emahtab/java-refresher

Java Refresher

Java Refresher

Empty array

int[] emptyArray = new int[0]; , but this won't even compile int[] compileError = new int[];

ArrayDeque Vs. LinkedList

java.util.ArrayDeque doesn't allow null while java.util.LinkedList does allow adding null.

Collections.reverse() - works fine with list of lists 👍

list = [[1,2], [3], [4,5] , [6]]
Collections.reverse(list) 
// [[6], [4,5], [3], [1,2]]

Convert an int to binary string

Integer.toBinaryString(2) => 10
Integer.toBinaryString(-2) => 11111111111111111111111111111110

String substring() method

String s = "a";
System.out.println(s.substring(1));   => "" (empty string)
System.out.println(s.substring(2));   => StringIndexOutOfBoundsException  

Concat String and char

String s = "a" + "hello".charAt(1);
System.out.println(s); //ae

**StringBuilder apppend **

StringBuilder sb = new StringBuilder();
sb.append('a');
sb.append(1);
sb.append("ABC");
System.out.println(sb); // a1ABC

List add(int index, T element)

List<Integer> list = new ArrayList<>();
list.add(1); list.add(2); list.add(3);
list.add(1, 5);
System.out.println(list);  // [1, 5, 2, 3]

List set(int index, T element)

List<Integer> list = new ArrayList<>();
list.add(1); list.add(2); list.add(3);
list.set(1, 5);
System.out.println(list);  // [1, 5, 3]

Set contains()

import java.util.*;
class Main {
  class Employee {
    int id;
    Employee(int id){
      this.id = id;
    }
  }
  public static void main(String[] args) {
    new Main().check();
  }

  private void check() {
    Employee e1 = new Employee(1);
    Employee e2 = new Employee(1);
    Set<Employee> set = new HashSet<>();
    set.add(e1);
    System.out.println(set.contains(e2)); // false
    set.add(e2);
    Employee e3 = e2;
    System.out.println(set.contains(e3)); // true
  }
}