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Java Tutorial

Java HOME Java Intro Java Get Started Java Syntax

Syntax Statements Code Challenge

Java Output

Print Text Print Numbers Code Challenge

Java Comments Java Variables

Variables Print Variables Multiple Variables Identifiers Constants (Final) Real-Life Examples Code Challenge

Java Data Types

Data Types Numbers Booleans Characters Real-Life Example Non-primitive Types The var Keyword Code Challenge

Java Type Casting Java Operators

Operators Arithmetic Assignment Comparison Logical Precedence Code Challenge

Java Strings

Strings Concatenation Numbers and Strings Special Characters StringBuilder Code Challenge

Java Math Java Booleans

Booleans Real-Life Example Code Challenge

Java If...Else

if else else if Short Hand If...Else Nested If Logical Operators Real-Life Examples Code Challenge

Java Switch

Switch Code Challenge

Java While Loop

While Loop Do/While Loop Real-Life Examples Code Challenge

Java For Loop

For Loop Nested Loops For-Each Loop Real-Life Examples Code Challenge

Java Break/Continue Java Arrays

Arrays Loop Through an Array Real-Life Examples Multidimensional Arrays Code Challenge

Java Methods

Java Methods Java Method Challenge Java Method Parameters

Parameters Return Values Code Challenge

Java Method Overloading Java Scope Java Recursion

Java Classes

Java OOP Java Classes/Objects Java Class Attributes Java Class Methods Java Class Challenge Java Constructors Java this Keyword Java Modifiers

Access Modifiers Non-Access Modifiers

Java Encapsulation Java Packages / API Java Inheritance Java Polymorphism Java super Keyword Java Inner Classes Java Abstraction Java Interface Java Anonymous Java OOP Visuals Java Enum

Enums Enum Constructor

Java User Input Java Date

Java Errors

Java Errors Java Debugging Java Exceptions Java Multiple Exceptions Java try-with-resources

Java Problem Solving

Java Problem Solving Java Build the Logic Java Test Cases

Java Projects

Java File Handling

Java Files Java Create Files Java Write Files Java Read Files Java Delete Files

Java I/O Streams

Java I/O Streams Java FileInputStream Java FileOutputStream Java BufferedReader Java BufferedWriter

Java Data Structures

Java Data Structures Java Collections Java List Java ArrayList Java LinkedList Java List Sorting Java Set Java HashSet Java TreeSet Java LinkedHashSet Java Map Java HashMap Java TreeMap Java LinkedHashMap Java Iterator Java Algorithms

Java Advanced

Java Wrapper Classes Java Generics Java Annotations Java RegEx Java Threads Java Lambda Java Optional Java Advanced Sorting

Java Cert

Java Certificate

Java How To's

Java How Tos

How Tos Add Two Numbers Swap Two Variables Even or Odd Number Reverse a Number Positive or Negative Square Root Area of Rectangle Celsius to Fahrenheit Sum of Digits Check Armstrong Num Random Number Count Words Count Vowels in a String Remove Vowels Count Digits in a String Reverse a String Palindrome Check Check Anagram Convert String to Array Remove Whitespace Count Character Frequency Sum of Array Elements Find Array Average Sort an Array Find Smallest Element Find Largest Element Second Largest Array Min and Max Array Merge Two Arrays Remove Duplicates Find Duplicates Shuffle an Array Factorial of a Number Fibonacci Sequence Find GCD Check Prime Number ArrayList Loop HashMap Loop Loop Through an Enum

Java Examples

Java Examples Java Videos Java Compiler Java Exercises Java Quiz Java Code Challenges Java Practice Problems Java Server Java Syllabus Java Study Plan Java Interview Q&A

Java Reference

Java Reference Java Keywords

assert abstract boolean break byte case catch char class continue const default do double else enum exports extends final finally float for goto if implements import instanceof int interface long module native new package private protected public return requires short static strictfp super switch synchronized this throw throws transient try var void volatile while

ArrayList Methods

add() addAll() clear() clone() contains ensureCapacity() forEach() get() indexOf() isEmpty() iterator() lastIndexOf() listIterator() remove() removeAll() removeIf() replaceAll() retainAll() set() size() sort() spliterator() subList() toArray() trimToSize()

Arrays Methods

compare() copyOf() deepEquals() equals() sort() fill() mismatch() length

BufferedReader

close() mark() markSupported() read() readLine() ready() reset() skip() lines() transferTo()

BufferedWriter

close() flush() newLine() write() append()

Character Methods

compare() getNumericValue() isAlphabetic() isDigit() isLetter() isLetterOrDigit() isLowerCase() isUpperCase() isWhitespace() toLowerCase() toString() toUpperCase() valueOf()

Collections Methods

sort() binarySearch() reverse() reverseOrder() shuffle() swap() frequency() max() min() disjoint() copy() fill() replaceAll() unmodifiableList() unmodifiableSet() unmodifiableMap()

Comparator Methods

compare() comparing() comparingInt() naturalOrder() reversed() reverseOrder() thenComparing() compareTo()

Date and Time

now() of() parse() getYear() getMonthValue() getMonth() getDayOfMonth() getDayOfWeek() plusDays() minusDays() plusMonths() plusYears() isBefore() isAfter() isLeapYear() lengthOfMonth() format() now() of() getHour() getMinute() plusHours() plusMinutes() now() of() toLocalDate() toLocalTime() format() ofPattern() format()

Enum Methods

compareTo() name() ordinal() toString() valueOf() values()

Errors & Exceptions

ArithmeticException ArrayIndexOutOfBoundsException ClassFormatError ClassNotFoundException ConcurrentModificationException FileNotFoundException IncompatibleClassChangeError InputMismatchException InterruptedException InvalidClassException IOException NegativeArraySizeException NoClassDefFoundError NoSuchFieldException NoSuchMethodException NullPointerException NumberFormatException RuntimeException StringIndexOutOfBoundsException TypeNotPresentException IllegalArgumentException IllegalStateException

File Methods

createNewFile() delete() exists() canRead() canWrite() isFile() isDirectory() length() getName() getAbsolutePath() getPath() getParent() list() listFiles() mkdir() mkdirs() renameTo() setReadOnly() setWritable() setReadable() setExecutable() canExecute()

FileInputStream

available() close() read() skip() getFD() getChannel()

FileOutputStream

close() flush() write() getFD() getChannel()

FileReader

read() ready() skip() close() mark() reset() markSupported() getEncoding() transferTo()

FileWriter

write() append() flush() close() getEncoding()

HashMap Methods

clear() clone() compute() computeIfAbsent() computeIfPresent() containsKey() containsValue() entrySet() forEach() get() getOrDefault() isEmpty() keySet() merge() put() putAll() putIfAbsent() remove() replace() replaceAll() size() values()

HashSet Methods

add() addAll() clear() clone() contains() isEmpty() iterator() remove() removeAll() size() spliterator() toArray()

Integer Methods

compare() compareTo() equals() intValue() max() min() parseInt() sum() toBinaryString() toHexString() toOctalString() toString() valueOf() MAX_VALUE MIN_VALUE

Iterator Methods

hasNext() next() remove()

LinkedHashMap Methods

LinkedHashSet Methods

add() addAll() clear() clone() contains() isEmpty() iterator() remove() removeAll() size() spliterator() toArray()

LinkedList Methods

add() addAll() addFirst() addLast() clear() clone() contains descendingIterator() element() forEach() get() getFirst() getLast() indexOf() isEmpty() iterator() lastIndexOf() listIterator() offer() offerFirst() offerLast() peek() peekFirst() peekLast() poll() pollFirst() pollLast() pop() push() remove() removeAll() removeFirst() removeFirstOccurrence() removeIf() removeLast() removeLastOccurrence() replaceAll() retainAll() set() size() sort() spliterator() subList() toArray()

Math Methods

abs() acos() addExact() asin() atan() atan2() cbrt() ceil() copySign() cos() cosh() decrementExact() exp() expm1() floor() floorDiv() floorMod() getExponent() hypot() IEEEremainder() incrementExact() log() log10() log1p() max() min() multiplyExact() negateExact() nextAfter() nextDown() nextUp() pow() random() rint() round() scalb() signum() sin() sinh() sqrt() subtractExact() tan() tanh() toDegrees() toIntExact() toRadians() ulp()

Optional Methods

empty() filter() flatMap() get() ifPresent() isPresent() map() of() ofNullable() orElse() orElseGet() orElseThrow()

Output Methods

print() printf() println()

Random Methods

ints() nextBoolean() nextDouble() nextFloat() nextGaussian() nextInt() nextLong() setSeed()

RegEx Methods

compile() flags() matcher() matches() pattern() quote() split() end() find() group() groupCount() lookingAt() matches() replaceAll() replaceFirst() reset() start()

Scanner Methods

close() delimiter() findInLine() findWithinHorizon() hasNext() hasNextBoolean() hasNextByte() hasNextDouble() hasNextFloat() hasNextInt() hasNextLine() hasNextLong() hasNextShort() locale() next() nextBoolean() nextByte() nextDouble() nextFloat() nextInt() nextLine() nextLong() nextShort() radix() reset() useDelimiter() useLocale() useRadix()

String Methods

charAt() codePointAt() codePointBefore() codePointCount() compareTo() compareToIgnoreCase() concat() contains() contentEquals() copyValueOf() endsWith() equals() equalsIgnoreCase() format() getBytes() getChars() hashCode() indexOf() intern() isEmpty() join() lastIndexOf() length() matches() offsetByCodePoints() regionMatches() replace() replaceAll() replaceFirst() split() startsWith() subSequence() substring() toCharArray() toLowerCase() toString() toUpperCase() trim() valueOf()

StringBuilder Methods

append() appendCodePoint() capacity() charAt() codePointAt() codePointBefore() codePointCount() delete() deleteCharAt() ensureCapacity() getChars() indexOf() insert() lastIndexOf() length() offsetByCodePoints() replace() reverse() setCharAt() setLength() subSequence() substring() toString() trimToSize()

System Methods

currentTimeMillis() nanoTime() exit() gc() getProperty() getProperties() setProperty() clearProperty() arraycopy() identityHashCode() lineSeparator() getenv() in out err

Thread Methods

currentThread() getName() getPriority() interrupt() isAlive() isInterrupted() join() run() setName() setPriority() sleep() start()

TreeMap Methods

firstKey() lastKey() firstEntry() lastEntry() floorKey() ceilingKey() lowerKey() higherKey() headMap() tailMap() subMap() pollFirstEntry() pollLastEntry() descendingMap() clear() clone() compute() computeIfAbsent() computeIfPresent() containsKey() containsValue() entrySet() forEach() get() getOrDefault() isEmpty() keySet() merge() put() putAll() putIfAbsent() remove() replace() replaceAll() size() values()

TreeSet Methods

first() last() floor() ceiling() lower() higher() headSet() tailSet() subSet() pollFirst() pollLast() descendingSet() add() addAll() clear() clone() contains() isEmpty() iterator() remove() removeAll() size() spliterator() toArray()

Practice Java Coding Problems

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Improve your skills by solving real Java coding problems.

Write code, submit, and get instant feedback.

Weekly Problem

Count a Character

Count how often one character appears in a word.

Easy 20 XP

Solve the Problem

main.py

first = input()
second = input()

# Count the character and print the total

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More Practice Problems

Problem	Description	Difficulty	XP
Say Hello	Read a name from input and print a greeting.	Easy	20 XP
Celsius to Fahrenheit	Convert a temperature from Celsius to Fahrenheit.	Easy	20 XP
Even or Odd	Check if a number is even or odd.	Easy	20 XP
Make a Username	Create a username and initials from a first and last name.	Easy	20 XP
Voting Age	Determine if a person is old enough to vote.	Easy	20 XP
Grade Calculator	Convert a score into a letter grade.	Easy	20 XP
Multiplication Table	Print the multiplication table for a given number.	Easy	20 XP
Factorial	Create a function that calculates the factorial of a number.	Easy	20 XP
Sum of Numbers	Read a list of numbers and calculate their sum.	Easy	20 XP
Area Calculator	Calculate the area of a rectangle, triangle, or circle.	Easy	20 XP
Shopping Receipt	Read item details from input and print a short receipt.	Easy	20 XP
Personal Info	Read personal details from input and display them.	Easy	20 XP
Swap Values	Read two values and print them in swapped order.	Easy	20 XP
Rectangle Border	Print a rectangle border made of stars.	Easy	20 XP
Repeat Message	Read a message and a number, then print the message that many times.	Easy	20 XP
Currency Exchange	Calculate a currency exchange from an amount and rate.	Easy	20 XP
BMI Calculator	Calculate Body Mass Index from weight and height.	Easy	20 XP
Circle Properties	Calculate the circumference and area of a circle.	Easy	20 XP
Simple Calculator	Perform basic arithmetic on two numbers.	Easy	20 XP
Discount Price	Calculate a discounted price from a price and discount percentage.	Easy	20 XP
Split the Bill	Split a total amount equally among a group of people.	Easy	20 XP
Digit Extractor	Extract the individual digits of a 3-digit number.	Easy	20 XP
Word Counter	Count the number of words in a sentence.	Easy	20 XP
Shout It Out	Convert a string to uppercase and print its length.	Easy	20 XP
First and Last	Print the first and last character of a word.	Easy	20 XP
Repeat String	Read a string and a number, then print the string repeated that many times.	Easy	20 XP
Range Checker	Check if a number falls within a given range.	Easy	20 XP
Password Check	Check if a password is long enough.	Easy	20 XP
Ticket Price	Determine the ticket type and price based on age.	Easy	20 XP
Positive Negative Zero	Check if a number is positive, negative, or zero.	Easy	20 XP
Smallest of Three	Find the smallest of three numbers.	Easy	20 XP
Countdown	Count down from a number to 1 and print Go!	Easy	20 XP
FizzBuzz	Implement the classic FizzBuzz challenge.	Easy	20 XP
Sum 1 to N	Calculate the sum of all numbers from 1 to N.	Easy	20 XP
Star Triangle	Print a right triangle made of stars.	Easy	20 XP
Tip Calculator	Create a tip calculator function for a restaurant bill.	Easy	20 XP
Power Function	Write a function that calculates the power of a number.	Easy	20 XP
Average Score	Read a list of scores and calculate their average.	Easy	20 XP
Count Matches	Count how many numbers in a list match a target value.	Easy	20 XP
Min and Max	Find the smallest and largest numbers in a list.	Easy	20 XP
Absolute Difference	Calculate the absolute difference between two numbers.	Easy	20 XP
Round Up or Down	Round a decimal number to the nearest whole number.	Easy	20 XP
Leap Year	Determine if a given year is a leap year.	Medium	50 XP
Day of Week	Convert a day number to a day name.	Medium	50 XP
Triangle Classifier	Classify a triangle based on its side lengths.	Medium	50 XP
Time Converter	Convert a number of seconds into hours, minutes, and seconds.	Medium	50 XP
Reverse Words	Reverse the order of words in a sentence.	Medium	50 XP
Palindrome Check	Check if a word reads the same forwards and backwards.	Medium	50 XP
Count Vowels	Count the number of vowels in a string.	Medium	50 XP
Title Case	Capitalize the first letter of each word in a sentence.	Medium	50 XP
Remove Duplicates	Remove consecutive duplicate characters from a string.	Medium	50 XP
Longest Word	Find the longest word in a sentence.	Medium	50 XP
Access Control	Determine if a person gets access based on role, age, and membership.	Medium	50 XP
Digit Sum	Calculate the sum of all digits in a number.	Medium	50 XP
Number Pyramid	Print a pyramid of numbers.	Medium	50 XP
Collatz Steps	Count the steps to reach 1 using the Collatz sequence.	Medium	50 XP
Prime Check	Check if a number is a prime number.	Medium	50 XP
Reverse Number	Reverse the digits of a number.	Medium	50 XP
Is Palindrome Function	Write a function that checks if a word is a palindrome.	Medium	50 XP
Min of Three	Write a function that returns the smallest of three numbers.	Medium	50 XP
Count Digits	Write a function that counts the number of digits in a number.	Medium	50 XP
Recursive Sum	Write a recursive function to sum numbers from 1 to N.	Medium	50 XP
Array Map Function	Write a function that doubles every number in a list.	Medium	50 XP
Find the Largest	Read a list of numbers and find the largest one.	Medium	50 XP
Reverse a List	Read a list of numbers and print them in reverse order.	Medium	50 XP
Second Largest	Find the second largest number in a list.	Medium	50 XP
Running Sum	Print the running (cumulative) sum of a list of numbers.	Medium	50 XP
Count Above Average	Count how many numbers are above the average.	Medium	50 XP
Longest Streak	Find the longest consecutive streak of the same number.	Medium	50 XP
Greatest Common Divisor	Find the greatest common divisor (GCD) of two numbers.	Medium	50 XP
Fibonacci	Print the first N numbers of the Fibonacci sequence.	Medium	50 XP
Prime Factors	Find all prime factors of a number.	Medium	50 XP
Power of Two	Check if a number is a power of 2.	Medium	50 XP
Sum of Squares	Calculate the sum of squares from 1 to N.	Medium	50 XP
LCM Calculator	Calculate the least common multiple of two numbers.	Medium	50 XP
Perfect Number	Check if a number is a perfect number.	Medium	50 XP
Sort Three Numbers	Read three numbers and print them in ascending order.	Medium	50 XP
Sort Numbers	Read a list of numbers and print them sorted.	Medium	50 XP
Sort Words	Read a list of words and print them sorted alphabetically.	Medium	50 XP
Letter Frequency	Count how many times each letter appears in a word.	Medium	50 XP
Most Common Char	Find the character that appears most often in a string.	Medium	50 XP
Digit Frequency	Count how many times each digit appears in a number.	Medium	50 XP
Caesar Cipher	Encrypt a message by shifting each letter in the alphabet.	Hard	100 XP
Anagram Check	Check if two words are anagrams of each other.	Hard	100 XP
Run Length Encoding	Compress a string using run-length encoding.	Hard	100 XP
Date Validator	Check if a given date is valid.	Hard	100 XP
Diamond Pattern	Print a diamond pattern of stars.	Hard	100 XP
Number to Binary	Convert a decimal number to binary.	Hard	100 XP
Pascal Triangle Row	Print a specific row of Pascal's triangle.	Hard	100 XP
Remove Duplicates	Remove duplicate numbers from a list while keeping order.	Hard	100 XP
Rotate Left	Rotate a list of numbers to the left by K positions.	Hard	100 XP
Merge Sorted	Merge two sorted lists into one sorted list.	Hard	100 XP
Pair Sum	Find the first pair of numbers that add up to a target.	Hard	100 XP
GCD Function	Write a recursive function to find the GCD of two numbers.	Hard	100 XP
Number Base Convert	Convert a number from one base to another.	Hard	100 XP
Bubble Sort	Implement bubble sort and count the number of swaps.	Hard	100 XP
Selection Sort	Implement selection sort and print each step.	Hard	100 XP
Insertion Sort	Implement insertion sort and print each step.	Hard	100 XP
Word Frequency	Count how many times each word appears in a sentence.	Hard	100 XP
Unique Elements Count	Count how many unique numbers appear in a list.	Hard	100 XP

Past Weekly Challenges

Problem	Description	Difficulty	XP
Double or Nothing	Double a number if it is positive, otherwise print zero.	Easy	20 XP
Balanced Brackets	Check if a string of brackets is balanced.	Medium	50 XP
Matrix Diagonal Sum	Calculate the sum of both diagonals of a square matrix.	Hard	100 XP
Sum of Digits	Calculate the sum of all digits in a given number.	Easy	20 XP
Find the Duplicate	Find the duplicate number in an array of integers.	Medium	50 XP
Maximum Subarray Sum	Find the contiguous subarray with the largest sum.	Hard	100 XP
Reverse String	Reverse a string without using built-in reverse functions.	Easy	20 XP
Count Consonants	Count the number of consonants in a string.	Medium	50 XP
Sum of Primes	Find the sum of all prime numbers up to N using a sieve.	Hard	100 XP
Capitalize Words	Capitalize the first letter of every word in a sentence.	Easy	20 XP
Two Sum	Find two numbers in a list that add up to a target.	Medium	50 XP
Matrix Transpose	Transpose an N×N matrix.	Hard	100 XP
Count Words	Count the number of words in a sentence.	Easy	20 XP
Common Characters	Find characters that appear in both strings, in sorted order.	Medium	50 XP
Spiral Order	Print the elements of a matrix in spiral order.	Hard	100 XP
Number Pattern	Print a right-aligned triangle of row numbers.	Easy	20 XP
Missing Number	Find the missing number in a sequence from 1 to N.	Medium	50 XP
Sort Descending	Read N numbers and sort them from largest to smallest without using a built-in sort.	Hard	100 XP
Palindrome Number	Check if a number reads the same forwards and backwards.	Easy	20 XP
Longest Common Prefix	Find the longest common prefix shared by all words.	Medium	50 XP
Stock Buy Sell	Find the maximum profit from a single stock buy and sell.	Hard	100 XP
Alternating Sum	Add and subtract numbers in turn.	Easy	20 XP
Move Zeros to End	Push every zero to the end, keeping the other numbers in order.	Medium	50 XP
Roman to Integer	Convert a Roman numeral into an ordinary number.	Hard	100 XP
Digit Product	Multiply all the digits of a number together.	Easy	20 XP
First Unique Character	Find the first character that appears only once.	Medium	50 XP
Longest Unique Substring	Measure the longest stretch with no repeated character.	Hard	100 XP
Repeat Each Character	Double every character in a word.	Easy	20 XP
Merge Words Alternately	Interleave the letters of two words.	Medium	50 XP
Merge Intervals	Combine intervals that overlap into single ranges.	Hard	100 XP

How it Works

Each challenge gives you starter code in Java. Your program reads input and prints the correct output.

When you submit, your code is tested against multiple hidden test cases with different inputs.

Your score is based on how many tests pass. Partial credit is given for partially correct solutions.

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