Computing New String - Memory Out Of Heap Error

Given a string, compute a new string where identical chars that are adjacent in the original string are separated from each other by a "*". My implementation :

package com.tcs.dash;
public class StringBuild {
public String edit(String userIp){
StringBuilder builder = new StringBuilder(userIp);
String replaceText = "";
for(int i = 0; i < builder.length() - 1; i++){
if(builder.charAt(i) == builder.charAt(i+1)){
replaceText = builder.charAt(i) + "*" + builder.charAt(i+1);
builder = builder.replace(i, i+1, replaceText);
}
}
return builder.toString();
}
}

I am getting error at line 13. An exception actually.

I/P given = aaaa

Console:

Exception in thread "main" java.lang.OutOfMemoryError: Java heap space
at java.util.Arrays.copyOf(Unknown Source)
at java.lang.AbstractStringBuilder.expandCapacity(Unknown Source)
at java.lang.AbstractStringBuilder.ensureCapacityInternal(Unknown Source)
at java.lang.AbstractStringBuilder.replace(Unknown Source)
at java.lang.StringBuilder.replace(Unknown Source)
at com.tcs.dash.StringBuild.edit(StringBuild.java:13)
at com.tcs.dash.StringBuildExample.main(StringBuildExample.java:14)