The specification for the method getReturnType() in javax.lang.model.element.ExecutableElement says:

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TypeMirror getReturnType()
    Returns the return type if this executable represents *a method*. Returns *NoType otherwise*. Returns VoidType if this executable is a method that does not return a value.

       ...
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  For a constructor in the following example VoidType is returned:

public class S_EE0004 extends TestSource {
    public S_EE0004(Integer param) {}
}