1Z0-771 flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

Integer cache range

Java caches Integer objects from -128 to 127. Autoboxing values inside this range returns the same reference, so == is true; outside it, == compares distinct objects.

var restrictions

var requires an initializer on the same declaration, cannot be null, and cannot be used for fields, method parameters, or return types. It is local-variable type inference, not dynamic typing.

Definite assignment

A local variable must be assigned on every path before use or the code fails to compile. Fields instead receive default values such as 0, false, or null.

Switch expression yield

In a switch expression, an arrow arm with a block must return a value using yield. Arrow arms never fall through, and the switch must be exhaustive.

Multi-catch rules

catch (IOException | SQLException e) handles several unrelated types. The parameter is implicitly final, and the listed types cannot be related by subclassing.

try-with-resources close order

Resources declared in a try-with-resources header are closed in reverse order of declaration, even when the body throws, using their AutoCloseable close method.

finally override behavior

A return or throw inside finally overrides any earlier return or exception from the try block, which is why returning from finally is discouraged.

Record compact constructor

A compact canonical constructor omits the parameter list, cannot assign fields directly, and runs validation before the implicit field assignments occur.

Default method conflict

When a class implements two interfaces declaring the same default method, it must override that method or the code fails to compile.

Sealed class permits clause

A sealed class lists permitted subclasses with permits; each subclass must be final, sealed, or non-sealed. Permits may be omitted when subclasses share the source file.

Static method hiding

Static methods are hidden, not overridden. The version invoked depends on the declared type of the reference, not the runtime class of the object.

Type erasure

Generic type parameters are erased at compile time. Consequently primitives cannot be type arguments, and instanceof List<String> or new T() is illegal.

PECS principle

Producer extends, Consumer super. List<? extends Number> can be read from, while List<? super Integer> can be written to, preserving type safety.

HashMap null rules

HashMap permits one null key and multiple null values. Hashtable and ConcurrentHashMap reject null keys and values with a NullPointerException.

TreeMap ordering

TreeMap sorts keys by natural ordering or a supplied Comparator and uses compareTo rather than equals to decide key identity and duplicates.

Stream single use

A stream pipeline can be consumed only once. Reusing it after a terminal operation throws IllegalStateException.

Intermediate vs terminal operations

Intermediate operations such as map, filter, and sorted are lazy and return a stream. Terminal operations such as collect, forEach, and count trigger execution.

orElse versus orElseGet

Optional.orElse always evaluates its argument, even when a value is present. orElseGet takes a Supplier and evaluates it only when the Optional is empty.

partitioningBy result

Collectors.partitioningBy always returns a Map with exactly two entries keyed Boolean.TRUE and Boolean.FALSE, unlike groupingBy which omits empty groups.

Thread start versus run

Calling start creates a new thread that then executes run. Calling run directly executes the method on the current thread without creating a new one.

volatile guarantees

volatile ensures visibility and ordering of reads and writes across threads but not atomicity, so incrementing a volatile int is still unsafe without synchronization.

Thread lifecycle states

Thread.getState returns NEW, RUNNABLE, BLOCKED, WAITING, TIMED_WAITING, or TERMINATED. BLOCKED means waiting for a monitor lock; WAITING means waiting indefinitely.

ExecutorService shutdown

shutdown stops accepting new tasks but lets queued tasks finish. shutdownNow interrupts running tasks and returns the list of tasks never started.

Serialization transient keyword

Fields marked transient are skipped during serialization and restored to default values on deserialization. The class must implement Serializable for any of this to apply.

Path versus File

Path is the NIO.2 abstraction for a file system location, created with Path.of. Files provides static operations such as exists, readAllLines, and move.

Files.walkFileTree

Files.walkFileTree traverses a directory tree with a FileVisitor, giving control over preVisitDirectory, visitFile, and postVisitDirectory, unlike the lazy Files.walk stream.

Module requires transitive

requires transitive re-exports a dependency so modules reading your module also read it. Plain requires keeps the dependency internal to your module.

Module opens directive

opens grants runtime deep reflection access to a package. Without it, setAccessible fails for non-exported packages in named modules.

Automatic modules

A plain JAR placed on the module path becomes an automatic module that exports all its packages and reads all other modules, easing migration.

Annotation retention policies

RetentionPolicy.SOURCE discards annotations after compilation, CLASS stores them in bytecode but hides them from reflection, and RUNTIME makes them reflection-visible.

getMethods versus getDeclaredMethods

getMethods returns public methods including inherited ones. getDeclaredMethods returns all methods declared in the class, including private ones, but excludes inherited methods.

Period versus Duration

Period is date-based, holding years, months, and days. Duration is time-based, holding seconds and nanoseconds, and is used with Instant and time values.