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Optimizing a financial analysis application

As a Java developer for a financial analysis application, you've identified that one of your report-generation methods is creating excessive garbage and triggering frequent garbage collections. The report concatenates thousands of data points into a single string. You need to refactor this code to be more GC-friendly while maintaining the same functionality.

This exercise is part of the course

Optimizing Code in Java

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Exercise instructions

  • Create a new StringBuilder instance of the necessary length.
  • For every value in dataPoints, add the formatted value in the string builder.
  • Convert the StringBuilder to a String and return it.

Hands-on interactive exercise

Have a go at this exercise by completing this sample code.

public class Main {
    public static String generateReportWithoutOptimization(double[] dataPoints) {
        String report = "";
        for (double value : dataPoints) {
            report += String.format("%.2f, ", value);
        }
        return report;
    }
    
    public static String generateReportWithOptimization(double[] dataPoints) {
        // Create a new string builder
        StringBuilder report = ____(dataPoints.length * 8);
        for (double value : dataPoints) {
            // For every value, add the formatted value to the string builder 
            report.____(String.format("%.2f, ", value));
        }
        // Convert to string before returning
        return report.____;
    }
    
    public static void main(String[] args) {
        double[] testData = new double[10000];
        for (int i = 0; i < testData.length; i++) {
            testData[i] = Math.random() * 100;
        }
        
        long startWithoutOptimization = System.nanoTime();
        String reportWithoutOptimization = generateReportWithoutOptimization(testData);
        long endWithoutOptimization = System.nanoTime();
        
        long startWithOptimization = System.nanoTime();
        String reportWithOptimization = generateReportWithOptimization(testData);
        long endWithOptimization = System.nanoTime();
        
        System.out.println("Time taken without optimization: " + (endWithoutOptimization - startWithoutOptimization) / 1_000_000 + " ms");
        System.out.println("Time taken with optimization: " + (endWithOptimization - startWithOptimization) / 1_000_000 + " ms");
    }
}
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