The last line was creating a problem. lastIndex would never be at -1, so there would be an infinite loop. This can be fixed by moving the last line of code into the if block.
String str = "helloslkhellodjladfjhello";
String findStr = "hello";
int lastIndex = 0;
int count = 0;
while(lastIndex != -1){
lastIndex = str.indexOf(findStr,lastIndex);
if(lastIndex != -1){
count ++;
lastIndex += findStr.length();
}
}
System.out.println(count);