How to Use Java String Class concat() Method

String concat() Method

Function Description

Java String class concat() method is used to combine the specified string at the end of the string. This method returns the concatenated string if specified string’s length is greater than zero otherwise it returns the same string.

For Example:

If String in which we want to combine the string is “Hello”

String to be concatenated is “” blank, then returned concatenated string will be “Hello”

If the String to be concatenated is “ Test”, then returned concatenated string will be “Hello Test”

Method Signature

public String concat(String str)

Parameter

str : A string to be concatenated at the end of the string

Return Type

A Concatenated String

Exception

Throws NullPointerException if the specified string to be concatenated is null(the str parameter is null)

Example of Java String Class concat() Method

public class ExampleStringConcat {

    public static void main(String[] args) {

          //String in which we want to concat another string

          String strHello = "Hello!";

          //String to be concatenated

          String strTech = "RoundTheTech";
         
          String concatStr = strHello.concat(strTech);
         
          System.out.println("String in which another string will be concatenated : "+strHello);

          System.out.println("String to be concatenated : "+strTech);

          System.out.println("After concat : "+concatStr);
         

          String str= "Welcome to String concat function ";

          //another way to use concat

          concatStr = strHello.concat(strTech).concat(". ").concat(str);

          System.out.println("\nString after concat : " + concatStr);

    }
 
}
 
 
Output:

String in which another string will be concatenated : Hello!

String to be concatenated :  RoundTheTech

After concat : Hello! RoundTheTech
 
String after concat : Hello! RoundTheTech. Welcome to String concat function
 

Example when NullPointer Exception is Thrown

public class ExampleStringConcat {

    public static void main(String[] args) {

          //String in which we want to concat another string

          String strHello = "Hello!";

          //String to be concatenated. It is null

          String strTech = null;
         
          //strTech is null, so concat() method will throw NullPointerException

          String concatStr = strHello.concat(strTech);
         
          System.out.println("String in which another string will be concatenated : "+strHello);

          System.out.println("String to be concatenated : "+strTech);

          System.out.println("After concat : "+concatStr);
         
         
          String str= "Welcome to String concat function ";

          //another way to use concat

          concatStr = strHello.concat(strTech).concat(". ").concat(str);

          System.out.println("\nString after concat : " + concatStr);

    }
 
}
 

Output:

Exception in thread "main" java.lang.NullPointerException

     at java.lang.String.concat(String.java:2027)

     at   com.java.test.ExampleStringConcat.main(ExampleStringConcat.java:11)

Other Methods of String Class

SequenceMethodDescription
1char charAt(int index)This method is used to return the character of the string at specified index. First character is at index 0 while last is at string length-1 . StringIndexOutOfBounds Exception is thrown if valid index value is not passed.
2int length()Returns the number of characters count  in  a string.
3static String format(String format, Object… args)Return the formatted String. It is a static method and formats the string as per passing format and argument(s).
4static String format(Locale l, String format, Object… args)Return the formatted String. It is a static method and formats the string as per passing format and argument(s) and locale.
5String substring(int beginIndex)Returns substring from the given index position to the end index of the string.
6String substring(int beginIndex, int endIndex)Returns substring from the specified begin index to the specified end index.
7boolean contains(CharSequence s)Returns boolean value(true/false). It searches the given sequence of characters within the string and returns true/false accordingly. Characters will be case sensitive while searching in string.
8static String join(CharSequence delimiter, CharSequence… elements)Returns joined string. This method joins each passing String using the specified delimiter. It is a static method.
9static String join(CharSequence delimiter, Iterable<? extends CharSequence> elements)Returns the joined string. This method joins each passing string object of an iterate object using the specified delimiter and returns the concatenated string. It is a static method.
10boolean equals(Object another)Returns boolean value(true/false). It checks the equality of string with the passing object.
11boolean isEmpty()Returns boolean value(true/false). It returns true if the string is empty otherwise returns false.
12String replace(char old, char new)Return the replaced string. It replaces all occurrences of the specified character with the new character.
13String replace(CharSequence old, CharSequence new)Return the replaced string. It replaces all occurrences of the specified Character sequence with the specified new Character sequence.
14String replaceAll(String regex, String replacement)Returns the replaced string. It replaces all occurrences of the sequence of characters with the given replacement string that are matching with the given regular expression.
15String replaceFirst(String regex, String replacement)Returns the replaced string. It replaces the first matching sequence of characters with the replacement string that matches with the given regular expression.
16static boolean equalsIgnoreCase(String another)Returns the boolean (true/false) value. It compares the passing string with string by ignoring the case. It  compares the content irrespective of lowercase or uppercase of the character(s).
17String[] split(String regex)Returns the array of strings resulting from splitting of the string     around matches of the given regular expression. String is breaked into strings from all  the matching regular expressions.
18String[] split(String regex, int limit)Returns the array of strings resulting from splitting of the string     around matches of the given regular expression. It breaks the string from all the matching regular expression upto the given limit.
19String intern()Returns an interned string. This method helps to compare two objects using == operator and checks in a string constant pool. it is faster for comparison.
20int indexOf(int ch)Returns the index value of first occurrence of the specified character within the string
21int indexOf(int ch, int fromIndex)Returns the index value of first occurrence of the specified character from the substring starting from specified index
22int indexOf(String substring)Returns the index value of first occurrence of the specified substring within the string.
23int indexOf(String substring, int fromIndex)Returns the index value of the first occurrence of the specified string within the substring starting from specified index to end of the string index.
24int lastIndexOf(int ch)Returns the index value of last occurrence of the specified character
25int lastIndexOf (int ch, int fromIndex)Returns the index value of last occurrence of the specified character, search starts at given fromIndex. 
26int lastIndexOf (String str)Returns the index value of last occurrence of the specified string.
27int lastIndexOf (String str, int fromIndexReturns the index value of last occurrence of the specified string, searches start from given fromIndex
28String toLowerCase()Returns all the characters of the string in lowercase .
29String toLowerCase(Locale l)Returns all the characters of the string in lowercase using specified locale.
30String toUpperCase()Returns all the characters of the string in uppercase.
31String toUpperCase(Locale l)Returns all the characters of the string in uppercase using specified locale.
32String trim()Returns the string after removing the white space from the beginning and end of the string.
33static String valueOf(int value)Return the converted string. It converts the objects and all primitive types of values into string. It is a static method.

In this article, we learned all about the concat() method of String class with examples. Apart from the concat() method, we got an idea about other String methods too. We will come up with more articles explaining other String methods description with examples. For more details please click on the respective methods link.

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