The Java Tutorials have been written for JDK 8. Examples and practices described in this page don't take advantage of improvements introduced in later releases and might use technology no longer available.
See Java Language Changes for a summary of updated language features in Java SE 9 and subsequent releases.
See JDK Release Notes for information about new features, enhancements, and removed or deprecated options for all JDK releases.
Autoboxing is the automatic conversion that the Java compiler makes between the primitive types and their corresponding object wrapper classes. For example, converting an int to an Integer, a double to a Double, and so on. If the conversion goes the other way, this is called unboxing.
Here is the simplest example of autoboxing:
Character ch = 'a';
The rest of the examples in this section use generics. If you are not yet familiar with the syntax of generics, see the Generics (Updated) lesson.
Consider the following code:
List<Integer> li = new ArrayList<>(); for (int i = 1; i < 50; i += 2) li.add(i);
Although you add the int values as primitive types, rather than Integer objects, to li, the code compiles. Because li is a list of Integer objects, not a list of int values, you may wonder why the Java compiler does not issue a compile-time error. The compiler does not generate an error because it creates an Integer object from i and adds the object to li. Thus, the compiler converts the previous code to the following at runtime:
List<Integer> li = new ArrayList<>(); for (int i = 1; i < 50; i += 2) li.add(Integer.valueOf(i));
Converting a primitive value (an int, for example) into an object of the corresponding wrapper class (Integer) is called autoboxing. The Java compiler applies autoboxing when a primitive value is:
Consider the following method:
public static int sumEven(List<Integer> li) { int sum = 0; for (Integer i: li) if (i % 2 == 0) sum += i; return sum; }
Because the remainder (%) and unary plus (+=) operators do not apply to Integer objects, you may wonder why the Java compiler compiles the method without issuing any errors. The compiler does not generate an error because it invokes the intValue method to convert an Integer to an int at runtime:
public static int sumEven(List<Integer> li) { int sum = 0; for (Integer i : li) if (i.intValue() % 2 == 0) sum += i.intValue(); return sum; }
Converting an object of a wrapper type (Integer) to its corresponding primitive (int) value is called unboxing. The Java compiler applies unboxing when an object of a wrapper class is:
The
Unboxing
example shows how this works:
import java.util.ArrayList; import java.util.List; public class Unboxing { public static void main(String[] args) { Integer i = new Integer(-8); // 1. Unboxing through method invocation int absVal = absoluteValue(i); System.out.println("absolute value of " + i + " = " + absVal); List<Double> ld = new ArrayList<>(); ld.add(3.1416); // Π is autoboxed through method invocation. // 2. Unboxing through assignment double pi = ld.get(0); System.out.println("pi = " + pi); } public static int absoluteValue(int i) { return (i < 0) ? -i : i; } }
The program prints the following:
absolute value of -8 = 8 pi = 3.1416
Autoboxing and unboxing lets developers write cleaner code, making it easier to read. The following table lists the primitive types and their corresponding wrapper classes, which are used by the Java compiler for autoboxing and unboxing:
Primitive type | Wrapper class |
---|---|
boolean | Boolean |
byte | Byte |
char | Character |
float | Float |
int | Integer |
long | Long |
short | Short |
double | Double |