = true;
        double price = 19.99;
    }
}

TIP

It is always best to declare the type of variables explicitly, because this prevents bugs from creeping into the code later on.

Type errors

Values can only be assigned and reassigned to variables of the correct type:

java

public class Main {
  public static void main(String[] args) {
    int price = 19.99; // error!

    String bookTitle = "Persepolis";
    bookTitle = 10; // error!
  }
}

output

./Main.java:3: error: incompatible types: possible lossy conversion from double to int
    int price = 20.0; // error!
                ^
./Main.java:6: error: incompatible types: int cannot be converted to String
    bookTitle = 10; // error!
                ^
2 errors
error: compilation failed

Adding strings

In Java, you can concatenate strings using the + operator.

java

public class Main {
    public static void main(String[] args) {
        String author = "Toni Morrison";
        String title = "Beloved";

        System.out.println(title + " was written by " + author);
    }
}

output

Beloved was written by Toni Morrison

Changing type

Java allows you to cast variables into different types.

The main reasons to do this are:

1. Turning variables into strings

   public class Main {
     public static void main(String[] args) {
       String bookCount = Integer.toString(17); // int to string
       String price = Double.toString(19.99); // double to string
       String isMember = Boolean.toString(false); // boolean to string
     }
   }

2. Parsing strings into variables of other types

   public class Main {
     public static void main(String[] args) {
       int bookCount = Integer.parseInt("17"); // string to number
       double price = Double.parseDouble("19.99"); // string to double
       boolean isMember = Boolean.parseBoolean("false"); // string to boolean
     }
   }

Type coercion

Generally, Java won't allow you to operate between two variables of different types, but there are some important exceptions.

java

public class Main {
    public static void main(String[] args) {
        String price1 = "10";
        int price2 = 5;
        double price3 = 9.99;

        System.out.println(price1 + price2); // Output: 105
        System.out.println(price2 + price3); // Output: 14.99
    }
}

output

105
14.99

WARNING

Type coercion can be useful, but it is also the source of lots of bugs.