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Table of Contents

Algorithm

Array

File

Math

Media

Networking

String

Class

I/O

Algorithm

Quicksort

publicstaticvoidquickSort(int[]arr,intleft,intright) {varpivotIndex =left + (right -left) /2;varpivotValue =arr[pivotIndex];vari =left;varj =right;while (i <=j) {while (arr[i] <pivotValue) {i++;            }while (arr[j] >pivotValue) {j--;            }if (i <=j) {vartmp =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);            }        }    }

Bubblesort

publicstaticvoidbubbleSort(int[]arr) {varlastIndex =arr.length -1;for(varj =0;j <lastIndex;j++) {for(vari =0;i <lastIndex -j;i++) {if(arr[i] >arr[i +1]) {vartmp =arr[i];arr[i] =arr[i +1];arr[i +1] =tmp;        }      }    }  }

Selectionsort

publicstaticvoidselectionSort(int[]arr) {varlen =arr.length;for (vari =0;i <len -1;i++) {varminIndex =i;for (varj =i +1;j <len;j++) {if(arr[j] <arr[minIndex])minIndex =j;            }vartmp =arr[minIndex];arr[minIndex] =arr[i];arr[i] =tmp;        }    }

Array

Generic two array concatenation

publicstatic <T>T[]arrayConcat(T[]first,T[]second) {varresult =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) {vartotalLength =first.length;for (vararray :rest) {totalLength +=array.length;        }varresult =Arrays.copyOf(first,totalLength);varoffset =first.length;for (vararray :rest) {System.arraycopy(array,0,result,offset,array.length);offset +=array.length;        }returnresult;    }

Check if all elements of array are equal

publicstatic <T>booleanallEqual(T[]arr) {returnArrays.stream(arr).distinct().count() ==1;  }

Find maximum integer from the array

publicstaticintfindMax(int[]arr) {returnArrays.stream(arr).reduce(Integer.MIN_VALUE,Integer::max);    }

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) {varall =newArrayList<File>();varlist =newFile(path).listFiles();if (list !=null) {// In case of access error, list is nullfor (varf :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 {varsrcFile =newFile(srcFilename);try (varfileOut =newFileOutputStream(zipFilename);varzipOut =newZipOutputStream(fileOut);varfileIn =newFileInputStream(srcFile);        ) {varzipEntry =newZipEntry(srcFile.getName());zipOut.putNextEntry(zipEntry);finalvarbytes =newbyte[1024];intlength;while ((length =fileIn.read(bytes)) >=0) {zipOut.write(bytes,0,length);            }        }    }

Zip multiple files

publicstaticvoidzipFiles(String[]srcFilenames,StringzipFilename)throwsIOException {try (varfileOut =newFileOutputStream(zipFilename);varzipOut =newZipOutputStream(fileOut);        ) {for (vari=0;i<srcFilenames.length;i++) {varsrcFile =newFile(srcFilenames[i]);try (varfileIn =newFileInputStream(srcFile)) {varzipEntry =newZipEntry(srcFile.getName());zipOut.putNextEntry(zipEntry);finalvarbytes =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) {varresult =1;for (varfactor =2;factor <=number;factor++) {result *=factor;        }returnresult;    }

Lottery

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

Greatest Common Divisor

publicstaticintgcd(inta,intb) {if (b ==0)returna;returngcd(b,a %b);      }

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 (vari =3;i *i <=number;i +=2) {if (number %i ==0) {returnfalse;}}returntrue;}

Media

Capture screen

publicstaticvoidcaptureScreen(Stringfilename)throwsAWTException,IOException {varscreenSize =Toolkit.getDefaultToolkit().getScreenSize();varscreenRectangle =newRectangle(screenSize);varrobot =newRobot();varimage =robot.createScreenCapture(screenRectangle);ImageIO.write(image,"png",newFile(filename));    }

Networking

HTTP GET

publicstaticHttpResponse<String>httpGet(Stringuri)throwsException {varclient =HttpClient.newHttpClient();varrequest =HttpRequest.newBuilder()                .uri(URI.create(uri))                .build();returnclient.send(request,BodyHandlers.ofString());    }

HTTP POST

publicstaticHttpResponse<String>httpPost(Stringaddress,HashMap<String,String>arguments)throwsIOException,InterruptedException{varsj =newStringJoiner("&");for(varentry :arguments.entrySet()) {sj.add(URLEncoder.encode(entry.getKey(),"UTF-8") +"="                  +URLEncoder.encode(entry.getValue(),"UTF-8"));        }varout =sj.toString().getBytes(StandardCharsets.UTF_8);varrequest =HttpRequest.newBuilder()                           .uri(URI.create(address))                           .headers("Content-Type","application/x-www-form-urlencoded; charset=UTF-8")                           .POST(BodyPublishers.ofByteArray(out))                           .build();returnHttpClient.newHttpClient().send(request,BodyHandlers.ofString());    }

String

Palindrome check

publicstaticbooleanisPalindrome(Strings) {varsb =newStringBuilder();for (varc :s.toCharArray()) {if (Character.isLetter(c)) {sb.append(c);            }        }varforward =sb.toString().toLowerCase();varbackward =sb.reverse().toString().toLowerCase();returnforward.equals(backward);    }

Reverse string

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

String to date

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

Anagram Check

publicbooleanisAnagram(Strings1,Strings2) {varl1 =s1.length();varl2 =s2.length();vararr1 =newint[256];vararr2 =newint[256];if (l1 !=l2) {returnfalse;}for (vari =0;i <l1;i++) {arr1[s1.charAt(i)]++;arr2[s2.charAt(i)]++;}returnArrays.equals(arr1,arr2);  }

Class

Get methods name

publicstaticList<String>getAllMethods(finalClass<?>cls) {varlist =newArrayList<String>();for (varmethod :cls.getDeclaredMethods()) {list.add(method.getName());        }returnlist;    }

Get fields name

publicstaticList<String>getAllFieldNames(finalClass<?>cls) {returnArrays.stream(cls.getFields())            .map(Field::getName)            .collect(Collectors.toList());  }

Find Levenshtein distance

publicstaticintfindLevenshteinDistance(Stringword1,Stringword2) {// If word2 is empty, removingint[][]ans =newint[word1.length() +1][word2.length() +1];for (inti =0;i <=word1.length();i++) {ans[i][0] =i;    }// if word1 is empty, addingfor (inti =0;i <=word2.length();i++) {ans[0][i] =i;    }// None is emptyfor (inti =1;i <=word1.length();i++) {for (intj =1;j <=word2.length();j++) {intmin =Math.min(Math.min(ans[i][j -1],ans[i -1][j]),ans[i -1][j -1]);ans[i][j] =word1.charAt(i -1) ==word2.charAt(j -1) ?ans[i -1][j -1] :min +1;      }    }returnans[word1.length()][word2.length()];  }

I/O

Read file by stream

publicstaticList<String>readFile(StringfileName)throwsFileNotFoundException {try (Stream<String>stream =newBufferedReader(newFileReader(fileName)).lines()) {returnstream.collect(Collectors.toList());    }  }

InputStream to String

publicstaticStringinputStreamToString(InputStreaminputStream)throwsIOException {try (varreader =newBufferedReader(newInputStreamReader(inputStream))) {varstringBuilder =newStringBuilder();vardata =reader.read();while (data != -1) {stringBuilder.append((char)data);data =reader.read();       }returnstringBuilder.toString();     }   }