String class in Java | Set 1

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 ith index.
    "GeeksforGeeks".charAt(3); // returns  ‘k’
  3. String substring (int i)Return the substring from the ith  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 ans 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 ans 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

  17. Program to illustrate all string  methods:

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    // 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);
      
            int out1 = s1.compareTo(s2);
            System.out.println("If s1 = s2 " + out);
      
            // 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 "
                                word1.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);
        
    }

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    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
    If s1 = s2 false
    Changing to lower Case geekyme
    Changing to UPPER Case GEEKYME
    Trim the word Learn Share Learn
    Original String feeksforfeeks
    Replaced f with g -> geeksgorgeeks
    

     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

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