Java Autoboxing - Auto-unboxing

Java 1.5 onward auto-boxing and auto-unboxing were added. This allows you to covert primitives and boxed types primitives back and forth with simple assignment vs calling special functions or constructors.

Lets look at some examples.

int primitiveInt = 1000;                   //primitive int
Integer wrapperInt = new Integer(1000);    // wrapper Integer, boxed type primitive.
Integer autoBoxedWrapperInteger = 1000;    // Autoboxing, primitive is autoboxed to Integer, an
                                            //Object is bing created
int autoUnboxedPrimitiveInt = wrapperInt;  //Auto-unboxing, primitive is being assigned a value by auto-unoxing

//Comparing a primitive to boxed type, boxed type is autoboxed to primitive so the results is
// two primitives are being compared using === as opposed to 2 Objects are being ref compared
// using ==
System.out.println("primitiveInt == wrapperInt  :" +
            (primitiveInt == wrapperInt ? true : false));
//Two object references are being compared using ==, therefore the result is false
System.out.println("wrapperInt == autoBoxedWrapperInteger  :" +
            (wrapperInt == autoBoxedWrapperInteger ? true : false));
System.out.println("primitiveInt == autoBoxedWrapperInteger  :" +
           (primitiveInt == autoBoxedWrapperInteger ? true : false));
System.out.println("primitiveInt == autoUnboxedPrimitiveInt  :" +
           (primitiveInt == autoUnboxedPrimitiveInt ? true : false));

Output

primitiveInt == wrapperInt  :true
wrapperInt == autoBoxedWrapperInteger  :false
primitiveInt == autoBoxedWrapperInteger  :true
primitiveInt == autoUnboxedPrimitiveInt  :true

Here are some tips based on reading from Effective Java 2nd edition.

• Prefer primitives over boxed primitives.


• When you mix primitives and boxed primitive in single opeartion - boxed-primitive is auto-unboxed.


• When mixing primitives and wrapper types in expression watch out for "==" comparision.


• Performance hit due to autoboxing and unboxing, a new object is created when autoboxing is performed.


• Unboxing can throw NullPointerException