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<code>Collection</code>s cannot hold primitive data types such as <code>int</code>, <code>long</code>, or <code>double</code>.{{sfn|Bloch|2018|loc=Chapter §5 Item 28: Prefer lists to arrays|pp=126-129}} Instead, <code>Collection</code>s can hold [[wrapper class]]es such as {{Javadoc|module=java.base|package=java.lang|class=Integer|monotype=y}}, {{Javadoc|module=java.base|package=java.lang|class=Long|monotype=y}}, or {{Javadoc|module=java.base|package=java.lang|class=Double|monotype=y}}.<ref name=":0">{{Cite book|title=Big Java Early Objects|last=Horstmann|first=Cay|year=2014}}</ref>
<code>Collection</code>s are generic and hence invariant, but arrays are [[Covariance and contravariance (computer science)|covariant]]. This can be considered an advantage of generic objects such as {{code|Collection}} when compared to arrays, because under circumstances, using the generic {{code|Collection}} instead of an array prevents run time exceptions by instead throwing a compile-time exception to inform the developer to fix the code. For example, if a developer declares an {{code|Object[]}} object, and assigns the {{code|Object[]}} object to the value returned by a new {{code|Long[]}} instance with a certain capacity, no compile-time exception will be thrown. If the developer attempts to add a {{code|String}} to this {{code|Long[]}} object, the java program will throw an {{code|ArrayStoreException}}. On the other hand, if the developer instead declared a new instance of a {{code|Collection<Object>}} as {{code|ArrayList<Long>}}, the Java compiler will (correctly) throw a compile-time exception to indicate that the code is written with incompatible and incorrect type, thus preventing any potential run-time exceptions.The developer can fix the code by instantianting {{code|Collection<Object>}} as an {{code|ArrayList<Object>}} object. If the code is using Java SE7 or later versions, the developer can
<code>Collection</code>s are generic and hence [[reification (computer science)|reified]], but arrays are not reified.{{sfn|Bloch|2018|loc=Chapter §5 Item 28: Prefer lists to arrays|pp=126-129}}
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