/Java-interview-cheat-sheet

Java-interview-cheat-sheet

Code Samples

Twosum:

class Solution {
    public int[] twoSum(int[] nums, int target) {
        HashMap<Integer, Integer> mp = new HashMap<Integer, Integer>();
        int[] res;
        for (int i = 0; i < nums.length; i++){
            int complement = target - nums[i];
            if (mp.containsKey(nums[i])){
                return new int[] {i, mp.get(nums[i])};
            } else {
                mp.put(complement, i);
            }
        }
        return new int[] {};
    }
}

Java language semantics:

  • A public variable is accessible from anywhere (well, anywhere where the class is accessible). A private variable is only accessible inside the class. A static variable belongs to the class rather than to an instance of a class.
  • Java int vs. Integer type: an int simply represents a whole number, while an Integer has additional properties and methods. The int is one of Java’s eight Java primitive types, while the Integer wrapper class is one of hundreds of components included in the Java API. For example, if you want to declare a hash map of integer key-value pairs, HashMap<Integer, Integer> mp = new HashMap<Integer, Integer>();
  • Take note of the new keyword to create a new HashMap, Set, ArrayList, etc.!
  • System.out.println("hi");
  • Syntactically, "pointers" don't really exist in Java.

Set

Set interface is declared as public interface Set extends Collection Set<Integer> s = new Set<Integer>(); List of common methods:

  • s.add(elem);
  • s.remove(elem);
  • s.isEmpty();
  • s.size();
  • s.toArray()
  • s.clear()

There's also SortedSet, TreeSet, EnumSet, HashSet,

Arraylist

import java.util.ArrayList
ArrayList<String> cars = new ArrayList<String>("BMW, Toyota");
cars.add("Jeep"); // add to list
cars.add("tank");
cars.get(1); // access list
cars.set(0, "Honda"); // change item in list
cars.remove(0); // remove elem at idx 0 in arraylist
cars.size()

Java string

String s = "fasd"
s.charAt(index)
s.toLowerCase()
s.toUpperCase()
s.isLetterOrDigit()
// If you would like to use these methods per Character in the string, use the Character class
Character.isLetterOrDigit(s.charAt(index))
Character.toLowerCase(s.charAt(index))