String Class in Java

A String is a sequence of character. It is one of most powerful class in java. String object is immutable, It means once you created a string object, Changes to it's contents or length is not possible.
Most frequent way of creating String object is..

String str = "Hello Java";
Here, "Hello Java" is called string literal. When we use String class in our program we should know few points about String in java.

1. String consist of sequence of characters. Each character in String is a 16-bit Unicode character.
2. "+" Operator is overloaded for String class and hence it can be use for concatenate to Strings in java, For Example
   
        String str = "Hello"+" Java";
3. In java a String can be created from a byte array, character array, from existing String, from StringBuffer, or from literal as shown above, Java provides a wide range of constructor for all these.
4. Every class has a method i.e toString() to provide string representation of it's object.
5. String index is start with 0.   

Note:- 

• String object is not an array of characters. 
• String object is immutable; its reference variable is not.

 

Constructors in String class:-

 

String class has several constuctors. some of them we discuss here.

• To ceate a String object from literals we can do this.

               String str = "Welcome";

• To create a empty String call it's default constructor. For example.

               String str = new String();

• To create a String object from a character array we can use following form of constructor

                String(char[] values); 

                For example.....
                    char[] data = {'a', 'b', 'c', 'd', 'e'};
                    String str = new String(data);
                By this constructor, A String will be created having string value "abcde".

• To create a String object from a byte array we can use following form of constructor

                String(byte[] values); 

                For example.....
                     byte[] data = {65, 66, 67, 68, 69, 70};
                    String str = new String(data);
                By this constructor, A String will be created having string value "ABCDE".

• To create a String from StringBuffer we can use following form of constructor

                String(StringBuffer buffer);

                For example...
                     StringBuffer buffer = new StringBuffer();
                             buffer.append('a');
                             buffer.append("bcd");
                     String str = new String(buffer);
                By this consturctor, A String will be created having string value "abcd".

•   To create a String from existing String, we can use following form of constructor

                   String(String str);

                   For example....
                        String s1 = "Hiiii Java";
                        String s2 = new String(s1); 

• To create a subrange of an character array as an input for making String, we can use following form of constructor
  
        String(char[] values, int starIndex, int numChars);
    Here startIndex spedifies the index at which the subrange begins, and numChars specifies the number of charectrer to use
  
        For example.....
              char[] data = {'a', 'b', 'c', 'd', 'e', 'f', 'g',' h'};
              String str = new String(data, 2, 3);
       By this this constructor, A String will be created having string value "cde".
            

Methods in String class:-

• String concat(String str) 

          Concatenates the specified string to the end of this string.

• char charAt(int index)

         Returns the character at the specified index.

• boolean endsWith(String value) 

        Tests if this string ends with the specified value.

• boolean equals(Object object)

        Compares this string to the specified object.

• boolean equalsIgnoreCase(String object)

        Compares this String to object, ignoring case considerations.

• int indexOf(String str, int fromIndex)

         Returns the index within this string of the first occurrence of the specified substring, starting at the specified index.

• int indexOf(String str)

         Returns the index within this string of the first occurrence of the specified substring.

• int indexOf(int ch, int fromIndex)

         Returns the index within this string of the first occurrence of the specified character, starting the search at the specified index.

int indexOf(int ch)        Returns the index within this string of the first occurrence of the specified character.   int lastIndexOf(int ch)        Returns the index within this string of the last occurrence of the specified character. int lastIndexOf(int ch, int fromIndex)       Returns the index within this string of the last occurrence of the specified character, searching backward starting at the specified index. int lastIndexOf(String str)       Returns the index within this string of the rightmost occurrence of the specified substring. int lastIndexOf(String str, int fromIndex)       Returns the index within this string of the last occurrence of the specified substring, searching backward starting at the specified index. int length()       Returns the length of this string. String replace(char oldChar, char newChar)       Returns a new string resulting from replacing all occurrences of oldChar in this string with newChar. String replaceAll(String regex, String replacement)       Replaces each substring of this string that matches the given regular expression with the given replacement. String replaceFirst(String regex, String replacement)       Replaces the first substring of this string that matches the given regular expression with the given replacement. String[] split(String regex)      Splits this string around matches of the given regular expression. boolean startsWith(String prefix)      Tests if this string starts with the specified prefix. boolean startsWith(String prefix, int toffset)      Tests if this string starts with the specified prefix beginning a specified index. CharSequence subSequence(int beginIndex, int endIndex)      Returns a new character sequence that is a subsequence of this sequence. String substring(int beginIndex)       Returns a new string that is a substring of this string. String substring(int beginIndex, int endIndex)       Returns a new string that is a substring of this string. char[] toCharArray()      Converts this string to a new character array. String toLowerCase()      Converts all of the characters in this String to lower case using the rules of the default locale.   String toLowerCase(Locale locale)      Converts all of the characters in this String to lower case using the rules of the given Locale. String toUpperCase()      Converts all of the characters in this String to upper case using the rules of the default locale. String toUpperCase(Locale locale)     Converts all of the characters in this String to upper case using the rules of the given Locale. String trim()      Returns a copy of the string, with leading and trailing whitespace omitted. String toString() This object (which is already a string!) is itself returned.