1Z0-830 Java 21 Interfaces, Default Methods, and Enums Guide
April 13, 2026
Study 1Z0-830 Java 21 Interfaces, Default Methods, and Enums: key concepts, common traps, and exam decision cues.
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Interface questions usually become lambda or conflict-resolution questions fast. Enum questions usually become “remember this is a real type” questions just as fast.
Interface-choice map
If the stem is mainly about…
Better first instinct
lambda target compatibility
confirm the type is a functional interface
inherited behavior conflict
trace default-method resolution explicitly
enum behavior
remember enums can have fields, methods, constructors, and interface implementations
What the exam is really testing
If the code shows…
Strong reading
single abstract method plus helpers
functional-interface legality is the core issue
multiple inherited defaults
conflict resolution is required
enum-specific method bodies
the enum is acting as a polymorphic type
Common traps
Trap
Better rule
thinking any interface works as a lambda target
there must be exactly one abstract method
forgetting default-method conflicts need resolution