Convert an Object to Map in Java: Jackson, Gson and Reflection
Convert an object to Map in Java with Jackson 3 convertValue(), JavaPropsMapper flat keys, Gson and reflection, plus nulls, records and Map back to object.
“Core Java” is Sun’s term, used to refer to Java SE, the standard edition and a set of related technologies, like the Java VM, CORBA, etc. This is mostly to differentiate from others like Java ME or Java EE. “Core Java” is Oracle’s definition and refers to subset of Java SE technologies.
The Core Java technologies and application programming interfaces (APIs) are the foundation of the Java Platform, Standard Edition (Java SE). They are used in all classes of Java programming, from desktop applications to Java EE applications.
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Tutorials
Filter a Map by a List of Keys in Java: Lookup vs Stream
Learn to filter a map by a list of keys in Java with get() lookups, streams and retainAll(), keep the key order and report the keys that are missing.
IllegalMonitorStateException in Java: Causes and Fixes
IllegalMonitorStateException in Java explained, with every cause from wait() and notify() outside synchronized to ReentrantLock unlock() and the fixes.
Round a Double to 2 Decimal Places in Java (or N Places)
Learn how to round a double to 2 decimal places in Java with BigDecimal, Math.round(), String.format() and DecimalFormat, plus rounding modes and pitfalls.
Invert a Map in Java: Swap Keys and Values, Even With Duplicates
Invert a map in Java by swapping keys and values with a loop or Collectors.toMap(), handle duplicate values with groupingBy(), and use Guava BiMap.
Java printf() and String.format(): Format Specifier Guide
Java printf examples for %s, %d, %.2f, width, flags, dates and locales, plus String.format(), formatted() and fixes for common format exceptions.
Java Thread join(): Timeouts, Interrupts and join vs yield
Java thread join() waits for another thread to finish. Learn join(long), join(Duration), interrupts, deadlocks, visibility and yield() vs join().
XmlAssert: AssertJ Assertions for XMLUnit With Examples
Learn XmlAssert, the AssertJ API of XMLUnit, to check XML in Java tests by XPath, compare documents, ignore order and whitespace, and validate XSD.
XMLUnit Tutorial: Compare XML in Java Tests With Examples
Learn XMLUnit 2 to compare XML in Java tests, with identical vs similar checks, element order, whitespace, node filters and DifferenceEvaluator examples.
JSONassert Guide: Compare JSON in Java Tests With Examples
Learn JSONassert to compare JSON in Java tests, with LENIENT vs STRICT modes, arrays, failure messages, ignored fields and a REST response example.
Java HashMap: Complete Guide With Examples
Java HashMap guide with examples for put, get, merge, compute, iteration, keys, internals and thread safety, plus links to every HashMap how-to.
Java Concurrency Guide: Threads, Executors and Virtual Threads
Java concurrency explained for Java 25 with thread safety, a race condition fix, a timeline from Java 1.0 to 25 and all our tutorials grouped by task.
Java Collections & Arrays Tutorial
Learn the Java Collections Framework with a Java 21 hierarchy diagram, how to choose List, Set, Map and Queue classes, and links to every tutorial.
Java ArrayList Guide: Methods, Capacity, subList and Examples
Learn the Java ArrayList with tested examples for get(), size() vs capacity, subList(), ensureCapacity(), sorting, streams and Java 21 getFirst().
Java List Interface: Methods, Implementations and Examples
The Java List interface explained with examples of List.of(), positional methods, Java 21 sequenced methods and how to choose an implementation.
Java Map: HashMap vs LinkedHashMap vs TreeMap and More
A Java Map stores unique keys with one value each. Quick put/get/merge/computeIfAbsent examples, a comparison of HashMap, LinkedHashMap, TreeMap, ConcurrentHashMap, EnumMap, Hashtable and Map.of(), and all our Map tutorials grouped by topic.
Java Case-Insensitive Map: TreeMap, Commons and Spring
Create a Java case insensitive map with TreeMap and CASE_INSENSITIVE_ORDER, Commons CaseInsensitiveMap or Spring LinkedCaseInsensitiveMap, side by side.
Create and Start a Thread in Java: Runnable, Builders, Executors
Learn to create and start a thread in Java with Runnable, Thread, Thread.ofPlatform(), virtual threads, ExecutorService, ThreadFactory and priority.
Java EnumMap with Examples: Usage, Streams and vs HashMap
Java EnumMap stores enum keys in an array by ordinal. Learn its constructors, null rules, key order, stream collectors and how it differs from HashMap.
Jayway JsonPath Tutorial: Read and Filter JSON in Java
Learn Jayway JsonPath 3 in Java with operators, filters, functions, typed reads with Jackson, missing-path options and updates, with runnable examples.
REST-assured POST Request and PUT Examples with JSON Body
REST-assured POST request examples with JSON body as String, Map, POJO or file, plus PUT and PATCH, headers, auth, response checks and extraction.
Remove Last Character from a String in Java (6 Ways)
How to remove last character from a string in Java with substring(), StringBuilder, regex and chop(), safe for null, empty strings and emoji.
REST-assured XML Request: Send and Validate XML Bodies
REST-assured XML request guide. Send XML from a String, file or JAXB object, set the charset, and validate responses with GPath, XPath and XmlPath.
REST-assured Tutorial: API Testing in Java with JUnit
REST-assured tutorial for API testing in Java with JUnit. Learn given/when/then, GPath checks, extraction, specifications, auth and schema validation.
Java Nested Map: Create, Update and Iterate a Map of Maps
Create a Java nested map with computeIfAbsent and streams, add, remove and iterate entries, read values safely, deep copy, flatten and choose alternatives.
Immutable vs Unmodifiable Map in Java: Differences and Examples
Immutable vs unmodifiable map in Java explained with Map.of(), Map.copyOf(), Collections.unmodifiableMap(), toUnmodifiableMap() and Guava ImmutableMap.
Collect Stream to Immutable List: toList vs toUnmodifiableList
Collect stream to immutable list in Java with toList() or Collectors.toUnmodifiableList(), how nulls differ, plus unmodifiable Set and Map collectors.
TreeMap vs HashMap in Java: Differences and When to Use Each
TreeMap vs HashMap in Java compared on ordering, speed, null keys, memory and key equality, with Java 25 examples and a table for picking the right map.
How to Kill a Thread in Java: interrupt(), Flags, Thread.stop
How to kill a thread in Java safely with interrupt() or a volatile flag, why Thread.stop() throws UnsupportedOperationException and how to stop tasks.
Apache POI: Append Rows to an Existing Excel File in Java
Append rows to an existing Excel sheet with Apache POI 5.5.1. We find the next free row, keep the date and number formats, add rows before a totals row and save through a temp file so a failed write never empties the spreadsheet.
Thread Name in Java: Set and Get the Name of a Thread
Set and get the thread name in Java with getName(), setName(), constructors and Thread builders, default names, executor thread names and threadId().
Apache POI SAX Parser: Read Large Excel Files in Java
Read large Excel files with the Apache POI SAX parser and XSSFSheetXMLHandler, handle missing cells and dates, import in batches, write with SXSSF.
Java IdentityHashMap: Reference Equality vs HashMap
Java IdentityHashMap compares keys with == instead of equals(). Learn how it differs from HashMap, the String and Integer traps and the Java 20 change.
Java ConcurrentMap: Atomic Methods and Implementations
Java ConcurrentMap with putIfAbsent, compute and merge examples, why get-then-put loses updates, null rules and ConcurrentHashMap vs ConcurrentSkipListMap.
How to Debug Java Streams: peek() and IntelliJ Stream Trace
Learn to debug Java streams with peek(), lambda breakpoints and the IntelliJ Trace Current Stream Chain action, and avoid the peek() and count() traps.
Java Wait for All Threads to Finish: 7 Ways With Examples
Java wait for all threads to finish with ExecutorService close(), invokeAll(), Future.get(), CountDownLatch, CompletableFuture.allOf() and join().
Java WeakHashMap: Weak Keys, GC Behavior and Use Cases
Learn how Java WeakHashMap removes entries after GC clears weak keys, the String and value-reference traps, WeakHashMap vs HashMap and real use cases.
Ternary Operator in Java: Syntax, Examples and Pitfalls
The ternary operator in Java with examples covering syntax, result types, the null unboxing trap, nested ternaries and when to use if-else instead.
Java Structured Concurrency with StructuredTaskScope (Java 25)
Java structured concurrency with the Java 25 preview API. Learn StructuredTaskScope.open(), fork, join, Joiners, timeouts, custom joiners and the flags.
Java Virtual Threads Guide with Examples (Java 25)
Java virtual threads explained for Java 25 with examples, carrier threads, the Java 24 pinning fix, when not to use them, pooling and ThreadLocal pitfalls.
EasyMock Tutorial: Mocking with EasyMock 5 and JUnit 5
EasyMock tutorial with JUnit 5. Create mocks, record expectations, replay and verify, mock void methods, use matchers, captures and nice or strict mocks.
MockWebServer with JUnit: Test Spring WebClient Calls
Test Spring WebClient with MockWebServer and JUnit. Queue responses, route with a Dispatcher, verify requests, and simulate errors, delays and timeouts.
WireMock Tutorial: Mock REST APIs in Java with WireMock 3
WireMock 3 tutorial for Java. Stub REST APIs in JUnit 5 tests, match requests, simulate delays and faults, verify calls and use stateful scenarios.
InputStream to OutputStream in Java: transferTo and Files.copy
Copy an InputStream to OutputStream in Java with transferTo(), a custom loop, Files.copy(), Guava or Commons IO, plus the closing and flushing rules.
Convert Reader to InputStream (and Back) in Java
Convert a Reader to InputStream in Java with a String, OutputStreamWriter or Commons IO ReaderInputStream, and back with InputStreamReader and a charset.
Java Create Directory: createDirectories() vs mkdirs()
Java create directory examples with Files.createDirectories() and createDirectory(), parent folders, permissions, mkdirs() pitfalls and race-free checks.
Count Lines in a File in Java (Fast and Correct Ways)
Count lines in a file in Java with Files.lines(), LineNumberReader, readAllLines() or a fast byte counter, with edge cases like a missing final newline.
Java Check If Directory Is Empty: Files.list() and More
Java check if directory is empty with Files.list() or newDirectoryStream(), closing the stream, files and missing paths, hidden files and subfolders.
File Creation Date in Java: BasicFileAttributes Guide
Read the file creation date in Java with BasicFileAttributes, format the FileTime, and see which platforms and JDK versions return the real value.
Symbolic Links in Java: Create, Read and Detect Symlinks
Create, detect and read symbolic links in Java with Files, control link following with NOFOLLOW_LINKS, compare hard links and switch releases atomically.