Use local variables and immutable data structures instead of shared mutable state

Medium

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JavaClean codeAdd examplecsharp

Don't mutate a method parameter. Instead compute the result in a local variable.

Examples

Example 1:

Negative

Incorrect implementation that violates the practice.

namespace Practices.LocalVariablesAndImmutable;

public class CountNamesHelper
{
    public static IDictionary<string, int> CountNames(IList<string> names, IDictionary<string, int> count)
    {
        foreach (string name in names)
        {
            if (!count.TryAdd(name, 1))
            {
                count[name]++;
            }
        }
        return count;
    }
}

Example 2:

Negative

Incorrect implementation that violates the practice.

package practices.java.use_local_immutable;

import java.util.List;
import java.util.Map;

public class CountNamesHelper {

    public static Map<String, Long> countNames(List<String> names, Map<String, Long> countByName) {
        for (String name : names) {
            long previousCount = countByName.getOrDefault(name, 0L);
            countByName.put(name, previousCount + 1);
        }
        return countByName;
    }
}