OO_Java Cheat Sheet by tarinya

switch (Ausdruck) {
case Wert1:
   Anweisungen;
   break;
case Wert2:
   Anweisungen;
   break;
default:
   Anweisungen;
}

car extends vehicle
@Override
drive(){}
Vehicle v = new Car();
//calls drive of Car not Vehicle
v.drive();

public boolean groupSum(int start, int[] nums, int target) {
  if (start >= nums.length) return (target == 0);
  if (groupSum(start + 1, nums, target - nums[start])) return true;
  if (groupSum(start + 1, nums, target)) return true;
  return false;}

Collection<String> validParentheses(int nofPairs){
  Collection<String> list = new ArrayList<>();
  if (nofPairs == 0) {list.add("");} else {
    for (int k = 0; k < nofPairs; k++) {
      Collection<String> infixes = validParentheses(k);
      Collection<String> suffixes = validParentheses(nofPairs - k - 1);
      for (String infix : infixes) {
        for (String suffix : suffixes) {
          list.add("(" + infix + ")" + suffix);}}}}
  return list;}

people
.stream()
.filter(p -> p.getAge() >= 18)
.map(p -> p.getLastName())
.sorted()
.forEach(System.out::println);

people.stream().mapToInt(p -> p.getAge())
.average().ifPresent(System.out::println);

Map<String, Integer> totalAgeByCity =
  people.stream()
  .collect(
     Collectors.groupingBy(Person::getCity,
     Collectors.summingInt(Person::getAge)));

private static void reverteText() throws FileNotFoundException, IOException{
  try (FileReader reader = new FileReader("input.txt");
     FileWriter writer = new FileWriter("outpur.txt")){
      int value = reader.read();
      String text = "";
      while (value >=0){
         text = (char)value + text;
         value = reader.read();}
      writer.write(text);}}

1 / 0 --> ArithmeticException: / by zero
1 / 0.0 --> Infinity
-1.0 / 0 --> -Infinity
0 / 0.0 --> NaN

int operate(int x, int y) { ... }
double operate(int x, double y) { ... }
double operate(double x, double y) { ... }

max = left > right ? left : right;

public enum Color {
   BLUE(1), RED(2);
   private final int code;
   private Color(int code) {
      this.code = code;}
   public int getColorValue() {
      return code;}}

OuputStream fos = new FileOutputStream("serial.bin");
try (ObjectOutputStream stream = new ObjectOutputStream(fos)){
   stream.writeObject(person);
}

try (BufferedInputStream inputBuffer = new bis(new FileInputStream(pathSource));
BufferedOutputStream outputB = new bos(new FileOutputStream(pathTarget))) {
   byte[] byteBuffer = new byte[4096];
   int value = inputBuffer.read(byteBuffer);
   while (value >= 0) {
      outputB.write(byteBuffer, 0, value);
      value = inputBuffer.read(byteBuffer);}}
catch (FileNotFoundException e) {
   System.out.println("File not found: "+e);
} catch (IOException e) {
   System.out.println("reading Error:"+e);}

String a = "OO", b = "Prog", c = "OOProg";
//true
a == "OO"; c == "OO" + "Prog"; (a+b).equals(c);
//false
a + b == c;

@Before
public void setUp() {...}
@Test (timeout = 500, expected = Exception.class)
public void testGoodName(){
  assertEquals(expected, actual);
  assertTrue(condition);}

@Override
public boolean equals(Object obj) {
   if(this == obj) {return true;}
   if (obj == null) {return false;}
   if (getClass() != obj.getClass()) {
      return false;}
   Student other = (Student)obj;
   return regNumber == other.regNumber;}}

@Override
public int hashCode() {
  return 31* firstName.hashCode()
   + 31 * lastName.hashCode();
}

@FunctionalInterface
interface Comparator<T> {
  int compare(T first, T second);
}