String class in Java | Set 1

Difficulty Level : Easy
Last Updated : 24 Nov, 2020

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

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
```
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
```
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
```
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
```
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
```
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
```

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

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
```
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
```

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

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

int length(): Returns the number of characters in the String.
```
"GeeksforGeeks".length();  // returns 13
```
Char charAt(int i): Returns the character at i^th index.
```
"GeeksforGeeks".charAt(3); // returns  ‘k’
```
String substring (int i): Return the substring from the i^th index character to end.
```
"GeeksforGeeks".substring(3); // returns “ksforGeeks”
```
String substring (int i, int j): Returns the substring from i to j-1 index.
```
 "GeeksforGeeks".substring(2, 5); // returns “eks”
```

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”

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
```
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
```
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
```

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

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

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.

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).

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

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

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

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

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”
```

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

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