Introduction to String - The Coding Shala

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Introduction to String

A String is a collection of Unicode characters and similar to the arrays. We can perform almost all the operations on the string that we used in an array.

Below is some function that we use frequently with Strings.

Compare Function

The String has its own compare function, unlike the == operator. We can compare two strings in the following ways:
  • we can use == operator, like string1 == string2.
  • we can use equals() method. like string1.equals(string). 
  • we can use compareTo() method. For example string1.compareTo(string2).
Note: The equals() method compares the value inside string while == operator compares the value of two object references to check if they refer to the same String instance or not. Both equals() and == returns true and false.

The Java String compareTo() method is used for comparing two string lexicographically. If both the strings are the same then this method returns 0 else it returns a positive or negative value. It returns a positive value if the first string is lexicographically greater than the second string else returns negative.

The following example explains the compare function: 

//String Example
public class Main{
 public static void main(String[] args) {
  //initializing string
  String str1 = "Hello World";
  System.out.println(str1);
  String str2 = str1;  //refer to the same object
  String str3 = new String(str1);  //another object with same value
  
  System.out.println("Compare with == :");
  System.out.println("str1 and str2 is: "+ (str1==str2));
  System.out.println("str1 and str3 is: "+ (str1==str3));
  
  System.out.println("Compare with equals() method: ");
  System.out.println("str1 and str2 is: "+ (str1.equals(str2)));
  System.out.println("str1 and str3 is: "+ (str1.equals(str3)));
  
  System.out.println("Compare with compareTo() method:");
  System.out.println("str1 and str2 is: "+ (str1.compareTo(str2)==0));  //0 for same
  System.out.println("str1 and str3 is: "+ (str1.compareTo(str3)==0));
 }
}
Output: 


Hello World
Compare with == :
str1 and str2 is: true
str1 and str3 is: false
Compare with equals() method: 
str1 and str2 is: true
str1 and str3 is: true
Compare with compareTo() method:
str1 and str2 is: true
str1 and str3 is: true

In Java, Strings are immutable which means we can't change the content of the string once it's initialized. 

The following example explains the basic operation on Strings: 

//String Example
public class Main{
 public static void main(String[] args) {
  //initializing string
  String str1 = "Hell";
  //str1[0] = 'F'; this will give error since strings are immutable
  
  //concat on string
  str1 = str1 + 'o';
  System.out.println(str1);
  
  str1 = str1 + " World";
  System.out.println(str1);
  
  //get char by index
  System.out.println("Char at index 1 is: "+ str1.charAt(1));
  
  //index of a char
  System.out.println("index of o char is: "+ str1.indexOf('o'));
  System.out.println("Last index of char o is: "+ str1.lastIndexOf('o'));
  
  //substring
  System.out.println(str1.substring(6));
  System.out.println(str1.substring(1,5));
 }
}
Output: 

Hello
Hello World
Char at index 1 is: e
index of o char is: 4
Last index of char o is: 7
World
ello

We know in Java, Strings are immutable so if we want to work with strings either we can convert string to char Array or can use another data structure like StringBuilder. The following example explains it: 
//String Example
public class Main{
 public static void main(String[] args) {
  //initializing string
  String str1 = "Fello";
  //str1[0] = 'F'; this will give error since strings are immutable
  
  System.out.println(str1);
  
  char[] st = str1.toCharArray();
  st[0] = 'H';
  System.out.println(st);
  
  //String Builder example
  StringBuilder sb = new StringBuilder();
  for(int i=0;i<str1.length();i++) {
   sb.append(str1.charAt(i));
  }
  System.out.println(sb.toString());
  System.out.println(sb.reverse());
 }
}
Output: 

Fello
Hello
Fello
olleF



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