adam bien's blog

Custom Map Updates without Null Checks: Map#merge 📎

The Java 1.8+ Map#merge method is useful for upserts of a Map with custom bevavior. Null checks are not required:


import java.util.HashMap;
import java.util.Map;
import static org.hamcrest.CoreMatchers.is;
import static org.hamcrest.MatcherAssert.assertThat;
import org.junit.Test;

public class MapMergeTest {

    @Test
    public void mergeInMap() {
        Map<String, Integer> map = new HashMap<>();
        int initial = 1;
	
        int result = map.merge("key", initial, (oldValue, newValue) -> oldValue + newValue);
        assertThat(result, is(1));

        int update = 42;
        int expected = initial + update;
        result = map.merge("key", update, (oldValue, newValue) -> oldValue + newValue);
        assertThat(result, is(expected));
    }
}

The Map#merge is equivalent to:


if (oldValue != null ) {
    if (newValue != null)
        map.put(key, newValue);
    else
        map.remove(key);
    } else {
    if (newValue != null)
        map.put(key, newValue);
    else
        return null;
    }

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