Guide to Stream.reduce() (#6372)

* Initial Commit * Update StreamReduceUnitTest.java * Update StreamReduceUnitTest.java * Update StreamReduceUnitTest.java
2019-02-23 23:20:37 -03:00
parent 37f9a70441
commit ca4440d792
4 changed files with 265 additions and 0 deletions
@@ -0,0 +1,62 @@
+package com.baeldung.streamreduce.application;
+
+import com.baeldung.streamreduce.entities.User;
+import com.baeldung.streamreduce.utilities.NumberUtils;
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.List;
+
+public class Application {
+
+    public static void main(String[] args) {
+        List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5, 6);
+        int result1 = numbers.stream().reduce(0, (a, b) -> a + b);
+        System.out.println(result1);
+        
+        int result2 = numbers.stream().reduce(0, Integer::sum);
+        System.out.println(result2);
+
+        List<String> letters = Arrays.asList("a", "b", "c", "d", "e");
+        String result3 = letters.stream().reduce("", (a, b) -> a + b);
+        System.out.println(result3);
+        
+        String result4 = letters.stream().reduce("", String::concat);
+        System.out.println(result4);
+
+        String result5 = letters.stream().reduce("", (a, b) -> a.toUpperCase() + b.toUpperCase());
+        System.out.println(result5);
+        
+        List<User> users = Arrays.asList(new User("John", 30), new User("Julie", 35));
+        int result6 = users.stream().reduce(0, (partialAgeResult, user) -> partialAgeResult + user.getAge(), Integer::sum);
+        System.out.println(result6);
+        
+        String result7 = letters.parallelStream().reduce("", String::concat);
+        System.out.println(result7);
+        
+        int result8 = users.parallelStream().reduce(0, (partialAgeResult, user) -> partialAgeResult + user.getAge(), Integer::sum);
+        System.out.println(result8);
+        
+        List<User> userList = new ArrayList<>();
+        for (int i = 0; i <= 1000000; i++) {
+            userList.add(new User("John" + i, i));
+        }
+        
+        long t1 = System.currentTimeMillis();
+        int result9 = userList.stream().reduce(0, (partialAgeResult, user) -> partialAgeResult + user.getAge(), Integer::sum);
+        long t2 = System.currentTimeMillis();
+        System.out.println(result9);
+        System.out.println("Sequential stream time: " + (t2 - t1) + "ms");
+        
+        long t3 = System.currentTimeMillis();
+        int result10 = userList.parallelStream().reduce(0, (partialAgeResult, user) -> partialAgeResult + user.getAge(), Integer::sum);
+        long t4 = System.currentTimeMillis();
+        System.out.println(result10);
+        System.out.println("Parallel stream time: " + (t4 - t3) + "ms");
+        
+        int result11 = NumberUtils.divideListElements(numbers, 1);
+        System.out.println(result11);
+        
+        int result12 = NumberUtils.divideListElementsWithExtractedTryCatchBlock(numbers, 0);
+        System.out.println(result12);
+    }    
+}
@@ -0,0 +1,25 @@
+package com.baeldung.streamreduce.entities;
+
+public class User {
+
+    private final String name;
+    private final int age;
+
+    public User(String name, int age) {
+        this.name = name;
+        this.age = age;
+    }
+
+    public String getName() {
+        return name;
+    }
+
+    public int getAge() {
+        return age;
+    }
+    
+    @Override
+    public String toString() {
+        return "User{" + "name=" + name + ", age=" + age + '}';
+    }
+}
@@ -0,0 +1,52 @@
+package com.baeldung.streamreduce.utilities;
+
+import java.util.List;
+import java.util.function.BiFunction;
+import java.util.logging.Level;
+import java.util.logging.Logger;
+
+public abstract class NumberUtils {
+
+    private static final Logger LOGGER = Logger.getLogger(NumberUtils.class.getName());
+
+    public static int divideListElements(List<Integer> values, Integer divider) {
+        return values.stream()
+                .reduce(0, (a, b) -> {
+                    try {
+                        return a / divider + b / divider;
+                    } catch (ArithmeticException e) {
+                        LOGGER.log(Level.INFO, "Arithmetic Exception: Division by Zero");
+                    }
+                    return 0;
+                });
+    }
+    
+    public static int divideListElementsWithExtractedTryCatchBlock(List<Integer> values, int divider) {
+        return values.stream().reduce(0, (a, b) -> divide(a, divider) + divide(b, divider));
+    }
+    
+    public static int divideListElementsWithApplyFunctionMethod(List<Integer> values, int divider) {
+        BiFunction<Integer, Integer, Integer> division = (a, b) -> a / b;
+        return values.stream().reduce(0, (a, b) -> applyFunction(division, a, divider) + applyFunction(division, b, divider));
+    }
+    
+    private static int divide(int value, int factor) {
+        int result = 0;
+        try {
+            result = value / factor;
+        } catch (ArithmeticException e) {
+            LOGGER.log(Level.INFO, "Arithmetic Exception: Division by Zero");
+        }
+        return result;
+    }
+    
+    private static int applyFunction(BiFunction<Integer, Integer, Integer> function, int a, int b) {
+        try {
+            return function.apply(a, b);
+        }
+        catch(Exception e) {
+            LOGGER.log(Level.INFO, "Exception occurred!");
+        }
+        return 0;
+    }
+}