Primitives and references
Name the primitivesbyte, short, int, long, float, double, boolean, char
Reassign a parameterOnly the local copy moves.
TrapA local int sits in the stack frame. An int field sits inside its object on the heap.
Done 0/0
To review 0
Old way, still asked about
The usual way today
Newer, Java 9 or later
The model
- A primitive variable holds its value. A reference variable points to a heap object, so
b = acopies the reference, not the object. - Java always passes by value. A method gets a copy of the reference, so it can change a shared object but not the caller's variable.
==compares references.equals()compares content, and equal objects must return the samehashCode().- The JVM runs the bytecode from
javac, compiles hot methods with the JIT, and frees unreachable objects with garbage collection. - Threads share one heap, so shared writes need synchronization.
Types & equality
what a variable holds and what == comparesBoxing, == and equals()
Box an
intInteger n = 100;Compare references
a == bCompare content
a.equals(b)TipString literals share one pooled object. new String always makes another.
Trap== on Integer only seems to work from -128 to 127, the cached range. Unboxing null throws Null.
String, StringBuilder, StringBuffer
Build text in one thread
new StringBuilder()Append across threads
new StringBuffer()Say why
String is immutableThe pool can share it, its hash never changes, and threads need no lock.Trap+= in a loop copies the text into a new String every time.
Classes & inheritance
how classes share code and hide itAccess, static and super
Allow only the class
privateAllow the packageNo modifier, package-private.
Also allow subclasses
protectedAllow all code
publicKeep one shared copy
static int count;Call the parent
super(name);super.save();TipA top-level class can only be public or package-private.
TrapA static method has no this, so it can't use instance fields directly.
The four pillars of OOP
Hide the internalsEncapsulation
Extend a classInheritance
Vary a call by typePolymorphism
Expose only essentialsAbstraction
Asked in interviews
AskedWhat are the four pillars of OOP?Overload, override, hide
Reuse a name with new parametersOverloading. The compiler picks.
Replace an inherited method
@Override. The object's class picks at runtime.Redeclare a static methodHiding. The declared type picks.
TrapA different return type alone isn't an overload. Static, private and final methods can't be overridden.
final, finally, finalize()
Block reassigning, overriding or extending
final int MAX = 100;Clean up before collection
finalize()Deprecated. Use try-with-resources or Cleaner.Tipfinally runs after try and catch, whether or not something threw.
TrapA final reference isn't an immutable object. A final List can't point to another list, but add still works.
Abstract class, interface, composition
Give unrelated classes a capability
interface FlyableImplement many types
implements A, BReuse by delegating
private Engine e;Say who owns the partAssociation owns nothing. Aggregation holds it. Composition owns it.
TipPrefer composition. A change in a parent can break every subclass.
TrapAn interface holds no instance state, only public static final constants. default methods add behaviour without it.
Collections
pick the structure and know its costPick a collection
Keep order
ArrayList, duplicates allowed.Reject duplicates
HashSet, O(1) on average.Keep elements sorted
TreeSet, O(log n).Read by index
ArrayList is O(1), LinkedList is O(n).Work at the ends
LinkedList is O(1), but ArrayDeque is often faster.TipMap doesn't extend Collection. The framework has two separate roots.
TrapInserting in the middle of a LinkedList is still O(n), unless an iterator already points there.
Inside HashMap
Pick the bucket
hash & (capacity - 1)Know when it resizesPast capacity times
0.75, it doubles and rehashes.TipA bucket only becomes a tree in a table of 64 or more. A smaller table resizes instead.
TrapOverride equals() without hashCode(), and a lookup with an equal key can return null.
Hashtable and ConcurrentHashMap
Read old synchronized code
Hashtable locks every method.Share a map across threads
ConcurrentHashMap . Reads take no lock.Say how it writesCompare-and-swap on an empty bucket. Otherwise it locks the first node.
TipHashMap allows one null key. Hashtable and Concurrent reject null.
TrapCheck-then-put isn't atomic, even on a synchronized map. Use computeIfAbsent or merge.
Sort, iterate and update
Set a natural order
compareTo(T other)Sort by a key
Comparator.comparing(E::name) Remove while iterating
iter.remove()Iterate a snapshot
CopyOnWriteArrayList Count words
map.merge(w, 1, Integer::sum)Make an unmodifiable list (9+)
List.of(1, 2)TipKeep natural order consistent with equals(). A TreeSet treats a compare result of 0 as a duplicate.
TrapAdding to an ArrayList in a loop throws Concurrent. The check is best-effort.
Exceptions
what you must catch and what gets lostChecked or unchecked
Recover from a failureChecked. Catch or declare it.
Signal a bugUnchecked. Handling is optional.
Trapcatch (Exception e) never catches an Error, and a normal application shouldn't try.
throw, throws and catch
Raise an exception
throw new E("x");Declare it to callers
throws IOExceptionMulti-catch two types
catch (A | B e)TrapA catch for a subclass after its superclass is a compile-time error. Order them from specific to general.
Try-with-resources and finally
Close automatically
try (var r = open())Read a
close() failuree.getSuppressed() TipA resource implements AutoCloseable. Several close in reverse order.
TrapA throw or return in finally replaces the exception from try, and nothing records the lost one.
Threads & locks
share memory without losing updatesCreate a thread
Start a new thread
new Thread(r).start(); Tell a process from a threadA process has its own memory. Threads share one heap.
TipImplement Runnable, so your class can still extend another.
Traprun() executes on the current thread, so nothing new starts. A second start() throws Illegal.
Locks and volatile
Lock a critical section
synchronized (o) { }Give up on a busy lock
lock.tryLock()Show a flag to all threads
volatile boolean done;TipRelease a ReentrantLock in finally. AtomicInteger avoids locks for one counter.
Trapcount++ is a read, an add and a write. volatile doesn't make it atomic, so threads still lose updates.
wait(), sleep() and deadlock
Release the lock and wait
obj.wait();Pause, keep all locks
Thread.sleep(1000); Prevent a deadlockTake locks in one order everywhere.
TrapA spurious wakeup needs no notify, so call wait() in a while loop that rechecks the condition.
Thread pools and virtual threads
Reuse a fixed set of threads
Executors.newFixedThreadPool(4) Start a virtual thread per task (21+)
Executors.newVirtualThreadPerTaskExecutor() Tipsubmit() returns a Future. Without shutdown(), the pool's threads keep the JVM running.
TrapVirtual threads are cheaper, not faster, so they help waiting work, not CPU work. Never pool them.
Latch, barrier, fork/join
Wait for N events, once
new CountDownLatch(3) Make threads meet at each phase
new CyclicBarrier(4) Split CPU work
RecursiveTask<T> TipFork/join uses work stealing. An idle worker takes tasks from a busy worker's queue.
TrapIf one thread at a barrier is interrupted or times out, the others get Broken.
JVM, memory & GC
what runs code and frees memoryJDK, JRE, JVM and class loading
Tell the three apartJVM runs bytecode, JRE adds libraries, JDK adds tools.
Name the load phasesLoading, linking, initialization.
Say who loads a classEach loader asks its parent first.
TipSince Java 11, Oracle ships no separate JRE. jlink builds a trimmed runtime.
JVM memory areas
Hold frames and localsThe stack, one per thread.
Hold objectsThe heap, shared by threads.
Hold class dataMethod area, in Metaspace.
TipDeep recursion throws Stack. A full heap throws Out.
TrapHotSpot keeps static variables on the heap with the Class object, not in Metaspace.
Garbage collection
Decide what is garbageTrace from GC roots. The rest is garbage.
Use the server default
-XX:+UseG1GCMaximize throughput
-XX:+UseParallelGC Pause under 1 ms (11+)
-XX:+UseZGCTipMost objects die young, so GC collects the young generation often. Java 14 removed CMS.
OutOfMemoryError, leaks, references
Dump the heap on error
-XX:+HeapDumpOnOutOfMemoryError Find what holds memoryA heap dump in Eclipse MAT.
Name common leaksStatic collections, unclosed resources, inner classes, thread locals.
Cache until memory runs low
new SoftReference<>(v) Drop once only weak references remain
new WeakReference<>(v) TipA phantom reference's get() always returns null, so it only schedules cleanup.
TrapRaise -Xmx only if the data really needs the space. If it's a leak, fix the reference.
Profiling and the JIT
Record a profile
-XX:StartFlightRecording Compile hot methods
C1 first, then C2 for the hottest code.TipMeasure before you optimize. Benchmarks need a warm-up, because the JIT compiles hot code after it runs.
Modern Java
lambdas, streams and what came after Java 8Lambdas and streams
people.stream()source .filter(p -> p.age() >= 18)intermediate .map(Person::name)intermediate .collect(Collectors.toList());terminal
Pass a function
x -> x * 2Shorten a lambda
String::toUpperCase Return a value that may be absent
Optional.ofNullable(v) TipA functional interface has one abstract method, so a lambda can implement it.
TrapIntermediate steps do nothing until the terminal one runs, and a stream can be used only once.
Records and generics
Declare a record (16+)
record P(int x) {}Say what erasure doesThe compiler checks type arguments, then removes them.
TipErasure rules out List<int>, new T() and instanceof List<String>.
TrapRecords are only shallowly immutable. Store a list field with List.copyOf.
After Java 8
Infer a type (10+)
var n = 10;Switch as a value (14+)
case SAT -> "Off";Test and cast (16+)
o instanceof Point pLimit subclasses (17+)
sealed class A permits BName the newest LTSJava 25, with scoped values.
Asked in interviews
AskedWhat are the key features added in Java since Java 8?Modules, annotations, reflection
Share a package
exports com.app.api;Keep an annotation for reflection
@Retention(RUNTIME)Call a method found at runtime
method.invoke(obj) TipA package a module doesn't export stays hidden, even its public classes.
TrapSince Java 17, set throws Inaccessible where a module doesn't open the package.
I/O, JDBC & transactions
files, objects and databasesBytes, characters and NIO
Read binary data
InputStreamRead text
ReaderServe many idle connections
SelectorTipThe default charset is UTF-8 since Java 18.
TrapReading an image through a Reader corrupts it, because it treats the bytes as text.
Serialize and copy objects
Leave a field out
transientPin the class version
serialVersionUID = 1LCopy an objectShallow shares nested objects. Deep copies them.
TipA version mismatch throws Invalid.
TrapNever deserialize untrusted data. The Java documentation calls it inherently dangerous.
JDBC connections
Borrow a connection
ds.getConnection() Pick a driverType 4, the pure Java thin driver.
TipUse try-with-resources, because close() returns the connection to the pool.
TrapDriver opens a new physical connection every time.
Safe SQL and transactions
Bind a value safely
ps.setString(1, name);Start a transaction
setAutoCommit(false) Name the guaranteesAtomicity, consistency, isolation, durability.
TipIn Spring, @Transactional handles the commit and rollback.
TrapA Statement built by joining strings lets input like ' OR '1'='1 change the query.
Spring Boot & microservices
what the framework doesSpring Boot and auto-configuration
Turn on auto-configuration
@SpringBootApplication Let your own bean win
@ConditionalOnMissingBean TipStarters pull in tested libraries. An embedded server runs the app with java -jar.
Stereotype annotations
Mark data access
@RepositoryReturn a view name
@ControllerReturn data directly
@RestControllerTipAll are specializations of @Component. @Repository also translates data exceptions.
Dependency injection and scopes
Inject a required beanA constructor, so the field can be
final.Keep one per container
singleton, the default.Create one per lookup
prototype, with no destruction callbacks.TrapA singleton that needs a request-scoped bean needs a scoped proxy. Without one, startup fails.
Spring Data JPA and Security
Get a query from a method name
findByLastName(name) Require a role
.hasRole("ADMIN") TipSpring Security runs as a chain of servlet filters in front of your controllers.
TrapLoading a list can run one more query per entity to fetch a lazy relation, the N+1 select problem.
Spring AOP and microservices
Wrap many methods
@Around("@annotation(Loggable)")Survive a failed callTimeouts, retries, circuit breakers.
Keep data consistentEventual consistency, often a saga.
Follow one requestDistributed tracing.
TrapOnly calls through the proxy get advice. A call on this skips the aspect, and private methods never get it.
Design patterns & testing
the usual patterns and how to testSingleton and Factory
Write a safe singleton
enum E { INSTANCE }Hide which class is built
Factory.create("dog") TrapDouble-checked locking needs a volatile field, or another thread can see a half-built object.
Observer and Strategy
Notify many of a change
observer.update(msg) Swap an algorithm
cart.setStrategy(s) TrapObservers that never detach leak memory, and one slow observer delays the rest.
Unit tests, integration tests, TDD
Test one class
@TestStart the whole context
@SpringBootTestFollow the TDD cycleRed, green, refactor.
TipA common stack is JUnit, Mockito and AssertJ. JUnit 6 is the current generation.
Mock with Mockito
Set a mock's answer
when(s.pay()).thenReturn(true); Check the call
verify(s).pay(); TrapA test that mocks everything only checks that the code calls its mocks.
Asked in interviews
AskedWhat is mocking and why is it useful?Nothing matches. Try a shorter word, or clear the search.
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