<div dir="ltr"><p style="margin:0px;font-stretch:normal;font-size:12px;line-height:normal;font-family:Helvetica;color:rgb(0,0,0)"><span style="-webkit-font-kerning: none;">Agreed that the use of a .ref or .val tag is an awkward deviation from familiar syntax.<span class="gmail-Apple-converted-space"> </span></span></p>
<p style="margin:0px;font-stretch:normal;font-size:12px;line-height:normal;font-family:Helvetica;color:rgb(0,0,0)"><span style="-webkit-font-kerning: none;">Use of a @NotNull annotation is one alternative. A contextual keyword (e.g. <i>notnull</i> or <i>nonulls</i>) is another.</span></p>
<p style="margin:0px;font-stretch:normal;font-size:12px;line-height:normal;font-family:Helvetica;color:rgb(0,0,0);min-height:14px"><span style="-webkit-font-kerning: none;"></span><br></p>
<p style="margin:0px;font-stretch:normal;font-size:12px;line-height:normal;font-family:Helvetica;color:rgb(0,0,0)"><span style="-webkit-font-kerning: none;">The key thing is that, whether keyword or annotation, it should only need to be specified when a field is defined. Every implication of a <i>notnull</i> specification should be capable of being generated by the compiler.</span></p>
<p style="margin:0px;font-stretch:normal;font-size:12px;line-height:normal;font-family:Helvetica;color:rgb(0,0,0);min-height:14px"><span style="-webkit-font-kerning: none;"></span><br></p>
<p style="margin:0px;font-stretch:normal;font-size:12px;line-height:normal;font-family:Helvetica;color:rgb(0,0,0)"><span style="-webkit-font-kerning: none;">Examples:</span></p>
<p style="margin:0px;font-stretch:normal;font-size:12px;line-height:normal;font-family:Helvetica;color:rgb(0,0,0);min-height:14px"><span style="-webkit-font-kerning: none;"></span><br></p>
<p style="margin:0px;font-stretch:normal;font-size:12px;line-height:normal;font-family:Helvetica;color:rgb(0,0,0)"><span style="-webkit-font-kerning: none;"><span class="gmail-Apple-converted-space"> </span>primitive class PhoneNumber {</span></p>
<p style="margin:0px;font-stretch:normal;font-size:12px;line-height:normal;font-family:Helvetica;color:rgb(0,0,0)"><span style="-webkit-font-kerning: none;"><span class="gmail-Apple-converted-space"> </span>// all fields are non-negative never-null integer values</span></p>
<p style="margin:0px;font-stretch:normal;font-size:12px;line-height:normal;font-family:Helvetica;color:rgb(0,0,0)"><span style="-webkit-font-kerning: none;"><span class="gmail-Apple-converted-space"> </span>}</span></p>
<p style="margin:0px;font-stretch:normal;font-size:12px;line-height:normal;font-family:Helvetica;color:rgb(0,0,0)"><span style="-webkit-font-kerning: none;"><span class="gmail-Apple-converted-space"> </span>value class ContactPhoneInfo {</span></p>
<p style="margin:0px;font-stretch:normal;font-size:12px;line-height:normal;font-family:Helvetica;color:rgb(0,0,0)"><span style="-webkit-font-kerning: none;"><span class="gmail-Apple-converted-space"> </span>private notnull PhoneNumber mobile; <span class="gmail-Apple-converted-space"> </span>// equivalent to PhoneNumber.val</span></p>
<p style="margin:0px;font-stretch:normal;font-size:12px;line-height:normal;font-family:Helvetica;color:rgb(0,0,0)"><span style="-webkit-font-kerning: none;"><span class="gmail-Apple-converted-space"> </span>private PhoneNumber landLine; <span class="gmail-Apple-converted-space"> </span>// optional, equivalent to PhoneNumber.ref</span></p>
<p style="margin:0px;font-stretch:normal;font-size:12px;line-height:normal;font-family:Helvetica;color:rgb(0,0,0)"><span style="-webkit-font-kerning: none;"><span class="gmail-Apple-converted-space"> </span>}</span></p>
<p style="margin:0px;font-stretch:normal;font-size:12px;line-height:normal;font-family:Helvetica;color:rgb(0,0,0)"><span style="-webkit-font-kerning: none;"><span class="gmail-Apple-converted-space"> </span>value class EmailContact {<span class="gmail-Apple-converted-space"> </span></span></p>
<p style="margin:0px;font-stretch:normal;font-size:12px;line-height:normal;font-family:Helvetica;color:rgb(0,0,0)"><span style="-webkit-font-kerning: none;"><span class="gmail-Apple-converted-space"> </span>private notnull String emailAddress; <span class="gmail-Apple-converted-space"> </span>// never missing</span></p>
<p style="margin:0px;font-stretch:normal;font-size:12px;line-height:normal;font-family:Helvetica;color:rgb(0,0,0)"><span style="-webkit-font-kerning: none;"><span class="gmail-Apple-converted-space"> </span>private ContactPhoneInfo phones; <span class="gmail-Apple-converted-space"> </span>// optional, may be missing</span></p>
<p style="margin:0px;font-stretch:normal;font-size:12px;line-height:normal;font-family:Helvetica;color:rgb(0,0,0)"><span style="-webkit-font-kerning: none;"><span class="gmail-Apple-converted-space"> </span>}</span></p>
<p style="margin:0px;font-stretch:normal;font-size:12px;line-height:normal;font-family:Helvetica;color:rgb(0,0,0);min-height:14px"><span style="-webkit-font-kerning: none;"></span><br></p>
<p style="margin:0px;font-stretch:normal;font-size:12px;line-height:normal;font-family:Helvetica;color:rgb(0,0,0)"><span style="-webkit-font-kerning: none;">The <i>notnull</i> modifier can be applied to any value class field type. If the field is a primitive or value object, it signals that the field value may be stored as a flattened value without needing an additional bit or reference to allow for a null value. But it also has other uses, including for identity-type fields like the String field in the above example.</span></p>
<p style="margin:0px;font-stretch:normal;font-size:12px;line-height:normal;font-family:Helvetica;color:rgb(0,0,0);min-height:14px"><span style="-webkit-font-kerning: none;"></span><br></p>
<p style="margin:0px;font-stretch:normal;font-size:12px;line-height:normal;font-family:Helvetica;color:rgb(0,0,0)"><span style="-webkit-font-kerning: none;">For example, if a field is a <i>notnull</i>, the compiler could autogenerate code throwing a NullPointerException (or equivalent) if a client attempts to instantiate the value object with a null value for that field. (Or better still, reject client code that attempts to supply an invalid null value). If a value object field is not a <i>notnull</i>, the compiler could autogenerate an <i>Optional</i> return value for the corresponding accessor.</span></p>
<p style="margin:0px;font-stretch:normal;font-size:12px;line-height:normal;font-family:Helvetica;color:rgb(0,0,0);min-height:14px"><span style="-webkit-font-kerning: none;"></span><br></p>
<p style="margin:0px;font-stretch:normal;font-size:12px;line-height:normal;font-family:Helvetica;color:rgb(0,0,0)"><span style="-webkit-font-kerning: none;">The advantage of the keyword over the annotation is that it can only be used in one place: a field definition, the only place it’s needed.</span></p>
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