1Z0-830 Java 21 Type Parameters, Bounds, Wildcards and Erasure Guide
April 13, 2026
Study 1Z0-830 Java 21 Type Parameters, Bounds, Wildcards and Erasure: key concepts, common traps, and exam decision cues.
Generics feel abstract until you turn them into three questions: what can I read, what can I write, and what survives after compilation?
Generic-choice map
| If the stem is mainly about… |
Better first instinct |
| source-only reading |
? extends T may be the right direction |
| sink-only writing |
? super T may be the right direction |
| exact container type |
remember invariance for generic types |
| runtime type information |
erasure limits what remains visible |
What the exam is really testing
| If the code shows… |
Strong reading |
List<Number> vs List<Integer> |
invariance is likely the real issue |
| wildcard reads and writes |
PECS is probably the elimination tool |
| runtime generic checks |
erasure limits what Java can know later |
Common traps
| Trap |
Better rule |
believing List<Integer> is a subtype of List<Number> |
generic invariance blocks that |
using ? extends T as a write-friendly target |
safe writes are intentionally restricted |
| assuming all generic facts remain available at runtime |
erasure changes what can be inspected |
Quiz
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Revised on Sunday, May 10, 2026