An array of characters functions similarly to a Java string. Consider the following scenario:

char[] ch={'c','o','d','e','r','z','p','y'};  
String s=new String(ch);  

is the same as:

String s="coderzpy";  

The Serializable, Comparable, and CharSequence interfaces are implemented by the Java String class, as shown in the diagram below.

Interface for CharSequences:

To represent a sequence of characters, the CharSequence interface is used. It is implemented by the String, StringBuffer, and StringBuilder classes. This means that these three classes can be used to create strings in Java.

Memory allotment of String

A String Object will be created in the string constant pool whenever it is created as a literal. This allows JVM to optimize String literal initialization.

The string can also be dynamically allocated by using the new operator. Strings that are dynamically allocated are given a new memory location in the heap. The string constant pool will not include this string.

For instance:

String str = new String("Coderzy");

Example to declare String :

// Java code to illustrate String
import java.io.*;
import java.lang.*;

class Test {
	public static void main(String[] args)
	{
		// Declare String without using new operator
		String s = "student at";

		// Prints the String.
		System.out.println("String s = " + s);

		// Declare String using new operator
		String s1 = new String("Coderzpy");

		// Prints the String.
		System.out.println("String s1 = " + s1);
	}
}

Output:

String s = student at
String s1 = Coderzpy

Note: also read about the Interface vs Abstract class

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Also read about the Strings in Java

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charAt() method:

The charAt() function in a string returns the character at the specified index.

class HelloWorld {
    public static void main(String[] args) {
        String myStr = "Hello";
        char result = myStr.charAt(0);
        System.out.println(result);
    }
}

Output:

H

equalsIgnoreCase() method:

equalsIgnoreCase() checks for equality between two Strings while ignoring their case.

        class HelloWorld {
    public static void main(String[] args) {
        String myStr = "Hello",str="Coder";
        boolean result = myStr.equalsIgnoreCase(str);
        System.out.println(result);
    }
}

Output:

false

indexOf() method:

  The index of the first occurrence of a substring or a character is returned by the indexOf() method.

 class HelloWorld {
    public static void main(String[] args) {
       String myStr = "Hello planet earth, you are a great planet.";
    System.out.println(myStr.indexOf("e", 5));
    }
}

Output:

10

length() method:

The length() function of a String returns the number of characters in it.

 class HelloWorld {
    public static void main(String[] args) {
       String myStr = "Hello planet earth, you are a great planet.";
    System.out.println(myStr.length());
    }
}

Output:

43

replace() method:

The replace() method in a string replaces all occurrences of a character with a new character.

public class Demo {
    public static void main(String[] args) {   
        String str = "coderzpy";
        System.out.println(str.replace('c','C'));
    }
}

Output:

Coderzpy

substring() method:

The substring() method returns a part of the string.

public class Demo {
    public static void main(String[] args) {
        String str = "ABCDEFGHIJKLMNO";
        System.out.println(str.substring(4));
        System.out.println(str.substring(4,7));        
    }
}

Output:

EFGHIJKLMNO
EFG

valueOf() method:

String class uses an overloaded version of valueOf() method for all primitive data types and for type Object.

public class Demo {
    public static void main(String[] args) {
         int num = 115;
         String s1 = String.valueOf(num);    //converting int to String
         s1+= "hello";
         System.out.println(s1);
         System.out.println("type of num : "+s1.getClass().getName());   
    }
}

Output:

115hello
type of num : java.lang.String

trim() method:

This method returns a string that has been stripped of any leading and trailing whitespaces.

public class Demo {
    public static void main(String[] args) {
        String str = "   hello coderzpy  ";
        System.out.println(str.trim());   
    }
}

Output:

hello coderzpy

endsWith() method:

the endsWith() method is used to determine whether the string ends with the specified suffix. If the suffix matches the string, it returns true; otherwise, it returns false.

public class Demo {
    public static void main(String[] args) {
        String a="Hello welcome to Coderzpy.com";  
        System.out.println(a.endsWith("m"));  
        System.out.println(a.endsWith("com"));       
    }
}
    

Output:

true
true

Note: also read about the String class methods in Java

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Constructors of StringBuffer class :

1. StringBuffer(): It saves 16 characters by not reallocating space.

StringBuffer s = new StringBuffer();

2. StringBuffer( int size): It accepts an integer argument that specifies the buffer’s size.

StringBuffer s = new StringBuffer(20);

3. StringBuffer(String str): It takes a string argument that sets the initial contents of the StringBuffer object and frees up space for 16 more characters without reallocating space.

StringBuffer s = new StringBuffer("studentscoding");

Important methods of StringBuffer class:

Example: length(),capacity(),append(), reverse(), replace(), ensurecapacity()

import java.io.*;

// Main class
class Demo {

	// main driver method
	public static void main(String[] args)
	{

		// Creating adn storing string by creating object of
		// StringBuffer
		StringBuffer s = new StringBuffer("Coderzpy");

		// Getting the length of the string
		int p = s.length();

		// Getting the capacity of the string
		int q = s.capacity();
			System.out.println("Length of string "+s+"="
						+ p);
		System.out.println(
			"Capacity of string "+s+"=" + q);
		s.append(".com");

		
		System.out.println("Length of string "+s+"="
						+ s.length());
		System.out.println(
			"Capacity of string "+s+"=" + s.capacity());
			System.out.println("reverse = "+s.reverse());
			
			
        s.deleteCharAt(7);
        // Returns string after deletion
        System.out.println("deleted "+s);
        
         s.replace(5, 8, "are");
 
       
        System.out.println("replaced with are"+s);

        s.ensureCapacity(40);
         	System.out.println(
			"Capacity of string "+s+"=" + s.capacity());
        
	}
}

Output:

Length of string Coderzpy=8
Capacity of string Coderzpy=24
Length of string Coderzpy.com=12
Capacity of string Coderzpy.com=24
reverse = moc.ypzredoC
deleted moc.ypzedoC
replaced with aremoc.yaredoC
Capacity of string moc.yaredoC=50

Note: also read about the Some examples of string class methods

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Constructors of StringBuilder class:

• StringBuilder(): It generates a blank String Builder with a 16-character initial capacity.
• StringBuilder(String str): The provided string is used to generate a String Builder.
• StringBuilder(int length): It generates a new String Builder that is empty and has the given capacity and length.

A simple example illustrates the use of a constructor:

// Java Code to illustrate StringBuilder

import java.util.*;
import java.util.concurrent.LinkedBlockingQueue;

public class Demo {
	public static void main(String[] argv) throws Exception
	{
		// Create a StringBuilder object
		// using StringBuilder() constructor
		StringBuilder str = new StringBuilder();

		str.append("Coderzpy");

		// print string
		System.out.println("String = " + str.toString());

		// create a StringBuilder object
		// using StringBuilder(CharSequence) constructor
		StringBuilder str1
			= new StringBuilder("AAAABBBCCCC");

		// print string
		System.out.println("String1 = " + str1.toString());

	}
}

Output:

String = Coderzpy
String1 = AAAABBBCCCC

Methods of StringBuilder class:

• public StringBuilder append(String s): This string is utilized to append to the provided string. Like append(char), append(boolean), append(int), append(float), append(double), etc., the append() function has many overloads.
• public StringBuilder insert(int offset, String s): It is used to insert the specified string with this string at the specified position. There are several other overloaded versions of the insert() method, including insert(int, char), insert(int, boolean), insert(int, int), insert(int, float), insert(int, double), etc.
• public StringBuilder replace(int startIndex, int endIndex, String str): The string from the startIndex and endIndex values are replaced using this method.
• public StringBuilder delete(int startIndex, int endIndex): It is used to remove the string from startIndex and endIndex that are supplied.
• public StringBuilder reverse(): It is used to reverse the string.
• public int capacity(): The current capacity is returned using it.
• public void ensureCapacity(int minimumCapacity):To make sure the capacity is at least equal to the required minimum, it is used.
• public char charAt(int index): The character at the requested place is returned using it.
• public int length(): The length of the string, or the total number of characters, is returned using this method.
• public String substring(int beginIndex): The substring starting at the supplied beginIndex is returned using it.
• public String substring(int beginIndex, int endIndex): It is used to retrieve the substring starting at the beginIndex and endIndex values supplied.

Simple code to illustrate the use of StringBuilder class methods:

// Java code to illustrate
// methods of StringBuilder

import java.util.*;
import java.util.concurrent.LinkedBlockingQueue;

public class Demo1 {
	public static void main(String[] argv)
		throws Exception
	{

		// create a StringBuilder object
		// with a String pass as parameter
		StringBuilder str
			= new StringBuilder("Coderzpy.com");

		// print string
		System.out.println("String = "
						+ str.toString());

		// reverse the string
		StringBuilder reverseStr = str.reverse();

		// print string
		System.out.println("Reverse String = "
						+ reverseStr.toString());

		// Append ', '(44) to the String
		str.appendCodePoint(44);

		// Print the modified String
		System.out.println("Modified StringBuilder = "
						+ str);

		// get capacity
		int capacity = str.capacity();

		// print the result
		System.out.println("StringBuilder = " + str);
		System.out.println("Capacity of StringBuilder = "
						+ capacity);
	}
}

Output:

String = Coderzpy.com
Reverse String = moc.ypzredoC
Modified StringBuilder = moc.ypzredoC,
StringBuilder = moc.ypzredoC,
Capacity of StringBuilder = 28

Note: also read about the Java StringBuffer class

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Constructors of StringToken: 

Let us consider ‘str’ as the string to be tokenized

1. StringTokenizer(String str): Delimiters that are typically used include newline, space, tab, carriage return, and form feed.
2. StringTokenizer(String str, String delim):  To tokenize the supplied string, a collection of delimiters called delim is used.
3. StringTokenizer(String str, String delim, boolean flag):The meaning of the first two parameters is the same, and the flag has the following function.

3.1: If the flag is false, delimiter characters serve to separate tokens

Example:

Input : if string --> "hello coderz" and Delimiter is " ", then 
Output:  tokens are "hello" and "coderz".

3.2: If the flag is true, delimiter characters are considered to be tokens.

Example:

Input : String --> is "hello coderz"and Delimiter is " ", then 
Output: Tokens --> "hello", " " and "coderz".

Methods of the StringTokenizer Class:

Example to illustrate the use of StringTokenizer class:

// Java Program to Illustrate StringTokenizer Class

// Importing required classes
import java.util.*;

// Main class
public class GFG {

	// Main driver method
	public static void main(String args[])
	{

		// Constructor 1
		System.out.println("Using Constructor 1 - ");

		// Creating object of class inside main() method
		StringTokenizer st1 = new StringTokenizer(
			"Hello Coderz, keep coding - keep cheering", " ");

		// Condition holds true till there is single token
		// remaining using hasMoreTokens() method
		while (st1.hasMoreTokens())

			// Getting next tokens
			System.out.println(st1.nextToken());

		// Constructor 2
		System.out.println("Using Constructor 2 - ");

		// Again creating object of class inside main()
		// method
		StringTokenizer st2 = new StringTokenizer(
			"JAVA : Code : String", " :");

		// If tokens are present
		while (st2.hasMoreTokens())

			// Print all tokens
			System.out.println(st2.nextToken());

		// Constructor 3
		System.out.println("Using Constructor 3 - ");

		// Again creating object of class inside main()
		// method
		StringTokenizer st3 = new StringTokenizer(
			"JAVA : Code : String", " :", true);

		while (st3.hasMoreTokens())
			System.out.println(st3.nextToken());
	}
}

Output:

Using Constructor 1 - 
Hello
Coderz,
keep
coding
-
keep
cheering
Using Constructor 2 - 
JAVA
Code
String
Using Constructor 3 - 
JAVA:
 
Code
 :
 
String

Note: also read about the StringBuilder class in Java

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