Java String concat() Method with Examples

Last Updated :20 Sep, 2025

The string concat() methodconcatenates (appends) a string to the end of another string. It returns the combined string. It is used for string concatenation in Java. It returns NullPointerException if any one of the strings is Null.

Java

classGFG{publicstaticvoidmain(Stringargs[]){// String InitializationStrings="Geeks";// Use concat() method for string concatenations=s.concat("forGeeks");System.out.println(s);}}

Output

GeeksforGeeks

Syntax of concat() Method

public Stringconcat(String s);

Parameters:

A string to be concatenated at the end of the other string.

Return Value:

Concatenated(combined) string.

Exception:

NullPointerException:When either of the string is Null.

Java String concat() Examples

There are many ways to use the concat() method in Java:

Combining Two Strings with concat() Method

The below example combines two strings using concat() method of string class.

Example:

Java

classGFG{publicstaticvoidmain(Stringargs[]){// String InitializationStrings1="Geeksfor";Strings2="Geeks";// Concatenate the strings s1 and s2 using the concat() method and store the result back in s1.s1=s1.concat(s2);System.out.println(s1);}}

Output

GeeksforGeeks

Sequential Concatenation of Multiple Strings

The below example shows sequential concatenation using String concat() method in Java.

Example:

java

publicclassGFG{publicstaticvoidmain(Stringargs[]){Strings1="Computer-";Strings2="Science-";// Combining above strings by passing one string as an argumentStrings3=s1.concat(s2);// Print and display temporary combined stringSystem.out.println(s3);Strings4="Portal";Strings5=s3.concat(s4);System.out.println(s5);}}

Output

Computer-Science-Computer-Science-Portal

Note: As perceived from the code we can do as many times as we want to concatenate strings bypassing older strings with new strings to be contaminated as a parameter and storing the resultant string in String datatype.

Handling NullPointerException in String concat()

The below example shows the NullPointerException in String concat() method.

Example:

Java

publicclassGFG{publicstaticvoidmain(Stringargs[]){Strings1="Computer-";Strings2=null;// Combining above strings by passing one string as an argumentStrings3=s1.concat(s2);// It will raise NullPointerExceptionSystem.out.println(s3);}}

Output

Exception in thread "main" java.lang.NullPointerException

Reversing a String Using concat() Method

We can reverse a string using the concat() method of string class. Below is the example to reverse a string in Java.

Example:

Java

publicclassReverseString{publicstaticvoidmain(String[]args){// Declare original string variableStringa="Geeks";// Declare another string variable and initialize it with an empty stringStringb="";// Iterate through each character in string "a" from the last index to the first.for(inti=a.length()-1;i>=0;i--){// Extract the current character at index "i" of the "a" stringcharch=a.charAt(i);// Convert the character to a String object using the "Character.toString" methodStringch1=Character.toString(ch);// Concatenate the converted character String to the end of the "b" stringb=b.concat(ch1);}System.out.println(""+a);System.out.println(""+b);}}