Part 25: String and its method in Java with Example

by Jesmin Akther | Jun 19, 2021 | JAVA

Java String

In Java, string is an object of sequence of char values like an array of characters. There are two ways to create String object:

String literal and new keyword

1) String Literal

Java String literal is created by using double quotes. For Example:

char[] names={‘d’,’r’,’a’,’f’,’t’,’s’,’b’,’o’,’o’,’k’,’.’,’c’,’o’,’m’};

• String site=new String(names); //String new keyword

Is same as:

String names =”draftsbook.com”; // String Literal

Each time create a string literal, the JVM checks the “string constant pool” first. If the string already in the pool, a reference to the pool instance is returned. If the string doesn’t exist in the pool, a new string instance is created and placed in the pool. For instance:

String site1=” draftsbook.com “;

String site2=” draftsbook.com “; //It doesn’t create a new instance

In the above example, only one object will be created. Firstly, JVM will not find any string object with the value “draftsbook.com “in string constant pool.  So, it will create a new object. After that it will find the string with the value “draftsbook.com “in the pool, it will not create a new object but will return the reference to the same instance. Keep in mind that String objects are stored in a special memory area known as the “string constant pool”. Why Java uses String literal? In order to make Java more memory efficient. Because no new objects are created if it exists already in the string constant pool. Learn about or easily understand String, StringBuffer read this writing What is the defference among String, StringBuffer and StringBuilder.

2) By new keyword

String site=new String(“draftsbook.com”);//creates two objects and one reference variable.

In such case, JVM will create a new string object in normal or non-pool or a heap memory in addition to the literal ” draftsbook.com ” will be positioned in the string constant pool. The variable site will refer to the object in a heap.Learn about or easily understand String, StringBuffer read this writing What is the defference among String, StringBuffer and StringBuilder.

Java String Example

public class ExString{ 

public static void main(String args[]){ 

String site1=" draftsbook.com ";//creating string by java string literal 

char[] names={'d','r','a','f','t','s','b','o','o','k',’.’,’c’,’o’,’m’}; 

String site2=new String(names);//converting char array to string 

String site3=new String("https://draftsbook.com/");//creating java string by new keyword 

System.out.println(site1); 

System.out.println(site2); 

System.out.println(site3); 

}

} 

OUTPUT

draftsbook

draftsbook.com

Home

Java String class allows a several number of methods to do operations on strings such as compare(), concat(), equals(), split(), length(), replace(), compareTo(), intern(), substring() etc.The java.lang.String class implements Serializable, Comparable and CharSequence interfaces.Learn about or easily understand String, StringBuffer read this writing What is the defference among String, StringBuffer and StringBuilder.

• CharSequence Interface

The interface is used to represent the sequence of characters. String, StringBuffer and StringBuilder classes implement it. It means, we can create strings in java by using these three classes.

• CharSequence in Java

The Java String is immutable which means it cannot be changed. Whenever alteration any string, a new instance is created. For mutable strings, you can use StringBuffer and StringBuilder classes.

• Java String class methods

The java.lang.String class provides many useful methods to perform operations on sequence of char values.Learn about or easily understand String, StringBuffer read this writing What is the defference among String, StringBuffer and StringBuilder.

Method and Description of Java:

• char charAt(int index) : This method returns char value for the particular index
• int length() : This method returns string length
• static String format(String format, Object… args): This method returns a formatted string.
• static String format(Locale l, String format, Object… args) : This method returns formatted string with given locale.
• String substring(int beginIndex) : This method returns substring for given begin index.
• String substring(int beginIndex, int endIndex): This method returns substring for given begin index and end index.
• boolean contains(CharSequence s) : This method returns true or false after matching the sequence of char value.
• static String join(CharSequence delimiter, CharSequence… elements): This method returns a joined string.
• static String join(CharSequence delimiter, Iterable<? extends CharSequence> elements) : This method returns a joined string.
• boolean equals(Object another): This method checks the equality of string with the given object.
• boolean isEmpty(): This method checks if string is empty.
• String concat(String str) : This method concatenates the specified string.
• String replace(char old, char new) : This method replaces all occurrences of the specified char value.
• String replace(CharSequence old, CharSequence new) : This method replaces all occurrences of the specified CharSequence.
• static String equalsIgnoreCase(String another): This method compares another string. It doesn’t check case.
• String[] split(String regex) : This method returns a split string matching regex.
• String[] split(String regex, int limit) : This method returns a split string matching regex and limit.
• String intern(): This method returns an interned string.
• int indexOf(int ch)  : This method returns the specified char value index.
• int indexOf(int ch, int fromIndex) : This method returns the specified char value index starting with given index.
• int indexOf(String substring): This method returns the specified substring index.
• int indexOf(String substring, int fromIndex): This method returns the specified substring index starting with given index.
• String toLowerCase(): This method returns a string in lowercase.
• String toLowerCase(Locale l): This method returns a string in lowercase using specified locale.
• String toUpperCase() : This methodreturns a string in uppercase.
• String toUpperCase(Locale l) : This method  returns a string in uppercase using specified locale.
• String trim(): This method removes beginning and ending spaces of this string.
• static String valueOf(int value): This methodconverts given type into string. It is an overloaded method.

Strings are actually are widely used in Java programming, are a sequence of characters. In Java programming language, strings are treated as objects. The Java platform provides the String class to create and manipulate strings.Learn about or easily understand String, StringBuffer read this writing What is the defference among String, StringBuffer and StringBuilder.

Creating Strings

The most direct way to create a string is to write −

String greeting = “Hello world!”;

Whenever invoke a string literal in code, the compiler creates a String object with its value in this case, “Hello world!’. Since with other object, can create String objects by using the new keyword and a constructor. The String class has 11 constructors that allow to provide the initial value of the string using different sources, such as an array of characters.

Example

public class stringExample {

   public static void main(String args[]) {

      char[] hArray = { 'h', 'e', 'l', 'l', 'o', '.' };

      String hString = new String(hArray); 

      System.out.println( hString );

   }

}

Output

hello.

The String class is immutable, so that once it is created a String object cannot be changed. If there is a necessity to make a lot of modifications to Strings of characters, then use String Buffer & String Builder Classes.

String Length

One accessor method that can use with strings is the length() method, which returns the number of characters contained in the string object. The following program is an example of length(), method String class.

Example

public class stringExample {

   public static void main(String args[]) {

      String palindrome = "Mim saw It as Sin";

      int lengthPalin = palindrome.length();

      System.out.println( "String Length is : " + lengthPalin);

   }

}

Output

String Length is: 17

Concatenating Strings : The String class includes a method for concatenating two strings,

string1.concat(string2);

This returns a new string that is string1 with string2 added to it at the end. can also use the concat() method with string literals, as in

“My name is “.concat(“Md”)

Strings are more commonly concatenated with the + operator, as in

“Hello,” + ” world” + “!”

OUTPUT

“Hello, world!”

Let us look at the following example

public class stringExample {

   public static void main(String args[]) {

      String stringSen = "saw It as ";

      System.out.println("Mim " + stringSen + "Sin");

   }

}

Output

Dot saw I was Mim

Creating Format Strings

The methods  printf() and format() are use to print output with formatted numbers. The String class has an equivalent class method, format(), that returns a String object rather than a PrintStream object. Using String’s static format() method allows you to create a formatted string that you can reuse, as opposed to a one-time print statement. For example, instead of:

Example

System.out.printf(“The value of the float variable is ” +

“%f, while the value of the integer ” +

“variable is %d, and the string ” +

“is %s”, floatVar, intVar, stringVar);

You can write as:

String fs;

fs = String.format(“The value of the float variable is ” +

“%f, while the value of the integer ” +

“variable is %d, and the string ” +

“is %s”, floatVar, intVar, stringVar);

System.out.println(fs);

String Methods

A few list of methods supported by String class are given below:

Method & Description:

• char charAt(int index)

The method is used to returns the character at the specified index.

• int compareTo(Object o)

The method is used to Compares this String to another Object.

• int compareTo(String anotherString)

The method is used to Compares two strings lexicographically.

• int compareToIgnoreCase(String str)

The method is used to Compares two strings lexicographically, ignoring case differences.

• String concat(String str)

The method is used to Concatenates the specified string to the end of this string.

• boolean contentEquals(StringBuffer sb)

The method is used to Returns true if and only if this String represents the same sequence of characters as the specified StringBuffer.

• static String copyValueOf(char[] data)

The method is used to Returns a String that represents the character sequence in the array specified.

• static String copyValueOf(char[] data, int offset, int count)

The method is used to Returns a String that represents the character sequence in the array specified.

• boolean endsWith(String suffix)

The method is used to Tests if this string ends with the specified suffix.

• boolean equals(Object anObject)

The method is used to Compares this string to the specified object.

• boolean equalsIgnoreCase(String anotherString)

The method is used to Compares this String to another String, ignoring case considerations.

• byte[] getBytes()

The method is used to Encodes this String into a sequence of bytes using the platform’s default charset, storing the result into a new byte array.

• byte[] getBytes(String charsetName)

The method is used to Encodes this String into a sequence of bytes using the named charset, storing the result into a new byte array.

• void getChars(int srcBegin, int srcEnd, char[] dst, int dstBegin)

The method is used to Copies characters from this string into the destination character array.

• int hashCode()

The method is used to Returns a hash code for this string.

• int indexOf(int ch)

The method is used to Returns the index within this string of the first occurrence of the specified character.

• int indexOf(int ch, int fromIndex)

The method is used to Returns the index within this string of the first occurrence of the specified character, starting the search at the specified index.

• int indexOf(String str)

The method is used to Returns the index within this string of the first occurrence of the specified substring.

• int indexOf(String str, int fromIndex)

The method is used to Returns the index within this string of the first occurrence of the specified substring, starting at the specified index.

• String intern()

The method is used to Returns a canonical representation for the string object.

• int lastIndexOf(int ch)

The method is used to Returns the index within this string of the last occurrence of the specified character.

• int lastIndexOf(int ch, int fromIndex)

The method is used to Returns the index within this string of the last occurrence of the specified character, searching backward starting at the specified index.

• int lastIndexOf(String str)

The method is used to Returns the index within this string of the rightmost occurrence of the specified substring.

• int lastIndexOf(String str, int fromIndex)

The method is used to Returns the index within this string of the last occurrence of the specified substring, searching backward starting at the specified index.

• int length()

The method is used to Returns the length of this string.

• boolean matches(String regex)

The method is used to Tells whether or not this string matches the given regular expression.

• boolean regionMatches(boolean ignoreCase, int toffset, String other, int ooffset, int len)

The method is used to Tests if two string regions are equal.

• boolean regionMatches(int offset, String other, int onset, int len)

The method is used to Tests if two string regions are equal.

• String replace(char oldChar, char newChar)

The method is used to Returns a new string resulting from replacing all occurrences of oldChar in this string with newChar.

• String replaceAll(String regex, String replacement

The method is used to Replaces each substring of this string that matches the given regular expression with the given replacement.

• String replaceFirst(String regex, String replacement)

The method is used to Replaces the first substring of this string that matches the given regular expression with the given replacement.

• String[] split(String regex)

The method is used to Splits this string around matches of the given regular expression.

• String[] split(String regex, int limit)

The method is used to Splits this string around matches of the given regular expression.

• boolean startsWith(String prefix)

The method is used to Tests if this string starts with the specified prefix.

• boolean startsWith(String prefix, int toffset)

The method is used to Tests if this string starts with the specified prefix beginning a specified index.

• CharSequence subSequence(int beginIndex, int endIndex)

The method is used to Returns a new character sequence that is a subsequence of this sequence.

• String substring(int beginIndex)

The method is used to Returns a new string that is a substring of this string.

• String substring(int beginIndex, int endIndex)

The method is used to Returns a new string that is a substring of this string.

• char[] toCharArray()

The method is used to Converts this string to a new character array.

• String toLowerCase()

The method is used to Converts all of the characters in this String to lower case using the rules of the default locale.

• String toLowerCase(Locale locale)

The method is used to Converts all of the characters in this String to lower case using the rules of the given Locale.

• String toString()

The method is used to This object (which is already a string!) is itself returned.

• String toUpperCase()

The method is used to Converts all of the characters in this String to upper case using the rules of the default locale.

• String toUpperCase(Locale locale)

The method is used to Converts all of the characters in this String to upper case using the rules of the given Locale.

• String trim()

The method is used to Returns a copy of the string, with leading and trailing whitespace omitted.

• static String valueOf(primitive data type x)

The method is used to Returns the string representation of the passed data type argument.