Data Types in Java

A kitchen has different containers for different things: a small jar for salt, a big tin for rice, a bottle for oil. You wouldn't pour oil into the salt jar. Java's data types are its containers. Every variable has a type that says what kind of value it holds and how much memory that takes, and the compiler checks that you never pour the wrong thing in.

Two Families of Types

The Eight Primitive Types

Ask Java for the Limits

Every primitive type has a wrapper class (Integer, Long, Double and so on) that knows its limits:

public class Limits {
    static void show(String type, long min, long max) {
        System.out.println(type + min + " to " + max);
    }
 
    public static void main(String[] args) {
        show("byte:  ", Byte.MIN_VALUE, Byte.MAX_VALUE);
        show("short: ", Short.MIN_VALUE, Short.MAX_VALUE);
        show("int:   ", Integer.MIN_VALUE, Integer.MAX_VALUE);
        show("long:  ", Long.MIN_VALUE, Long.MAX_VALUE);
        show("char:  ", Character.MIN_VALUE, Character.MAX_VALUE);
    }
}

byte:  -128 to 127
short: -32768 to 32767
int:   -2147483648 to 2147483647
long:  -9223372036854775808 to 9223372036854775807
char:  0 to 65535

How it works

1. Lines 2 to 4 define a small helper method, show, that prints one line. You'll write your own methods in "Methods in Java".
2. Each wrapper class has MIN_VALUE and MAX_VALUE constants, passed to show on lines 7 to 11.
3. show takes long values, so every smaller type, even char, is converted to a number automatically. That's widening, explained in "Type Casting and Type Conversion".
4. These limits are the same on every computer, which is part of Java's platform independence.

Try it yourself: add System.out.println(Double.MAX_VALUE); to main. How is such a large number printed?

Reference Types

Anything that isn't one of the eight primitives is a reference type. The variable holds a reference, and the object lives on the heap:

public class References {
    public static void main(String[] args) {
        String city = "Pune";
        int[] marks = {78, 92, 86};
        System.out.println(city + " has " + city.length() + " letters");
        System.out.println("First mark: " + marks[0]);
    }
}

Pune has 4 letters
First mark: 78

Reference types can do more than hold a value: city.length() asks the String object a question. Primitives can't do that; they're just values. A reference variable can also hold null, meaning "no object"; a primitive can't.

Common Mistakes

Using int for big numbers. India's population, a phone number or milliseconds since 1970 don't fit in an int. Use long.

Thinking String is a primitive. It's a class, so it's a reference type. That's why it starts with a capital S.

Using char for text. A char holds a single character in single quotes: 'A'. Text uses String and double quotes: "A".

Interview Corner

Practice

1. Easy: Pick the best type for each: number of passengers on a train, distance between two cities in km with decimals, whether a ticket is canceled, the class letter of a coach (A, B or S).
2. Harder: Store India's population (about 1.43 billion) in a variable and multiply it by 2. Try it with int first, then long. What changes?

Key Takeaways