Java Data Types

The two families of types

Primitive types are the simplest building blocks. They store a raw value directly, they are always lowercase, and there are exactly eight of them. Reference types, by contrast, store the address of an object that lives elsewhere in memory; String, arrays, and every class you write are reference types.

• Primitive types: byte, short, int, long, float, double, char, and boolean.
• Reference types: String, arrays, and objects of any class.
• Primitives can never be null, while reference types can.
• Primitive names are lowercase; reference type names start with a capital letter.

The eight primitive types

Each primitive type reserves a fixed amount of memory, which sets the range of values it can hold. Knowing these sizes helps you pick a type that is large enough for your data without wasting space.

Choosing a type in practice

Most everyday code uses just a few of these. Reach for int for whole numbers, double for anything with a decimal point, boolean for yes-or-no flags, and String for text. The narrower types like byte and short exist mainly to save memory when you have huge amounts of data.

public class Main {
  public static void main(String[] args) {
    int items = 12;
    double weight = 4.75;
    char section = 'B';
    boolean inStock = true;
    System.out.println(items + " items, " + weight + " kg, section " + section + ", in stock: " + inStock);
  }
}

Default values

When a primitive is a field of a class and you do not set it, Java gives it a sensible default: zero for numbers, false for boolean, and the null character for char. Local variables inside methods, however, get no default and must be assigned before use.

public class Main {
  public static void main(String[] args) {
    String message = "Data types in Java";
    System.out.println(message.length());
  }
}