Java String repeat() Method

Java String repeat() method returns a new string whose value is the concatenation of this string given number of times.

Java String repeat(int count) Important Points

• If the count is 0, the empty string is returned.
• If the count is negative, IllegalArgumentException is thrown.
• If the count is 1, the reference to this string is returned.
• If this string is empty, then an empty string is returned.
• This utility method was added to String class in Java 11 release.
• This method internally calls Arrays.fill() and System.arraycopy() methods to create the new string.

String repeat() Method Examples

Let’s look at some examples of String repeat() method.

jshell> String str = "Hello";
str ==> "Hello"

jshell> String repeatedStr = str.repeat(3);
repeatedStr ==> "HelloHelloHello"

Let’s look at the exception when the count is negative.

jshell> "Hi".repeat(-2)
|  Exception java.lang.IllegalArgumentException: count is negative: -2
|        at String.repeat (String.java:3240)
|        at (#13:1)

Let’s see what happens when the count is 0.

jshell> "Java".repeat(0)
$14 ==> ""

Let’s see what happens when the source string is empty.

jshell> "".repeat(10)
$15 ==> ""

jshell> "".repeat(0)
$16 ==> ""

jshell> "".repeat(-2)
|  Exception java.lang.IllegalArgumentException: count is negative: -2
|        at String.repeat (String.java:3240)
|        at (#17:1)

jshell> 

Let’s see what happens when the count is 1.

jshell> String s1 = "Java";
s1 ==> "Java"

jshell> String s2 = s1.repeat(1)
s2 ==> "Java"

jshell> s1 == s2
$20 ==> true

The output of == operator confirms that both the string variables are referring to the same string object.

OutOfMemoryError when repeat count is huge

If you look at the repeat() method implementation, there is a condition to check if the resulting string length will be too big? If yes, then throw OutOfMemoryError.

if (Integer.MAX_VALUE / count < len) {
    throw new OutOfMemoryError("Repeating " + len + " bytes String " + count +
            " times will produce a String exceeding maximum size.");
}

Let’s see if we get this error by providing a very large value for the repeat count.

jshell> "Hello".repeat(Integer.MAX_VALUE)
|  Exception java.lang.OutOfMemoryError: Repeating 5 bytes String 2147483647 times will produce a String exceeding maximum size.
|        at String.repeat (String.java:3255)
|        at (#22:1)

jshell> "Hello".repeat(Integer.MAX_VALUE - 2)
|  Exception java.lang.OutOfMemoryError: Repeating 5 bytes String 2147483645 times will produce a String exceeding maximum size.
|        at String.repeat (String.java:3255)
|        at (#23:1)

Conclusion

Java String repeat() is a simple utility method that can be used to concatenate a string specific number of times. It may produce OutOfMemoryError if the resulting string length is too large.

References:

• String repeat() API Doc