Boxing requires an exact match between the wrapper class and the primitive type. : Boxing Unboxing « Utility Classes « SCJP

8.10.15.Boxing requires an exact match between the wrapper class and the primitive type.

public class MainClass{

    public static void main(String[] argv){

        Double wd = 123d;

    }

}

8.10.Boxing Unboxing
	8.10.1.	Rules for overloading methods using widening, boxing, and var-args
	8.10.2.	Boxing, ==, and equals()
	8.10.3.	Where Boxing Can Be Used
	8.10.4.	Wrapper reference variables can be null
	8.10.5.	Overloading Method Matching
	8.10.6.	Overloading with Boxing and Var-args
	8.10.7.	Predict the output of the following
	8.10.8.	boxing beat var-args
	8.10.9.	Overloading When Combining Widening and Boxing
	8.10.10.	A boxing operation followed by a widening operation in order to match an invocation to a method.
	8.10.11.	Overloading in Combination with Var-args
	8.10.12.	Autoboxing and auto-unboxing are compiler modifications that permit previously illegal combinations of wrappers and primitives.
	8.10.13.	The wrapper on the left-hand side must correspond exactly to the primitive type on the right-hand side.
	8.10.14.	The following autoboxing assignment is illegal
	8.10.15.	Boxing requires an exact match between the wrapper class and the primitive type.
	8.10.16.	Boxing also occurs when a method's return type is a wrapper.
	8.10.17.	Unboxing is the automatic extraction of a wrapped value.
	8.10.18.	Unboxing the parameter

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