String class in Java

String is a sequence of characters. In java, objects of String are immutable which means a constant and cannot be changed once created.

Creating a String

There are two ways to create string in Java:

• String literal

  String s = “GeeksforGeeks”;

• Using new keyword

  String s = new String (“GeeksforGeeks”);

Constructors

1. String(byte[] byte_arr) – Construct a new String by decoding the byte array. It uses the platform’s default character set for decoding.
   Example:

   byte[] b_arr = {71, 101, 101, 107, 115};
   String s_byte =new String(b_arr); //Geeks
   

2. String(byte[] byte_arr, Charset char_set) – Construct a new String by decoding the byte array. It uses the char_set for decoding.
   Example:

   byte[] b_arr = {71, 101, 101, 107, 115};
   Charset cs = Charset.defaultCharset();
   String s_byte_char = new String(b_arr, cs); //Geeks
   

3. String(byte[] byte_arr, String char_set_name) – Construct a new String by decoding the byte array. It uses the char_set_name for decoding.
   It looks similar to the above constructs and they appear before similar functions but it takes the String(which contains char_set_name) as parameter while the above constructor takes CharSet.
   Example:

   byte[] b_arr = {71, 101, 101, 107, 115};
   String s = new String(b_arr, "US-ASCII"); //Geeks
   

4. String(byte[] byte_arr, int start_index, int length) – Construct a new string from the bytes array depending on the start_index(Starting location) and length(number of characters from starting location).
   Example:

   byte[] b_arr = {71, 101, 101, 107, 115};
   String s = new String(b_arr, 1, 3); // eek
   

5. String(byte[] byte_arr, int start_index, int length, Charset char_set) – Construct a new string from the bytes array depending on the start_index(Starting location) and length(number of characters from starting location).Uses char_set for decoding.
   Example:

   byte[] b_arr = {71, 101, 101, 107, 115};
   Charset cs = Charset.defaultCharset();
   String s = new String(b_arr, 1, 3, cs); // eek
   

6. String(byte[] byte_arr, int start_index, int length, String char_set_name) – Construct a new string from the bytes array depending on the start_index(Starting location) and length(number of characters from starting location).Uses char_set_name for decoding.
   Example:

   byte[] b_arr = {71, 101, 101, 107, 115};
   String s = new String(b_arr, 1, 4, "US-ASCII"); // eeks
   

7. String(char[] char_arr) – Allocates a new String from the given Character array
   Example:

   char char_arr[] = {'G', 'e', 'e', 'k', 's'};
   String s = new String(char_arr); //Geeks
   

8. String(char[] char_array, int start_index, int count) – Allocates a String from a given character array but choose count characters from the start_index.
   Example:

   char char_arr[] = {'G', 'e', 'e', 'k', 's'};
   String s = new String(char_arr , 1, 3); //eek
   

9. String(int[] uni_code_points, int offset, int count) – Allocates a String from a uni_code_array but choose count characters from the start_index.
   Example:

   int[] uni_code = {71, 101, 101, 107, 115};
   String s = new String(uni_code, 1, 3); //eek
   

10. String(StringBuffer s_buffer) – Allocates a new string from the string in s_buffer
    Example:

    StringBuffer s_buffer = new StringBuffer("Geeks");
    String s = new String(s_buffer); //Geeks
    

11. String(StringBuilder s_builder) – Allocates a new string from the string in s_builder
    Example:

    StringBuilder s_builder = new StringBuilder("Geeks");
    String s = new String(s_builder); //Geeks
    

String Methods

1. int length(): Returns the number of characters in the String.

   "GeeksforGeeks".length();  // returns 13

2. Char charAt(int i): Returns the character at i^th index.

   "GeeksforGeeks".charAt(3); // returns  ‘k’

3. String substring (int i): Return the substring from the i^th  index character to end.

   "GeeksforGeeks".substring(3); // returns “ksforGeeks”

4. String substring (int i, int j): Returns the substring from i to j-1 index.

    "GeeksforGeeks".substring(2, 5); // returns “eks”

5. String concat( String str): Concatenates specified string to the end of this string.

    String s1 = ”Geeks”;
    String s2 = ”forGeeks”;
    String output = s1.concat(s2); // returns “GeeksforGeeks”
   

6. int indexOf (String s): Returns the index within the string of the first occurrence of the specified string.

    String s = ”Learn Share Learn”;
    int output = s.indexOf(“Share”); // returns 6
   

7. int indexOf (String s, int i): Returns the index within the string of the first occurrence of the specified string, starting at the specified index.

    String s = ”Learn Share Learn”;
    int output = s.indexOf("ea",3);// returns 13
   

8. Int lastIndexOf( String s): Returns the index within the string of the last occurrence of the specified string.

    String s = ”Learn Share Learn”;
    int output = s.lastIndexOf("a"); // returns 14
   

9. boolean equals( Object otherObj): Compares this string to the specified object.

    Boolean out = “Geeks”.equals(“Geeks”); // returns true
    Boolean out = “Geeks”.equals(“geeks”); // returns false
   

10. boolean  equalsIgnoreCase (String anotherString): Compares string to another string, ignoring case considerations.

     Boolean out= “Geeks”.equalsIgnoreCase(“Geeks”); // returns true
     Boolean out = “Geeks”.equalsIgnoreCase(“geeks”); // returns true

11.  int compareTo( String anotherString): Compares two string lexicographically.

     int out = s1.compareTo(s2);  // where s1 and s2 are
                                 // strings to be compared
    
     This returns difference s1-s2. If :
     out < 0  // s1 comes before s2
     out = 0  // s1 and s2 are equal.
     out > 0   // s1 comes after s2.
    

12. int compareToIgnoreCase( String anotherString): Compares two string lexicographically, ignoring case considerations.

     int out = s1.compareToIgnoreCase(s2);  
    // where s1 and s2 are 
    // strings to be compared
    
     This returns difference s1-s2. If :
     out < 0  // s1 comes before s2
     out = 0   // s1 and s2 are equal.
     out > 0   // s1 comes after s2.
    

    Note- In this case, it will not consider case of a letter (it will ignore whether it is uppercase or lowercase).

13. String toLowerCase(): Converts all the characters in the String to lower case.

    String word1 = “HeLLo”;
    String word3 = word1.toLowerCase(); // returns “hello"
    

14. String toUpperCase(): Converts all the characters in the String to upper case.

    String word1 = “HeLLo”;
    String word2 = word1.toUpperCase(); // returns “HELLO”
    

15. String trim(): Returns the copy of the String, by removing whitespaces at both ends. It does not affect whitespaces in the middle.

    String word1 = “ Learn Share Learn “;
    String word2 = word1.trim(); // returns “Learn Share Learn”
    

16.  String replace (char oldChar, char newChar): Returns new string by replacing all occurrences of oldChar with newChar.

    String s1 = “feeksforfeeks“;
    String s2 = “feeksforfeeks”.replace(‘f’ ,’g’); // returns “geeksgorgeeks”
    

    Note:- s1 is still feeksforfeeks and s2 is geeksgorgeeks

Program to illustrate all string  methods:

// Java code to illustrate different constructors and methods
// String class.

import java.io.*;
import java.util.*;
class Test
{
    public static void main (String[] args)
    {
        String s= "GeeksforGeeks";
        // or String s= new String ("GeeksforGeeks");

        // Returns the number of characters in the String.
        System.out.println("String length = " + s.length());

        // Returns the character at ith index.
        System.out.println("Character at 3rd position = "
                           + s.charAt(3));

        // Return the substring from the ith  index character
        // to end of string
        System.out.println("Substring " + s.substring(3));

        // Returns the substring from i to j-1 index.
        System.out.println("Substring  = " + s.substring(2,5));

        // Concatenates string2 to the end of string1.
        String s1 = "Geeks";
        String s2 = "forGeeks";
        System.out.println("Concatenated string  = " +
                            s1.concat(s2));

        // Returns the index within the string
        // of the first occurrence of the specified string.
        String s4 = "Learn Share Learn";
        System.out.println("Index of Share " +
                           s4.indexOf("Share"));

        // Returns the index within the string of the
        // first occurrence of the specified string,
        // starting at the specified index.
        System.out.println("Index of a  = " +
                           s4.indexOf('a',3));

        // Checking equality of Strings
        Boolean out = "Geeks".equals("geeks");
        System.out.println("Checking Equality  " + out);
        out = "Geeks".equals("Geeks");
        System.out.println("Checking Equality  " + out);

        out = "Geeks".equalsIgnoreCase("gEeks ");
        System.out.println("Checking Equality " + out);

        //If ASCII difference is zero then the two strings are similar
        int out1 = s1.compareTo(s2);
        System.out.println("the difference between ASCII value is="+out1);
        // Converting cases
        String word1 = "GeeKyMe";
        System.out.println("Changing to lower Case " +
                            word1.toLowerCase());

        // Converting cases
        String word2 = "GeekyME";
        System.out.println("Changing to UPPER Case " +
                            word2.toUpperCase());

        // Trimming the word
        String word4 = " Learn Share Learn ";
        System.out.println("Trim the word " + word4.trim());

        // Replacing characters
        String str1 = "feeksforfeeks";
        System.out.println("Original String " + str1);
        String str2 = "feeksforfeeks".replace('f' ,'g') ;
        System.out.println("Replaced f with g -> " + str2);
    }
}

Output :

String length = 13
Character at 3rd position = k
Substring ksforGeeks
Substring = eks
Concatenated string = GeeksforGeeks
Index of Share 6
Index of a = 8
Checking Equality false
Checking Equality true
Checking Equality false
the difference between ASCII value is=-31
Changing to lower Case geekyme
Changing to UPPER Case GEEKYME
Trim the word Learn Share Learn
Original String feeksforfeeks
Replaced f with g -> geeksgorgeeks

For Set – 2 you can refer: Java.lang.String class in Java | Set 2

 This article is contributed by Rahul Agrawal. Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above