Claude Code
performance
Memory Overflow from String Concatenation in Loop
Processing large files or datasets causes out-of-memory errors. Profiling shows massive string accumulation. The issue: concatenating strings in a loop creates new string object each iteration.
Simple string concatenation is inefficient when done repeatedly.
Error Messages You Might See
java.lang.OutOfMemoryError: Java heap space
Memory usage spikes when processing large data
Application crashes with string concatenation
Common Causes
- Using += in loop to build strings: str += item creates new object each iteration
- String concatenation inside tight loop processing millions of items
- No buffer/accumulator, just repeatedly building larger strings
- Serialization to JSON or XML using simple string concatenation
- Log message building with repeated concatenation
How to Fix It
Use StringBuilder (Java) or StringBuffer for accumulation: StringBuilder sb = new StringBuilder(); sb.append(item). In loops: always use StringBuilder. For JSON: use library like Jackson, Gson. For logging: use parameterized messages: log.info("Item: {}", item) not "Item: " + item. Test with large data: if memory spikes, likely concatenation issue.
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Get HelpFrequently Asked Questions
Why is += slow in loops?
Each += creates new String. For 1000 items: creates 1000 intermediate strings. StringBuilder creates 1 string.
How much faster is StringBuilder?
10-100x faster for 1000s of concatenations. Order of magnitude better for large datasets.
When should StringBuilder be used?
Always in loops. Even 10+ concatenations. Single concatenation outside loop is fine: str = a + b + c.