Guide to Stream.reduce() (#6480)

* Initial Commit

* Update Application.java

* Update Application.java

java-streams-2/src/main/java/com/baeldung/reduce/benchmarks/JMHStreamReduceBenchMark.java

@@ -0,0 +1,52 @@
+package com.baeldung.reduce.benchmarks;
+
+import com.baeldung.reduce.entities.User;
+import java.util.ArrayList;
+import java.util.List;
+import org.openjdk.jmh.annotations.Benchmark;
+import org.openjdk.jmh.annotations.BenchmarkMode;
+import org.openjdk.jmh.annotations.Mode;
+import org.openjdk.jmh.annotations.Scope;
+import org.openjdk.jmh.annotations.State;
+import org.openjdk.jmh.runner.Runner;
+import org.openjdk.jmh.runner.RunnerException;
+import org.openjdk.jmh.runner.options.Options;
+import org.openjdk.jmh.runner.options.OptionsBuilder;
+
+@State(Scope.Thread)
+@BenchmarkMode(Mode.AverageTime)
+public class JMHStreamReduceBenchMark {
+    
+    private final List<User> userList = createUsers();
+
+    public static void main(String[] args) throws RunnerException {
+        
+        Options options = new OptionsBuilder()
+                .include(JMHStreamReduceBenchMark.class.getSimpleName()).threads(1)
+                .forks(1).shouldFailOnError(true).shouldDoGC(true)
+                .jvmArgs("-server").build();
+        new Runner(options).run();
+    }
+    
+    private List<User> createUsers() {
+        List<User> users = new ArrayList<>();
+        for (int i = 0; i <= 1000000; i++) {
+            users.add(new User("John" + i, i));
+        }
+        return users;
+    }
+
+    @Benchmark
+    public Integer executeReduceOnParallelizedStream() {
+        return this.userList
+                .parallelStream()
+                .reduce(0, (partialAgeResult, user) -> partialAgeResult + user.getAge(), Integer::sum);
+    }
+
+    @Benchmark
+    public Integer executeReduceOnSequentialStream() {
+        return this.userList
+                .stream()
+                .reduce(0, (partialAgeResult, user) -> partialAgeResult + user.getAge(), Integer::sum);
+    }
+}