KITFORMA · EDITORIAL GUIDE
Why can I read a List<? extends Number> but not add an Integer to it?
KitForma editorial guide. The unknown element type might be a more specific subtype than Integer. Knowing the upper bound is enough to read, but not to manufacture a valid element.
Step-by-step guidance
The unknown element type might be a more specific subtype than Integer. Knowing the upper bound is enough to read, but not to manufacture a valid element.
If your algorithm owns the output, create a separate
List<Number> output = new ArrayList<>();, copy the source values and append the new integer there. If it only writes integers to a caller-owned collection, declare a lower-bounded destination parameter. Avoid unchecked casts: a Double-only source is a useful counterexample showing why the cast is unsafe.
Compile with unchecked warnings enabled and test a source whose element type differs from your appended value. Copying the container does not copy mutable elements deeply; ownership of the contained objects still needs a separate rule.Sources and verification
Sources checked:
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