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Yash edited this page May 23, 2025 · 8 revisions

String

The String class represents character strings. All string literals in Java programs, such as "abc", are implemented as instances of this class.

public String intern()

A pool of strings, initially empty, is maintained privately by the class String.

When the intern method is invoked, if the pool already contains a string equal to this String object as determined by the equals(Object) method, then the string from the pool is returned. Otherwise, this String object is added to the pool and a reference to this String object is returned.

String ReverseObject, memory address, {equals (==)} 
public class ReverseObject {
  public static void main(String[] args) {
    // String is immutable Object, once created we cannot change the object
    //The SCP is an area inside the heap memory. It contains the unique strings. 
    //In order to put the strings in the string pool, one needs to call the intern().
    String s1_scp = "Yash";
    System.out.println("SCP intern():"+s1_scp.intern());
    String s2_scp = "Yash";
    System.out.println("SCP intern():"+s2_scp.intern());
    
    String s3_h = new String("Yash");
    String s4_h = new String("Yash");
    
    System.out.println("Memory Address & Content :"+ (s1_scp == s2_scp));
    System.out.println("Memory Address & Content :"+ (s3_h   == s4_h));
    
    System.out.println("Content :"+ s1_scp.equals(s2_scp) );
    System.out.println("Content :"+ s3_h.equals(s4_h));
    
    String str = "Yashwanth";
    stringReverse(str, false);
    stringReverse(str, true);
    
    int num = 1234;
    reverseNumberString(num);
    reverseNumber(num);
    
    StringBuffer buff = new StringBuffer(str);
    System.out.println("String Buffer : "+ buff.reverse());
    
    // Use List interface to reverse string.
    char[] ch = str.toCharArray();
    List<Character> reverse = new LinkedList<>();
    for (int i = ch.length - 1; i >= 0; i--) {
      reverse.add(ch[i]);
    }
    System.out.println("Reverse of a String with LIST : "+ reverse);
  }
  
  public static String stringReverse(String str, boolean isDistinct) {
    char[] ch = str.toCharArray();
    String reverse = "";
    for (int i = ch.length - 1; i >= 0; i--) {
      if ( isDistinct && !(reverse.indexOf(ch[i]) > -1) ) // Remove duplicates
        reverse += ch[i];
      if (!isDistinct) reverse += ch[i];
    }
    System.out.println("Reverse of a String: "+ reverse +", DISTINCT:"+isDistinct);
    return reverse;
  }

  public static int reverseNumberString(int num) {
    String str = new Integer(num).toString();
    char[] ch = str.toCharArray();
    String reverse = "";
    for (int i = ch.length - 1; i >= 0; i--) {
        reverse += ch[i];
    }
    System.out.println("Reverse of a String Number : "+ reverse);
    return Integer.parseInt(reverse);
  }

  public static int reverseNumber(int num) {
    int temp = num;
    int rev = 0;
    int sum = 0; // sumOfDigits
    while (temp > 0 ) {
      rev = (rev * 10) + temp % 10;
      sum += temp % 10;
      temp = temp / 10;
    }
    System.out.println("Reverse of a Number : "+ rev);
    System.out.println("Sum of Digits of a Number : "+ sum);
    
    if(rev == num) {
      System.out.println("Polyndrome Number : [121 = 121]");
    }
    return rev;
  }
}
Hash Code Collision in Java Strings - The behavior of String's hashCode(), equals(), and == operator — specifically using a known hash code collision. 
String[Aa], Hash[2112], System Hash[366712642]
String[BB], Hash[2112], System Hash[1829164700]
📄 Java Class: **`StringHashCollisionExample.java`**

This example illustrates how Java's String class behaves when comparing strings using:

  • == (reference equality) == checks memory address (identity).
  • .equals() (logical equality) .equals() checks logical equality (content)
  • .hashCode() (hashing logic used in collections) .hashCode() is used mainly for hashing (e.g. in HashMap, HashSet), and can collide for different objects. 
/**
 * Demonstrates the difference between ==, equals(), and hashCode() using Strings in Java.
 * Also shows an example of a hash code collision between two distinct strings: "Aa" and "BB".
 *
 * Key Concepts:
 * - `==` checks reference equality (same object in memory)
 * - `equals()` checks logical/content equality
 * - `hashCode()` provides a numeric representation used in hash-based collections like HashMap
 *
 * ⚠️ Even if two strings have the same hash code, they may not be equal.
 *
 * Usage:
 * - Run this class as a Java application to see the behavior printed in the console.
 * 
 * @author 🔐 Yash
 */
public class StringHashCollisionExample {

  public static void main(String[] args) {
    String s1 = "Aa";
    String s2 = "BB";
    
    System.out.println("String s1 = \"Aa\";");
    System.out.println("String s2 = \"BB\";\n");
    
    System.out.println("s1 == s2: " + (s1 == s2));
    System.out.println("s1.equals(s2): " + s1.equals(s2));
    System.out.println("s1.hashCode(): " + s1.hashCode());
    System.out.println("s2.hashCode(): " + s2.hashCode());
    
    System.out.println("\nSummary:");
    if (s1.hashCode() == s2.hashCode()) {
      System.out.println("- Hash codes are equal (collision): " + s1.hashCode());
    }
    if (!s1.equals(s2)) {
      System.out.println("- But strings are not equal: \"" + s1 + "\" != \"" + s2 + "\"");
    }
  }
}

⚠️ Important Notes:

  • "Aa" and "BB" have the same hash code: 2112.
  • They are not equal by content (equals() returns false).
  • They are not the same object in memory (== returns false).
  • This is a classic example of a hash collision in Java.

🔬 Why does it happen?

Java computes the hash code for a string like this:

// s[0]*31^(n-1) + s[1]*31^(n-2) + ... + s[n-1]
public int hashCode() {
    int h = 0;
    for (int i = 0; i < value.length; i++) {
        h = 31 * h + value[i];
    }
    return h;
}

Using this logic:

"Aa" → 65*31 + 97 = 2112

"BB" → 66*31 + 66 = 2112

ASCII Codes Table ascii.cl

ASCII (Decimal) Hex ASCII Character
65 41 A
66 42 B
97 61 a
98 62 b

❗ Important Note: Even if two objects have the same hash code, they are not guaranteed to be equal, and == doesn’t care about hash codes at all. It’s only about whether they’re the exact same object.

String Trim special chars Unicode's 1.1, general_punctuation, invisible-characters-unwanted 

String s1 = "My Sample Space Data", s2 = "My Sample Space Data";
System.out.format("S1: %s\n", java.util.Arrays.toString(s1.getBytes()));
System.out.format("S2: %s\n", java.util.Arrays.toString(s2.getBytes()));
Trim example with Unicodes chars 
public class StringUtil {
  public static void main(String[] args) {
    //String text = "  Hello World!  \uFEFF\u200B\u00A0 "; // Example string with various spaces
    String text = "    Hello   World! -  "+getUnicodeSpaces(); // \u2060-\u237D-\uFEFF
    
    // Print the original and trimmed text
    System.out.println("Original text: \"" + text + "\"");
    System.out.println("Original text: \"" + text.trim() + "\""); // Space, TabSpace
    System.out.println("Trimmed text: \"" + trimSpaces(text) + "\"");
    System.out.println("Trimmed text: \"" + trimAdvanced(text, getUnicodeSpaces()) + "\"");
    
  }
  
  //@Data - Only supported on the class, @Setter - On Class/Fields
  @AllArgsConstructor @NoArgsConstructor @Getter
  enum Unicodes {
    WHITESPACE("\u0020", ".", "SP - WhiteSpace"),
    
    // # FORMAT chars
    ENSP ("\u2002", "ENSP", "EN SPACE"),
    EMSP ("\u2003", "EMSP", "EM SPACE"),
    EMSP3("\u2004", "3/EMSP", "THREE-PER-EM SPACE"),
    EMSP4("\u2005", "4/EMSP", "FOUR-PER-EM SPACE"),
    EMSP6("\u2006", "6/EMSP", "SIX-PER-EM SPACE"),
    FSP  ("\u2007", "FSP", "FIGURE SPACE"),
    PSP  ("\u2008", "PSP", "PUNCTUATION SPACE"),
    THSP ("\u2009", "THSP", "THIN SPACE"),
    
    HSP  ("\u200A", "HSP", "HAIR SPACE"),
    ZWSP ("\u200B", "ZWSP", "ZERO WIDTH SPACE"),
    ZWNJ ("\u200C", "ZWNJ", "ZERO WIDTH NON-JOINER"),
    ZWJ  ("\u200D", "ZWJ", "ZERO WIDTH JOINER"),
    LRM  ("\u200E", "LRM", "LEFT-TO-RIGHT MARK"),
    RLM  ("\u200F", "RLM", "RIGHT-TO-LEFT MARK"),
    
    LS  ("\u2028", "LS", "LINE SEPARATOR"),
    PS  ("\u2029", "PS", "PARAGRAPH SEPARATOR"),
    LRE ("\u202A", "LRE", "LEFT-TO-RIGHT EMBEDDING"),
    RLE ("\u202B", "RLE", "RIGHT-TO-LEFT EMBEDDING"),
    PDF ("\u202C", "PDF", "POP DIRECTIONAL FORMATTING"),
    LRO ("\u202D", "LRO", "LEFT-TO-RIGHT OVERRIDE"),
    RLO ("\u202E", "RLO", "RIGHT-TO-LEFT EMBEDDING"),
    NNBSP("\u202F", "NNBSP", "ZERO WIDTH SPACE"),
    
    MMSP("\u205F", "MMSP", "MEDIUM MATHEMATICAL SPACE"),
    WJ  ("\u2060", "WJ",   "WORD JOINER"),
    ZWNBSP("\uFEFF", "ZWNBSP", "ZERO WIDTH NO-BREAK SPACE"),
    
    // # INVISIBLE chars
    FA  ("\u2061", "(FA)",   "Function application"),
    IT  ("\u2062", "(IT)",   "invisible times"),
    IS  ("\u2063", "(IS)",   "invisible separator"),
    IP  ("\u2064", "(IP)",   "invisible plus"),
    
    // # DEPRECATED chars
    ISS  ("\u206A", "ISS",   "INHIBIT SYMMETRIC SWAPPING"),
    ASS  ("\u206B", "ASS",   "ACTIVATE SYMMETRIC SWAPPING"),
    IAFS ("\u206C", "IAFS",   "INHIBIT ARABIC FORM SHAPING"),
    AAFS ("\u206D", "AAFS",   "ACTIVATE ARABIC FORM SHAPING"),
    NADS ("\u206E", "NADS",   "NATIONAL DIGIT SHAPES"),
    NODS ("\u206F", "NODS",   "NOMINAL DIGIT SHAPES"),
    
    NBSP ("\u00A0", "NBSP", "NO-BREAK SPACE (&nbsp;)"),
    
    BACKSPACE1("\u0008", "BKSP", "BACKSPACE"),
    BACKSPACE("\u2408", "BS", "SYMBOL FOR BACKSPACE (␈)"),
    SPACE("\u2420", "SP", "SYMBOL FOR SPACE (␠)"),
    
    IDSP("\u3000", "IDSP", "IDEOGRAPHIC SPACE"),
    
    IHSPACE("\u303F", "ZWSP", "IDEOGRAPHIC HALF FILL SPACE"),
    TAB("  ", "TAB", "TAB SPACE"),
    ;
    
    private String CODE, NAME, DESCRIPTION;
  }
  
  public static String getUnicodeSpaces() {
    // Get the size of the enum
    //System.out.println("Size of enum: " + Unicodes.values().length);
    
    // Append all CODE values to a string
    StringBuilder codesStringBuilder = new StringBuilder();
    for (Unicodes unicode : Unicodes.values()) {
      codesStringBuilder.append(unicode.getCODE());
    }
    String allCodes = codesStringBuilder.toString();
    //System.out.println("All codes: " + allCodes);
    return allCodes;
  }
  
  // Define a regular expression to match all kinds of spaces
  public static String trimSpaces(String text) {
    //String spaceTrimRegexLeading = "^["+ Pattern.quote(getUnicodeSpaces()) +"]+"; // 
    // Match leading and trailing spaces
    String spaceTrimRegex = "^[" + Pattern.quote(getUnicodeSpaces()) + "]+|[" + Pattern.quote(getUnicodeSpaces()) + "]+$";
    return text.replaceAll(spaceTrimRegex, "");
  }
  public static String trimAdvanced(String value, String skipString) {
    Objects.requireNonNull(value);
    
    int strLength = value.length();
    if (strLength == 0) return value;
    
    int len = value.length();
    int st = 0;
    char[] val = value.toCharArray();
    
    // Trim leading characters specified in skipString
    while ((st < len) && (skipString.indexOf(val[st]) >= 0)) {
      st++;
    }
    
    // Trim trailing characters specified in skipString
    while ((st < len) && (skipString.indexOf(val[len - 1]) >= 0)) {
      len--;
    }
    
    // Return trimmed substring
    return (st > len) ? "" : ((st > 0) || (len < strLength)) ? value.substring(st, len) : value;
  }
}
String PayLoad Validation 
<dependency>
    <groupId>org.json</groupId>
    <artifactId>json</artifactId>
    <version>20240303</version>
</dependency>

PayLoad

{
  "employees": {
    "employee": {
        "id": "1",
        "firstName": "Tom",
        "postalCode": [
          10
          ],
        "address": {
          "pin": "123"
        },
        
        "lastName": "Cruise"
      }
  }
}
import org.apache.commons.lang3.StringUtils;

public class StringPayload {

  public static String payLoadInSingleLine(String payload) {
    // Convert payLoad string to byte array
    byte[] bytes = payload.getBytes(StandardCharsets.UTF_8);
    // Create ByteArrayInputStream from byte array
    ByteArrayInputStream inputStream = new ByteArrayInputStream(bytes);
    // Create InputStreamReader from ByteArrayInputStream
    InputStreamReader reader = new InputStreamReader(inputStream, StandardCharsets.UTF_8);
    // Now you can use the reader for further processing
    String requestBody = new BufferedReader(reader).lines().reduce(String::concat).get();
    return requestBody;
  }
  public static void main(String[] args) {
    String payload = "";

    String requestBody = payLoadInSingleLine(payload);
    System.out.println(requestBody.length() +" - requestBody :"+requestBody);
    
    // Replace multiple spaces with a single space using regular expression
    String result = requestBody.replaceAll("\\s+", " ");
    System.out.println(result.length() +" - Replace multiple spaces :"+result);
    
    System.out.println("isValidJson(jsonStr): " + isValidPayload(result));
    System.out.println("getJsonString(jsonStr): " + getJsonString(result));
    System.out.println("validateAndConvertJsonToMap(jsonStr): " + validateAndConvertJsonToMap(result));
  }

  public static Map<String, Object> validateAndConvertJsonToMap(String json) { // readValue
    try {
      if(json != null)
        return new com.fasterxml.jackson.databind.ObjectMapper()
            .readValue(json, 
            new com.fasterxml.jackson.core.type.TypeReference<java.util.HashMap<String, Object>>() {});
    } catch (Exception e) {
      System.err.println("Invalid JSON: " + e.getMessage());
    }
    return null; // Return null if the JSON is invalid
  }
  
  public static boolean isValidPayload(String payload) { // readTree
    ObjectMapper objectMapper = new com.fasterxml.jackson.databind.ObjectMapper();
    objectMapper.configure(com.fasterxml.jackson.databind.DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false);
    try {
      objectMapper.readTree(payload);
      return true; // Valid JSON
    } catch (com.fasterxml.jackson.core.JsonProcessingException e) {
      System.out.println("--- JsonProcessingException:"+ e.getLocalizedMessage());
      return Boolean.FALSE; // Invalid JSON
    }
  }
  private static String getJsonString(String payload) {
    System.out.println("Parse the JSON string to either JSONObject or JSONArray");
    Object json;
    try {
      json = new org.json.JSONObject( payload );
    } catch (org.json.JSONException e) {
      try {
        System.out.println("===== JSON-Object::"+ e.getLocalizedMessage());
        json = new org.json.JSONArray( payload );
      } catch (org.json.JSONException ex) {
        System.out.println("===== JSON-Array::"+ ex.getLocalizedMessage());
        //throw new IllegalArgumentException("Invalid JSON format");
        System.err.println("=== Invalid JSON Ex:"+ payload);
        return payload;
      }
    }
    return json.toString();
  }
}
function isValidJson(jsonString) {
    try {
        JSON.parse(jsonString);
        return { isValid: true, errorMessage: null }; // JSON is valid
    } catch (error) {
        return { isValid: false, errorMessage: error.message }; // JSON is invalid
    }
}

const payload = ``;

const validationResult = isValidJson(payload);
console.log("Is valid JSON:", validationResult.isValid);
if (!validationResult.isValid) {
    console.log("Error message:", validationResult.errorMessage);
}

intern() « Before saving in String Constant Pool(SCP) it invokes intern() method to check object availability with same content in pool using equals method. If String-copy is available in the Pool then returns the reference. Otherwise, String object is added to the pool and returns the reference.
hash() « String @override hash() of Object class to generate same hash code for sequence of characters.
equals and == « == operator is to check the hash code where as equals() method is to check the content of a String.
immutable «


What's the difference between length and length()

int[] myArray = new int[10];
String myString = "hello world!";
List<int> myList = new ArrayList<int>();

myArray.length    //gives the length of the array
myString.length() //gives the length of the string
myList.size()     //gives the length of the list

String reverse program.

public static String reverseStr(String str) {
    //String str = "Yashwanth"; //https://stackoverflow.com/a/59166517/5081877
    String collect = IntStream.range(0, str.length())
            .boxed().sorted(Collections.reverseOrder())
            .map(i -> { 
                System.out.println(i);
                return String.valueOf(str.charAt(i)); 
                })
            .distinct().collect(Collectors.joining("")); //htnawsY
            //.collect(Collectors.joining("")); //htnawhsaY
            //.collect(Collectors.joining(".")); //h.t.n.a.w.h.s.a.Y
    System.out.println("Reverse Str:"+collect);
    return collect;
}

In a class-based object-oriented language, in general, state is carried by instances, methods are carried by classes, and inheritance is only of structure and behavior. Basic, Refactoring Techniques

Method signature: It consists of method name and parameter list (number/type/order of the parameters). methodName(parametersList y). An instance method in a subclass with the same signature and return type as an instance method in the super-class overrides the super-class's method.

Java OOP concepts
Class  - Collection of a common features of a group of object [static/instance Fields, blocks and Methods]
Object - Instance of a class (instance fields)
Abstraction - Process of hiding complex info and providing required info like API, Marker Interfaces ...
Encapsulation(Security) - Class Binding up with data members(fields) and member functions.
Inheritance  (Reusability by placing common code in single class)
 1. Multilevel - {A -> B -> C} 2. Multiple - Diamond problem {A <- (B) -> C} [Java not supports] 3. Cyclic {A <-> B} [Java not supports]
 * Is-A   Relation - Class A extends B
 * Hash-A Relation - Class A { B obj = new B(); } - (Composition/Aggregation)
Polymorphism (Flexibility) 1. Compile-Time Overloading    2. Runtime Overriding  [Greek - "many forms"]


int[] arr = {1,2,3}; int arrLength = arr.length;   // Fixed length of sequential blocks to hold same data type
String str = "Yash"; int strLength = str.length(); // Immutable Object value can't be changed.

List<?> collections = new ArrayList<String>(); int collectionGroupSize = collections.size();
Map<?, ?> mapEntry = new HashMap<String, String>();
Set<?> keySet = mapEntry.keySet(); // Set of Key's
Set<?> entrySet = mapEntry.entrySet(); // Set of Entries [Key, Value]

// Immutable Objects once created they can't be modified. final class Integer/String/Employee
Integer val = Integer.valueOf("100"); String str2 = String.valueOf(100); // Immutable classes
final class Employee { // All Wrapper classes, java.util.UUID, java.io.File ...
    private final String empName; // Field as Final(values can be assigned only once) Only getter functions.
    public Employee(String name) { this.empName = name; }
}

Native Java Code for Hashtable.h, Hashtable.cpp SQL API.

You can check your current JDK and JRE versions on your command prompt respectively,

  • JDK javac -version [C:\Program Files\Java\jdk1.8.0_121\bin] o/p: javac 1.8.0_121
  • JRE java -version [C:\Program Files\Java\jdk1.8.0_121\bin] o/P: java version "1.8.0_102"

JAVA_HOME - Must be set to JDK otherwise maven projects leads to compilation error. [ERROR] No compiler is provided in this environment. Perhaps you are running on a JRE rather than a JDK? C:\Softwares\OpenJDK\, 7-zip

Fatal error compiling: invalid target release: JRE and JDK must be of same version 1.8.0.XXX

Disable TLS 1.0 and 1.1security-libs/javax.net.ssl: TLS 1.0 and 1.1 are versions of the TLS protocol that are no longer considered secure and have been superseded by more secure and modern versions (TLS 1.2 and 1.3).

Core Java

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