Java Snippets

00_index.md

Data types

Why use wrapper classes?

http://stackoverflow.com/questions/3579035/why-are-there-wrapper-classes-in-java

• So that a null value is possible
• To include in a Collection
• To treat generically / polymorphically as an Object along with other Objects

Quick notes

• Always use Double over Float
• Integer Wrapper Class is autoboxed when compared until from -128 to (and with) 127

Integer j1 = 2;
Integer j2 = 2;
System.out.println( j1 == j2 );   // true
Integer k1 = 127;
Integer k2 = 127;
System.out.println( k1 == k2 );   // true
Integer l1 = 128;
Integer l2 = 128;
System.out.println( l1 == l2 );   // false
Integer m1 = 1000;
Integer m2 = 1000;
System.out.println( m1 == m2 );   // false

• convert primitive type to wrapper class, use WrapperClass.valueOf()

// implicit conversion
Integer x = 100;

// explicit conversion
Integer x = Integer.valueOf(100);

• Convert wrapper class to primitive type

Integer x = Integer.valueOf(10);
// implicit conversion
int y = x;
// explicit conversion
int y = x.intValue();

• Integer.parseInt(String) returns a primitive int
• Integer.valueOf(String) return a Integer wrapper class

arraylist-to-array.java

array.toArray(new Type[array.size()]);

// if Type = int
array.stream().mapToInt(i -> i).toArray();

create-arraylist.java

private ArrayList<Type> type = new ArrayList<>();

custom-sort.java

Collections.sort(SortList, new Comparator<User>(){
  public int compare(User p1, User p2) {
    int flag = user0.getAge().compareTo(user1.getAge());
    
    if (flag == 0) {
      return user0.getName().compareTo(user1.getName());
    } else {
      return flag;
    }
  }
});

main.java

public static void main(String[] args) {
  // TODO code application logic here
}

read-file.java

File file = new File("c:/temp/data.txt");
FileReader fr = new FileReader(file);
BufferedReader br = new BufferedReader(fr);
String line;
while((line = br.readLine()) != null){
  if(line.contains("password")){
    System.out.println(line);
  }
}

br.close();
fr.close();

sleep.java

Thread.sleep(1000);

split-by-words.java

text.split("\\W+");

start-browser.java

Desktop.getDesktop().browse(new URI(url));

str-to-char.java

// If your string contains exactly one character the simplest way to convert it to a character is probably to call the charAt method:

char c = s.charAt(0);

vending-machine.java

public class VendingMachine {
  int[] coins;

  public VendingMachine(int[] coins) {
    this.coins = coins;
  }
  
  public int[] getCoinsForExchange(int exchange) {
    ArrayList<Integer> finalCoins = new ArrayList<>();
    
    for(int i = 0; i < coins.length; i++) {
      int coin = coins[i];
      int diff = exchange / coin;
      System.out.format("%s, %s\n", i, diff);
      if (diff <= 1) {
        for(int j = 0; j < diff; j++) {
          finalCoins.add(coin);
          exchange = exchange - coin;
        }
      }
    }
    
    return finalCoins.stream().mapToInt(i -> i).toArray();
  }
}