How to contribute

Table of Contents

Algorithm

Array

File

Math

Media

Networking

String

Algorithm

Quicksort

publicstaticvoidquickSort(int[]arr,intleft,intright) {intpivotIndex =left + (right -left) /2;intpivotValue =arr[pivotIndex];inti =left,j =right;while (i <=j) {while (arr[i] <pivotValue) {i++;            }while (arr[j] >pivotValue) {j--;            }if (i <=j) {inttmp =arr[i];arr[i] =arr[j];arr[j] =tmp;i++;j--;            }if (left <i) {quickSort(arr,left,j);            }if (right >i) {quickSort(arr,i,right);            }        }    }

Array

Generic two array concatenation

publicstatic <T>T[]arrayConcat(T[]first,T[]second) {T[]result =Arrays.copyOf(first,first.length +second.length);System.arraycopy(second,0,result,first.length,second.length);returnresult;    }

Generic N array concatenation

publicstatic <T>T[]nArrayConcat(T[]first,T[]...rest) {inttotalLength =first.length;for (T[]array :rest) {totalLength +=array.length;        }T[]result =Arrays.copyOf(first,totalLength);intoffset =first.length;for (T[]array :rest) {System.arraycopy(array,0,result,offset,array.length);offset +=array.length;        }returnresult;    }

File

List directories

publicstaticFile[]listDirectories(Stringpath) {returnnewFile(path).listFiles(File::isDirectory);    }

List files in directory

publicstaticFile[]listFilesInDirectory(finalFilefolder) {returnfolder.listFiles(File::isFile);    }

List files in directory recursively

publicstaticList<File>listAllFiles(Stringpath) {List<File>all =newArrayList<>();File[]list =newFile(path).listFiles();if (list !=null) {// In case of access error, list is nullfor (Filef :list) {if (f.isDirectory()) {all.addAll(listAllFiles(f.getAbsolutePath()));                }else {all.add(f.getAbsoluteFile());                }            }        }returnall;    }

Read lines from file to string list

publicstaticList<String>readLines(Stringfilename)throwsIOException {returnFiles.readAllLines(newFile(filename).toPath());    }

Zip file

publicstaticvoidzipFile(StringsrcFilename,StringzipFilename)throwsIOException {FilesrcFile =newFile(srcFilename);try (FileOutputStreamfileOut =newFileOutputStream(zipFilename);ZipOutputStreamzipOut =newZipOutputStream(fileOut);FileInputStreamfileIn =newFileInputStream(srcFile);        ) {ZipEntryzipEntry =newZipEntry(srcFile.getName());zipOut.putNextEntry(zipEntry);finalbyte[]bytes =newbyte[1024];intlength;while ((length =fileIn.read(bytes)) >=0) {zipOut.write(bytes,0,length);            }        }    }

Zip multiple files

publicstaticvoidzipFiles(String[]srcFilenames,StringzipFilename)throwsIOException {try (FileOutputStreamfileOut =newFileOutputStream(zipFilename);ZipOutputStreamzipOut =newZipOutputStream(fileOut);        ) {for (inti=0;i<srcFilenames.length;i++) {FilesrcFile =newFile(srcFilenames[i]);try (FileInputStreamfileIn =newFileInputStream(srcFile)) {ZipEntryzipEntry =newZipEntry(srcFile.getName());zipOut.putNextEntry(zipEntry);finalbyte[]bytes =newbyte[1024];intlength;while ((length =fileIn.read(bytes)) >=0) {zipOut.write(bytes,0,length);                    }                }            }        }    }

Math

Fibonacci

publicstaticintfibonacci(intn) {if (n <=1)returnn;elsereturnfibonacci(n-1) +fibonacci(n-2);    }

Factorial

publicstaticintfactorial(intnumber) {intresult =1;for (intfactor =2;factor <=number;factor++) {result *=factor;        }returnresult;    }

Lottery

publicstaticInteger[]performLottery(intnumNumbers,intnumbersToPick) {List<Integer>numbers =newArrayList<>();for(inti =0;i <numNumbers;i++) {numbers.add(i+1);        }Collections.shuffle(numbers);returnnumbers.subList(0,numbersToPick).toArray(newInteger[numbersToPick]);    }

Prime

publicstaticbooleanisPrime(intnumber) {if (number <3) {returntrue;}// check if n is a multiple of 2if (number %2 ==0) {returnfalse;}// if not, then just check the oddsfor (inti =3;i *i <=number;i +=2) {if (number %i ==0) {returnfalse;}}returntrue;}

Media

Capture screen

publicstaticvoidcaptureScreen(Stringfilename)throwsAWTException,IOException {DimensionscreenSize =Toolkit.getDefaultToolkit().getScreenSize();RectanglescreenRectangle =newRectangle(screenSize);Robotrobot =newRobot();BufferedImageimage =robot.createScreenCapture(screenRectangle);ImageIO.write(image,"png",newFile(filename));    }

Networking

HTTP GET

publicstaticinthttpGet(URLaddress)throwsIOException {HttpURLConnectioncon = (HttpURLConnection)address.openConnection();returncon.getResponseCode();    }

String

Palindrome check

publicstaticbooleanisPalindrome(Strings) {StringBuildersb =newStringBuilder();for (charc :s.toCharArray()) {if (Character.isLetter(c)) {sb.append(c);            }        }Stringforward =sb.toString().toLowerCase();Stringbackward =sb.reverse().toString().toLowerCase();returnforward.equals(backward);    }

Reverse string

publicstaticStringreverseString(Strings) {returnnewStringBuilder(s).reverse().toString();    }

String to date

publicstaticDatestringToDate(Stringdate,Stringformat)throwsParseException {SimpleDateFormatsimpleDateFormat =newSimpleDateFormat(format);returnsimpleDateFormat.parse(date);    }