Strings in Java

Most of the data that transmits on internet is in the form of groups of characters, such group of characters are called strings. A string represents a group of characters. In java, string is an object of string class and not a character array as inn case of c/c++ programming languages.

In java, string is a class in java.lang package, but in java all classes are considered as data types. So, we can take string as both a class and a data type.

String s = "Java";

Here, s is a variable of the data type ‘String’.

Creating Strings #

There are three ways to create strings in java

• We can create a string just by assigning a group of characters to a string type variable

String s; // declaring string type variable
s = "Java"; // assigning a group of characters to it

• We can create an object to string class by allocating memory using new operator. This is just like creating an object to any class as given below

String s = new String("java");

• We can also create by converting the character array into strings.

char arr[] = {'J','a','v','a'}; 
String s = new String(arr); // string object is created and array name is passed to it

Here, the string object s contains the string Java, which means all the characters of the array are copied into the string.

String Class Methods #

• concat(String s)
• int length()
• char charAt(int i)
• int compareTo(String s)
• int compareToIgnoreCase(String s)
• boolean equals(String s)
• boolean equalsIgnoreCase(String s)
• boolean startsWith(String s)
• boolean endsWith(String s)
• int indexOf(String s)
• string replace(char c1, char c2)
• string substring(int i)
• string toLowerCase()
• string toUpperCase()
• string trim()

concat(String s) #

This method concatenates or joins two or more strings and return the resultant string as a result. Here the concat is the method name.

Example:

s1 = "Hello"
s2 = "Java"
String s3 = s1.concat(s2); // output will be "Hello Java"

int length() #

This method returns the length or number of characters of a string. For example, s1.length() gives the number of characters in the String s1.

Example: s1.length() gives the number of characters in the string

char charAt(int i) #

This method returns the character at the specified location i.

Example: Suppose we call this method as s1.charAt(5), then it gives the 5th character in the String s1.

int compareTo(String s) #

This method is useful to compare two strings and to know which string is bigger or smaller.

Example: If we compare s1 and s2, if both are equal it will give 0 as output, if s1 is greater it will return positive number and if s2 is greater it will return negative number as output.

int compareToIgnoreCase(String s) #

This is same as compareTo() method, but this does not take the case of string into consideration.

Example: for this method Java and java are one and the same as it ignores the case.

boolean equals(String s) #

This method returns true if two strings are same, otherwise false, this is case-sensitive.

boolean equalsIgnoreCase(String s) #

This is same as preceding method but, it performs case-insensitive commparison.

boolean startsWith(String s) #

This method returns true if a string is beginning with the substring s. It is case-sensitive.

boolean endsWith(String s) #

This method tests the ending of a string, if the string ends with sub string s, then it returns true. This method is also case-sensitive.

int indexOf(String s) #

This method is called in the form, s1.indexOf(s2), and it returns an integer value.

Example:

s1 = "This is a book";
s2 = "is"
int n = s1.indexOf(s2); 
/* this will search for substring "is" in the main string "This is a book" and 
returns the output as `2` as "is" can be seen at the 2nd position. We can 
observe that "is" is present at 2nd and 5th positions but it will return only 
the first ocurring one.*/

string replace(char c1, char c2) #

This method replaces all the occurrences of character c1 by a new character c2.

Example: s1 = "Hello" and we use replace method as s1.replace(’l’,‘x’), this will give output as “Hexxo”.

string substring(int i) #

This method is useful to extract sub string from a main string. It returns a new string consisting of all characters from the position i until the end of the string.

Example: s1.substring(5) returns characters starting from 5th character till the end of s1.

string toLowerCase() #

This method converts all the characters of the string into lower case, and returns that lower-cased string.

string toUpperCase() #

This method converts all the characters of the string into upper case, and returns that upper-cased string.

string trim() #

This method removes spaces from the beginning and ending of a string.

Example: if a string is written as " Hello Java “, the trim method will remove the unnecessary spaces before and after the string and the output will be “Hello Java”.