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
}
}