Java

Flatten list of lists

Product product1 = Product.builder().tags(asList("tag1", "tag2")).build();
Product product2 = Product.builder().tags(asList("tag2", "tag3")).build();
List<Product> products = asList(product1, product2);
List<String> listOfTags = products.stream()
                            .flatMap(product -> product.getTags().stream())
                            .collect(Collectors.toList());

Given you have a set of lists, if you want to combine all the lists into one big list, use flatMap.

Merge two lists

List<Integer> listOfOdds = asList(1,3,5,7,9);
List<Integer> listOfEvens = asList(2,4,6,8,10);
List<Integer> listOfAll = Stream.concat(
                            listOfOdds.stream(),
                            listOfEvens.stream()
                          ).collect(Collectors.toList());

Given two lists, to merge them into one list, use Stream.concat

Reading files

Using Scanner

Scanner scanner = new Scanner(MyClass.class.getClass().getResourceAsStream("/file.txt"));
while(scanner.hasNext()) {
    System.out.println(scanner.nextLine());
}

If file file.txt is on the classpath, this will use a classloader to create an InputStream to the file:

this can only be used if the context is non-static. If reading from a static method or context, use MyClass.class.getResourceAsStream(...)

Non-null checks (Java 1.7+)

public class Car {
    private final Engine engine;
    private final Transmission transmission;

    public Car(Engine engine, Transmission transmission) {
        this.engine = Objects.requireNonNull(engine, "Engine cannot be null");
        this.transmission = Objects.requireNonNull(transmission, "Transmission cannot be null");
    }

    ...
}

In JDK7, Objects.requireNonNull was introduced to be able to check if an object is null or not. It can be used to fail fast when wiring up objects at runtime.

Hash Functions

Java Secure Coding Practices

Hashing Passwords with Java

Cryptographic Hash Functions

Lean On…	Avoid… (birthday, collision attacks, crypto-broken)
SHA-2 (> 256)	MD5
SHA-3	SHA1
PBKDF2WithHmacSHA1	-
PBKDF2WithHmacSHA512	-
Bcrypt (std. in Spring Security)	-

Generate Excel workbook, spreadsheet, and table with formatting

As of version Apache POI 4.0.0,

XSSFWorkbook workbook = new XSSFWorkbook();
XSSFSheet sheet = workbook.createSheet("Architecture");

//  < v4.0.0
XSSFTable table = sheet.createTable();

// >= 4.0.0
XSSFTable table = sheet.createTable(null);
CTTable cttable = table.getCTTable();

cttable.setDisplayName("Table1");
cttable.setId(1);
cttable.setName("Test");
cttable.setRef("A1:C11");
cttable.setTotalsRowShown(false);

CTTableStyleInfo styleInfo = cttable.addNewTableStyleInfo();
styleInfo.setName("TableStyleMedium2");
styleInfo.setShowColumnStripes(false);
styleInfo.setShowRowStripes(true);

CTTableColumns columns = cttable.addNewTableColumns();
columns.setCount(3);
for (int i = 1; i <= 3; i++) {
  CTTableColumn column = columns.addNewTableColumn();
  column.setId(i);
  column.setName("Column" + i);
}

for (int r = 0; r < 2; r++) {
  XSSFRow row = sheet.createRow(r);
  for(int c = 0; c < 3; c++) {
    XSSFCell cell = row.createCell(c);
    if(r == 0) { //first row is for column headers
      //content **must** be here for table column names
      cell.setCellValue("Column"+ (c+1));
    } else {
      cell.setCellValue("Data.")
    }
  }
}    

try (FileOutputStream outputStream =
      new FileOutputStream("ExcelTableTest.xlsx")) {
  workbook.write(outputStream);
}