javaScope errorMedium

Unintended Object Retention Causing Memory Leak

Time: 0:00

BugFind the bug and fix the code

What it should do

Process data and return transformed result. Example: processRetainedData(data) -> transformedData

What happens instead

Application experiencing increased memory usage over time without apparent cause.

public class DataProcessor {
    private List<String> data;

    public DataProcessor(List<String> data) {
        this.data = data;
    }

    public List<String> processRetainedData() {
        return data.stream().map(new DataTransformer()::transform).collect(Collectors.toList());
    }

    private class DataTransformer {
        public String transform(String s) {
            return s.toUpperCase();
        }
    }
}

Test a custom input to verify an edge case (e.g. empty lists, negative numbers, duplicates).

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This bug belongs to the scope and closure bugs pattern

Every callback reports the last value, a counter never increments, UnboundLocalError on a variable you can see.

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public class DataProcessor { private List<String> data; public DataProcessor(List<String> data) { this.data = data; } public List<String> processRetainedData() { return data.stream().map(new DataTransformer()::transform).collect(Collectors.toList()); } private class DataTransformer { public String transform(String s) { return s.toUpperCase(); } } }

Unintended Object Retention Causing Memory Leak

What it should do

What happens instead

More scope and closure bugs to practise

Unintended Object Retention Causing Memory Leak

What it should do

What happens instead

More scope and closure bugs to practise