In Java there are two type of variables: Primitive type and Reference type. Conversion of primitive type to its corresponding wrapper Object is called Autoboxing and Wrapper Object to its corresponding primitive type is known as Unboxing.
Method Overloading with Autoboxing
In method overloading, you may come across a situation where a signature takes reference type or a primitive type as a formal argument. The compiler first searches a method with parameter(s) of the same data type(s). If you are using wrapper class Object as an actual argument and compiler does not find the method with parameter(s) of the same reference type (i.e. class or interface type), then it starts searching a method with parameter(s) having the corresponding primitive data type.
Primitive type int formal argument :10 Reference type Integer formal argument :15 Primitive type long formal argument :100
Method Overloading with Widening
If compiler fails to find any method corresponding to autoboxing, then it starts searching a method parameter(s) of the widened primitive data type.
In the example below, we are invoking overloaded method with primitive(int) formal argument that has the same data type as actual argument’s data type. We are invoking another method with argument of Long wrapper Object. Compiler starts searching for the method having the same reference type (Long wrapper class). Since there is no method having with parameter of Long wrapper class. So, It searches for method which can accept the parameter bigger than long primitive data type as an argument. In this case, it finds a method with float primitive data type and invokes it.
Primitive type int formal argument :10 Primitive type float formal argument :100.0
Method Overloading with Widening and Boxing Together
What happens if widening and boxing happen together? What method invocation will compiler be able to do?
Widening of primitive type has taken priority over boxing and var-args. But widening and boxing of primitive type can not work together.
25: error: incompatible types: byte cannot be converted to Integer c.method(val); ^ Some messages have been simplified; recompile with -Xdiags:verbose to get full output 1 error
But boxing followed by widening is acceptable if this is passed to a reference of type Object. See the following example for this.
Primitive type byte formal argument :5
Method Overloading with Var-args argument
Widening of primitive type gets more priority over var-args.
Widening type long formal argument :5
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