String class in Java

The 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:

1. String literal

String s = “neveropen”;

2. Using new keyword

String s = new String (“neveropen”);

String Constructors in Java

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 in Java

1. int length()

Returns the number of characters in the String.

"neveropen".length();  // returns 13

2. Char charAt(int i)

Returns the character at i^th index.

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

3. String substring (int i)

Return the substring from the i^th  index character to end.

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

4. String substring (int i, int j)

Returns the substring from i to j-1 index.

 "neveropen".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 “neveropen”

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(“neveropen”); // 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(“neveropen”); // 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 “neveropengorneveropen”

Note: s1 is still feeksforfeeks and s2 is neveropengorneveropen

17. String contains(string):

Returns true if string contains contains the given string

String s1="neveropen"
String s2="neveropen"
s1.contains(s2) // return true

Example of String Constructor and String Methods

Below is the implementation of the above mentioned topic:

Output :

String length = 13
Character at 3rd position = k
Substring ksforGeeks
Substring = eks
Concatenated string = neveropen
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 -> neveropengorneveropen

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