Java Core & Fundamentals Tutorial
Java tutorial for beginners on Java 25. Install the JDK, run a first program and follow a 12-step learning path plus an index of every Java topic.
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Could Not Find or Load Main Class in Java: Causes and Fixes
Fix Error could not find or load main class in Java by reading the Caused by line. Package, classpath, typos, missing JARs, Main-Class and IDE cases.
Java Console Input and Output: System.out, Console and IO
Java console input and output with System.out, System.err, printf, Scanner, Java 25 IO, the Console class, hidden passwords and the null console case.
Java Examples: Programs by Task, Add Two Integers and More
Java examples grouped by task, from a program to add two integers with overflow checks to number, string, console and JVM behavior programs.
Average of N Numbers in Java: Loop, Array, List and Streams
Calculate the average of N numbers in Java from user input, arrays and lists, and avoid integer division, overflow and empty input bugs.
Set Classpath from Command Line in Java (Windows, Linux, macOS)
Set classpath from command line in Java with -cp and CLASSPATH on Windows, Linux and macOS, with wildcards, quoting, argument files and the -jar trap.
Count Vowels and Consonants in a String in Java
Count vowels and consonants in Java with a loop, switch, streams and regex, plus y, digits, accents and locale pitfalls, with tested examples.
Reverse Words in a String in Java (6 Ways with Code)
Reverse words in a string in Java with split() and join, List.reversed(), a loop, streams, two pointers and StringUtils, with extra spaces and punctuation.
Sort a String Array Alphabetically in Java
Sort a String array alphabetically in Java with Arrays.sort, ignore case, reverse it, use Collator for accents, and handle null elements.
Swap Two Numbers in Java With and Without a Temp Variable
Swap two numbers in Java with a temp variable, arithmetic, XOR or one line, and learn where each trick fails and why a swap method cannot work.
Prime Numbers From 2 to N in Java (Sieve of Eratosthenes)
Find prime numbers from 2 to N in Java with the Sieve of Eratosthenes, check one number fast, get the first N primes and test big numbers.
Deficient Number in Java: Check, Deficiency and Examples
Check a deficient number in Java with a loop or the faster square-root method, compute the deficiency, and classify perfect and abundant numbers.
Pronic Number in Java: Check n(n+1) With or Without a Loop
Check a pronic number in Java with a loop, the square root or the 4x+1 test, handle long values safely, and generate the n(n+1) sequence.
Harshad Number in Java: Niven Number Check With Examples
Check a Harshad number in Java with a digit-sum loop that never divides by zero, try other bases, count Harshad numbers and build a lucky-draw rule.
Happy Number in Java: HashSet and Floyd Cycle Detection
Check a happy number in Java with a HashSet, Floyd cycle detection or the stop-at-4 shortcut, and see why every unhappy number reaches the same cycle.
Disarium Number in Java: Program, Examples and Full List
Check a disarium number in Java with exact integer math, see why 89, 135 and 518 qualify, list all 20 of them and compare them with Armstrong numbers.
Compare Float and Double in Java: Epsilon, compare(), BigDecimal
Learn to compare float and double in Java with a relative tolerance, Double.compare(), NaN and -0.0 rules, float vs double precision and BigDecimal.
sun.misc.Unsafe in Java: Deprecation, VarHandle and FFM API
sun.misc.Unsafe explained for Java 25 with what it does, the JEP 471 and 498 deprecation steps, warnings, and how to move to VarHandle and the FFM API.
Integer Cache in Java: Wrapper Class Caching (-128 to 127)
How the Integer cache in Java works, why 127 == 127 but 128 does not, cache ranges of all wrapper classes, AutoBoxCacheMax and the equals() fix.
