Java Basics:String Methods-6

 

Java String hashCode()

The Java String hashCode() method returns a hash code for the string.

A hashcode is a number (object's memory address) generated from any object, not just strings. This number is used to store/retrieve objects quickly in a hashtable.

The syntax of the string hashCode() method is:

string.hashCode()

Here, string is an object of the String class.

---

hashCode() Parameters

The matches() method doesn't take any parameters.

---

hashCode() Return Value

• returns the hashcode, which is an int value, of the string

The hash code is computed using formula:

s[0]*31(n-1) + s[1]*31(n-2) + ... + s[n-1]

where,

• s[0] is the first element of string s, s[1] is the second element and so on.
• n is the length of the string

---

Example: Java String hashCode()

class Main {
  public static void main(String[] args) {
    String str1 = "Java";
    String str2 = "Java Programming";
    String str3 = "";

    System.out.println(str1.hashCode()); // 2301506
    System.out.println(str2.hashCode()); // 1377009627

    // hash code of empty string is 0
    System.out.println(str3.hashCode()); // 0

  }
}

Note: For two strings to be equal, their hash code also must be equal.

Java String join()

The Java String join() method returns a new string with the given elements joined with the specified delimiter.

The syntax of the string join() method is either:

String.join(CharSequence delimiter, 
            Iterable elements)

or

String.join(CharSequence delimiter, 
            CharSequence... elements)

Here, ... signifies there can be one or more CharSequence.

Note: join() is a static method. You do not need to create a string object to call this method. Rather, we call the method using the class name String.

---

join() Parameters

The join() method takes two parameters.

• delimiter - the delimiter to be joined with the elements
• elements - elements to be joined

Notes:

• You can pass any class that implements CharSequence to join().
• If an iterable is passed, its elements will be joined. The iterable must implement CharSequence.
• String, StringBuffer, CharBuffer etc. are CharSequence as these classes implement it.

---

join() Return Value

• returns a string

---

Example 1: Java String join() With CharSequence()

class Main {
  public static void main(String[] args) {
    String result;

    result = String.join("-", "Java", "is", "fun");
    System.out.println(result);  // Java-is-fun

  }
}

Here, we have passed three strings Java, is and fun to the join() method. These string are joined using the - delimiter.

---

Example 2: Java String join() With Iterable

import java.util.ArrayList;

class Main {
  public static void main(String[] args) {
    ArrayList<String> text = new ArrayList<>();

    // adding elements to the arraylist
    text.add("Java");
    text.add("is");
    text.add("fun");

    String result;

    result = String.join("-", text);
    System.out.println(result);  // Java-is-fun

  }
}

Here, an ArrayList of String type is created. The elements of ArrayList are joined using the - delimiter.

Java String replaceFirst()

The Java String replaceFirst() method replaces the first substring that matches the regex of the string with the specified text.

The syntax of the replaceFirst() method is:

string.replaceFirst(String regex, String replacement)

Here, string is an object of the String class.

---

replaceFirst() Parameters

The replaceFirst() method takes two parameters.

• regex - a regex (can be a typical string) that is to be replaced
• replacement - the first matching substring is replaced with this string

---

replaceFirst() Return Value

• The replaceFirst() method returns a new string where the first occurrence of the matching substring is replaced with the replacement string.

---

Example 1: Java String replaceFirst()

class Main {
  public static void main(String[] args) {
      String str1 = "aabbaaac";
      String str2 = "Learn223Java55@";

      // regex for sequence of digits
      String regex = "\\d+";

      // the first occurrence of "aa" is replaced with "zz"
      System.out.println(str1.replaceFirst("aa", "zz")); // zzbbaaac

      // replace the first sequence of digits with a whitespace
      System.out.println(str2.replaceFirst(regex, " ")); // Learn Java55@
  }
}

In the above example, "\\d+" is a regular expression that matches a sequence of digits. To learn more, visit Java regex.

---

Escaping Characters in replaceFirst()

The replaceFirst() method can take a regex or a typical string as the first argument. It is because a typical string in itself is a regex.

In regex, there are characters that have special meaning. These metacharacters are:

\ ^ $ . | ? * + {} [] ()

If you need to match substring containing these metacharacters, you can escape these characters using \.

// Program to the first + character
class Main {
  public static void main(String[] args) {
    String str = "a+a-++b";

    // replace the first "+" with "#"
    System.out.println(str.replaceFirst("\\+", "#")); // a#a-++b

  }
}

---

Java String subSequence()

The Java String subSequence() method returns a character sequence (a subsequence) from the string.

The syntax of the subSequence() method is:

string.subSequence(int startIndex, int endIndex)

Here, string is an object of the String class.

---

subSequence() Parameters

The subSequence() method takes two parameters.

• startIndex - the starting index
• endIndex - the ending index

---

subSequence() Return Value

• The subSequence() method returns a CharSequence.

---

Example: Java String subSequence()

class Main {
  public static void main(String[] args) {
    String str = "Java Programming";

    System.out.println(str.subSequence(3, 8)); // a Pro
  }
}

Java String toCharArray()

The Java String toCharArray() method converts the string to a char array and returns it.

The syntax of the toCharArray() method is:

string.toCharArray(int startIndex, int endIndex)

Here, string is an object of the String class.

---

toCharArray() Parameters

The toCharArray() method doesn't take any parameters.

---

toCharArray() Return Value

• returns a char array

---

Example: Java String toCharArray()

class Main {
  public static void main(String[] args) {
    String str = "Java Programming";

    // creating a char array
    char[] result;

    result = str.toCharArray();
    System.out.println(result); // Java Programming
  }
}

Here, the length of the char array result will be equal to the length of the string str.