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Type Casting and Wrapper Classes

Convert between numeric types, and use Integer, Double and friends.

Java is strict about types, but it lets you convert values from one type to another. Some conversions happen automatically; others need an explicit cast.

Widening (automatic)

Converting a smaller type to a larger one is always safe, so Java does it for you: byte → short → int → long → float → double.

Java
Open in playground
public class Main {
    public static void main(String[] args) {
        int apples = 12;
        long bigger = apples;        // int to long
        double precise = bigger;     // long to double
        System.out.println(bigger);
        System.out.println(precise);

        char letter = 'A';
        int code = letter;           // a char is stored as a number
        System.out.println(code);
    }
}
Output
12
12.0
65

Narrowing (explicit cast)

Going to a smaller type can lose information, so you must write the target type in brackets.

Java
Open in playground
public class Main {
    public static void main(String[] args) {
        double price = 99.99;
        int rupees = (int) price;            // decimals are cut off, not rounded
        System.out.println(rupees);
        System.out.println(Math.round(price)); // rounds instead

        int big = 300;
        byte small = (byte) big;             // byte holds -128..127, so it wraps
        System.out.println(small);

        System.out.println((char) 66);
    }
}
Output
99
100
44
B

Type promotion in expressions

In arithmetic, byte, short and char are promoted to int, and if any operand is long, float or double, the whole expression becomes that type.

Java
Open in playground
public class Main {
    public static void main(String[] args) {
        byte a = 10, b = 20;
        int sum = a + b;               // a + b is an int, not a byte
        System.out.println(sum);
        System.out.println(7 / 2);     // int / int
        System.out.println(7 / 2.0);   // int / double
        System.out.println('a' + 1);   // char promoted to int
        System.out.println((char) ('a' + 1));
    }
}
Output
30
3
3.5
98
b

Wrapper classes and autoboxing

Each primitive has a matching wrapper class: int → Integer, double → Double, char → Character, boolean → Boolean, and so on. Collections such as ArrayList can only hold objects, so wrappers are needed there. Java converts automatically: this is called autoboxing (primitive to object) and unboxing (object to primitive).

Java
Open in playground
import java.util.ArrayList;

public class Main {
    public static void main(String[] args) {
        Integer boxed = 42;                  // autoboxing
        int plain = boxed + 8;               // unboxing
        System.out.println(plain);

        ArrayList<Double> temps = new ArrayList<>();
        temps.add(31.5);                     // double boxed to Double
        System.out.println(temps);

        System.out.println(Integer.parseInt("123") + 1);
        System.out.println(Double.parseDouble("2.5") * 2);
        System.out.println(Integer.toBinaryString(10));
        System.out.println(Integer.MAX_VALUE);
        System.out.println(Character.isDigit('7'));
        System.out.println(String.valueOf(3.14) + "!");
    }
}
Output
50
[31.5]
124
5.0
1010
2147483647
true
3.14!
Java
Open in playground
public class Main {
    public static void main(String[] args) {
        Integer a = 127, b = 127;
        Integer c = 1000, d = 1000;
        System.out.println(a == b);
        System.out.println(c == d);
        System.out.println(c.equals(d));
    }
}
Output
true
false
true