Benchmark of String.join vs stream reduction

AllocationBenchmark.java

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package org.elasticsearch.benchmark.routing.allocation;

import org.elasticsearch.Version;
import org.elasticsearch.cluster.ClusterName;
import org.elasticsearch.cluster.ClusterState;
import org.elasticsearch.cluster.metadata.IndexMetaData;
import org.elasticsearch.cluster.metadata.MetaData;
import org.elasticsearch.cluster.node.DiscoveryNodes;
import org.elasticsearch.cluster.routing.RoutingTable;
import org.elasticsearch.cluster.routing.ShardRoutingState;
import org.elasticsearch.cluster.routing.allocation.AllocationService;
import org.elasticsearch.common.settings.Settings;
import org.openjdk.jmh.annotations.Benchmark;
import org.openjdk.jmh.annotations.BenchmarkMode;
import org.openjdk.jmh.annotations.Fork;
import org.openjdk.jmh.annotations.Measurement;
import org.openjdk.jmh.annotations.Mode;
import org.openjdk.jmh.annotations.OutputTimeUnit;
import org.openjdk.jmh.annotations.Param;
import org.openjdk.jmh.annotations.Scope;
import org.openjdk.jmh.annotations.Setup;
import org.openjdk.jmh.annotations.State;
import org.openjdk.jmh.annotations.Warmup;

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
import java.util.concurrent.TimeUnit;

@Fork(3)
@Warmup(iterations = 10)
@Measurement(iterations = 10)
@BenchmarkMode(Mode.AverageTime)
@OutputTimeUnit(TimeUnit.MILLISECONDS)
@State(Scope.Benchmark)
@SuppressWarnings("unused") //invoked by benchmarking framework
public class AllocationBenchmark {
    // Do NOT make any field final (even if it is not annotated with @Param)! See also
    // http://hg.openjdk.java.net/code-tools/jmh/file/tip/jmh-samples/src/main/java/org/openjdk/jmh/samples/JMHSample_10_ConstantFold.java

    // we cannot use individual @Params as some will lead to invalid combinations which do not let the benchmark terminate. JMH offers no
    // support to constrain the combinations of benchmark parameters and we do not want to rely on OptionsBuilder as each benchmark would
    // need its own main method and we cannot execute more than one class with a main method per JAR.

    private class WrappedString {
        private final String s;

        public WrappedString(String s) {
            this.s = s;
        }

        @Override
        public String toString() {
            return s;
        }
    }

    private List<WrappedString> tasks;

    @Setup
    public void setUp() {
        StringBuffer testStringBuffer = new StringBuffer();
        for (int i = 0; i < 100; i++) {
            testStringBuffer.append('x');
        }
        String testString = testStringBuffer.toString();

        tasks = new ArrayList<>(10000);
        for (int i = 0; i < 10000; i++) {
            tasks.add(new WrappedString(testString));
        }
    }

    @Benchmark
    public String measureStreamReduce() {
        return tasks.stream().map(WrappedString::toString).reduce((s1,s2) -> {
            if (s1.isEmpty()) {
                return s2;
            } else if (s2.isEmpty()) {
                return s1;
            } else {
                return s1 + ", " + s2;
            }
        }).orElse("");
    }

    @Benchmark
    public String measureStringJoin() {
        return String.join(", ", tasks.stream().map(t -> (CharSequence)t.toString()).filter(t -> t.length() == 0)::iterator);
    }
}