• java.lang -> String, StringBuilder, StringBuffer (synchronized, slow), Math
• java.io -> File, BufferedInputStream, BufferedReader
• java.math -> BigDeciman, BigInteger, RoundingMode
• java.util -> Collections, ArrayList, HashMap, LinkedList, UUID, TimeZone, Objects

ASCII values:

• a-z = 97 - 122
• A-Z = 65 - 90
• 0-9 = 48 - 57

     var visited = new int[128];
     for(int i=0; i<128;i++){
       visited[i] = -1;
     }

     System.out.println("Has char r been visited?" + visited['r']);
     System.out.println("ASCII value of char a is: " + (int) Character.valueOf('a'));

• Implicit (Widening) Casting: Automatic When converting a smaller primitive type to a larger primitive type (e.g., int to long). No data loss.
• Explicit (Narrowing) Casting: Manual When converting a larger primitive type to a smaller type (e.g., double to int). Requires explicit casting syntax (targetType) and can result in data loss.

Order of Size (Narrow to Wide): byte > short > char > int > long > float > double

String Parses static methods:

• Integer.parseInt(String str) -> int;
• Long.parseLong(String str) -> long;
• Double.parseDouble(String str) -> double;
• Float.parseFloat(String str) -> float;

• Reverse string: Use StringBuilder.reverse(); StringBuffer is thread-safe and for that reason its slow. or you can also use pointer from length() to 0.

Useful String methods:

• str.toCharArray() -> char[]
• str.indexOf("m") -> int
• str.indexOf('m', 1) -> int
• str.indexOf('m', 1, 24) -> int
• str.lastIndexOf('m'); -> int
• str.startsWith("abc") -> boolean
• str.endsWith("xyz") -> boolean
• str.charAt(5)
• str.codePointAt(0) - returns ascii value of char at that index a-z = 97 - 122; A-Z = 65 - 90; 0-9 = 48 - 57
• str.toUpperCase(); -> String
• str.toLowerCase(); -> String
• str.length(); -> int
• str.hashcode();
• str.substring(int beginingIndex) -> String
• str.substring(int beginIndex, int endIndex) -> String
• str.concat("xyz") -> String
• str.replace("abc", "xyz") -> String
• str.replaceFirst(String regex, String replacement) -> String
• str.replaceAll(String regex, String replacement) -> String
• str.contains("abc") -> bool
• str.matches(String regex) -> bool
• str.isEmpty() -> bool " " -> false
• str.isBlank() -> " " -> true
• str.split(String regex, int limit) -> String[]
• str.split(String regex) -> String[]
• str.trim() -> removes spaces
• str.strip() -> removes spaces, even the unicode spaces like \u2000 (space)

• Math.random()
• Math.max(a,b)
• Math.min(a,b)
• Math.abs(-1) = 1
• Math.pow(a,b) = a^b
• Math.log()
• Math.log10()
• Math.sqrt(4) = 2
• Math.cbrt(9) = 3
• Math.round(10.45) = 10
• Math.floot(20.6) = 20
• Math.ceil(20.1) = 21

1. HashSet & TreeSet (Set Interface)

        Set<Character> ch = new HashSet();
        ch.add('a');
        ch.remove('a');
        ch.contains('a');
        ch.isEmpty();
        ch.size();
        Character[] alternative = ch.toArray(new Character[0]);

2. HashMap & TreeMap (Map Interface)

        Map<String,String> aMap = new HashMap<>();
        aMap.put("abc", "123");
        aMap.get("abc");
        aMap.remove("abc");
        aMap.containsKey("abc");
        aMap.size();
        for (Map.Entry<String, Integer> entry : map.entrySet()) {
            String key = entry.getKey();
            Integer value = entry.getValue();
            
            System.out.println("Key: " + key + ", Value: " + value);
        }

3. ArrayList (List Interface)

        List<String> aList = new ArrayList();
        aList.add("asf");
        aList.get(0);
        aList.getFirst();
        aList.getLast();
        aList.remove("asf");
        aList.remove(0);
        aList.removeFirst();
        aList.removeLast();
        aList.size();
        aList.sort(null);

4. LinkedList / ArrayDeque (Queue & Deque Interfaces)

5. PriorityQueue (Queue Interface)

HINTS TO REMEMBER

• Collections.swap(aList, 0, 0);
• Collections.replaceAll(null, null, null);
• Collections.max(null);
• Collections.min(null);
• Collections.copy(null, null);
• Collections.indexOfSubList(aList, alt);

Sorting:

• Collections.sort(list); — Sorts an ArrayList in natural ascending order.
• Collections.sort(list, Collections.reverseOrder()); — Sorts in descending order.
• Arrays.sort(arr); — Sorts primitive arrays efficiently (O(N \log N)).

Searching:

• Collections.binarySearch(list, key); — Fast lookup on a sorted list (O(\log N)).

Reversing & Shuffling:

• Collections.reverse(list); — Reverses elements in-place.
• Collections.frequency(list, target); — Counts occurrences of an element.

Primitive Wrapper Conversions:

• List list = Arrays.asList(1, 2, 3); (Fixed-size).
• List.of(1, 2, 3); (Java 9+, Immutable).

Read File:

import java.nio.file.Files;
import java.nio.file.Path;
import java.util.List;

String content = Files.readString(Path.of("input.txt"));

// OR

List<String> lines = Files.readAllLines(Path.of("input.txt"));
for (String line : lines) {
    // Process each line
}

Write File:

import java.nio.file.Files;
import java.nio.file.Path;
import java.util.List;

// Write a full string
Files.writeString(Path.of("output.txt"), "Hello, Interviewer!");

// Write a list of lines
List<String> lines = List.of("Line 1", "Line 2", "Line 3");
Files.write(Path.of("output.txt"), lines);

import java.net.URI;
import java.net.http.HttpClient;
import java.net.http.HttpRequest;
import java.net.http.HttpResponse;

public class HttpPostExample {
    public static void main(String[] args) throws Exception {
        HttpClient client = HttpClient.newHttpClient();

        String jsonPayload = "{\"name\": \"Interviewee\", \"score\": 100}";

        HttpRequest request = HttpRequest.newBuilder()
                .uri(URI.create("https://httpbin.org/post"))
                .header("Content-Type", "application/json")
                .header("Accept", "application/json")
                .POST(HttpRequest.BodyPublishers.ofString(jsonPayload))
                .build();

        HttpResponse<String> response = client.send(
                request, 
                HttpResponse.BodyHandlers.ofString()
        );

        System.out.println("Status Code: " + response.statusCode());
        System.out.println("Response Body: " + response.body());
    }
}