Source code for the article Guide to Java 8 Functional Interfaces (#637)

core-java-8/src/test/java/com/baeldung/functionalinterface/FunctionalInterfaceTest.java

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+package com.baeldung.functionalinterface;
+
+import java.util.Arrays;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+import java.util.function.Function;
+import java.util.stream.Collectors;
+import java.util.stream.Stream;
+
+import org.junit.Test;
+
+import static org.junit.Assert.*;
+
+public class FunctionalInterfaceTest {
+
+    @Test
+    public void whenPassingLambdaToComputeIfAbsent_thenTheValueGetsComputedAndPutIntoMap() {
+
+        Map<String, Integer> nameMap = new HashMap<>();
+        Integer value = nameMap.computeIfAbsent("John", s -> s.length());
+
+        assertEquals(new Integer(4), nameMap.get("John"));
+        assertEquals(new Integer(4), value);
+
+    }
+
+    @Test
+    public void whenPassingMethodReferenceToComputeIfAbsent_thenTheValueGetsComputedAndPutIntoMap() {
+
+        Map<String, Integer> nameMap = new HashMap<>();
+        Integer value = nameMap.computeIfAbsent("John", String::length);
+
+        assertEquals(new Integer(4), nameMap.get("John"));
+        assertEquals(new Integer(4), value);
+
+    }
+
+    public byte[] transformArray(short[] array, ShortToByteFunction function) {
+        byte[] transformedArray = new byte[array.length];
+        for (int i = 0; i < array.length; i++) {
+            transformedArray[i] = function.applyAsByte(array[i]);
+        }
+        return transformedArray;
+    }
+
+    @Test
+    public void whenUsingCustomFunctionalInterfaceForPrimitives_thenCanUseItAsLambda() {
+
+        short[] array = {(short) 1, (short) 2, (short) 3};
+        byte[] transformedArray = transformArray(array, s -> (byte) (s * 2));
+
+        byte[] expectedArray = {(byte) 2, (byte) 4, (byte) 6};
+        assertArrayEquals(expectedArray, transformedArray);
+
+    }
+
+    @Test
+    public void whenUsingBiFunction_thenCanUseItToReplaceMapValues() {
+
+        Map<String, Integer> salaries = new HashMap<>();
+        salaries.put("John", 40000);
+        salaries.put("Freddy", 30000);
+        salaries.put("Samuel", 50000);
+
+        salaries.replaceAll((name, oldValue) -> {
+            if (name.equals("Freddy")) {
+                return oldValue;
+            } else {
+                return oldValue + 10000;
+            }
+        });
+
+        assertEquals(new Integer(50000), salaries.get("John"));
+        assertEquals(new Integer(30000), salaries.get("Freddy"));
+        assertEquals(new Integer(60000), salaries.get("Samuel"));
+
+    }
+
+
+    @Test
+    public void whenPassingLambdaToThreadConstructor_thenLambdaInferredToRunnable() {
+
+        Thread thread = new Thread(() -> System.out.println("Hello From Another Thread"));
+        thread.start();
+
+    }
+
+    @Test
+    public void whenUsingSupplierToGenerateNumbers_thenCanUseItInStreamGenerate() {
+
+        int[] fibs = {0, 1};
+        Stream<Integer> fibonacci = Stream.generate(() -> {
+            int result = fibs[1];
+            int fib3 = fibs[0] + fibs[1];
+            fibs[0] = fibs[1];
+            fibs[1] = fib3;
+            return result;
+        });
+
+        List<Integer> fibonacci5 = fibonacci.limit(5)
+                .collect(Collectors.toList());
+
+        assertEquals(new Integer(1), fibonacci5.get(0));
+        assertEquals(new Integer(1), fibonacci5.get(1));
+        assertEquals(new Integer(2), fibonacci5.get(2));
+        assertEquals(new Integer(3), fibonacci5.get(3));
+        assertEquals(new Integer(5), fibonacci5.get(4));
+
+    }
+
+    @Test
+    public void whenUsingConsumerInForEach_thenConsumerExecutesForEachListElement() {
+
+        List<String> names = Arrays.asList("John", "Freddy", "Samuel");
+        names.forEach(name -> System.out.println("Hello, " + name));
+
+    }
+
+    @Test
+    public void whenUsingBiConsumerInForEach_thenConsumerExecutesForEachMapElement() {
+
+        Map<String, Integer> ages = new HashMap<>();
+        ages.put("John", 25);
+        ages.put("Freddy", 24);
+        ages.put("Samuel", 30);
+
+        ages.forEach((name, age) -> System.out.println(name + " is " + age + " years old"));
+
+    }
+
+    @Test
+    public void whenUsingPredicateInFilter_thenListValuesAreFilteredOut() {
+
+        List<String> names = Arrays.asList("Angela", "Aaron", "Bob", "Claire", "David");
+
+        List<String> namesWithA = names.stream()
+                .filter(name -> name.startsWith("A"))
+                .collect(Collectors.toList());
+
+        assertEquals(2, namesWithA.size());
+        assertTrue(namesWithA.contains("Angela"));
+        assertTrue(namesWithA.contains("Aaron"));
+
+    }
+
+    @Test
+    public void whenUsingUnaryOperatorWithReplaceAll_thenAllValuesInTheListAreReplaced() {
+
+        List<String> names = Arrays.asList("bob", "josh", "megan");
+
+        names.replaceAll(String::toUpperCase);
+
+        assertEquals("BOB", names.get(0));
+        assertEquals("JOSH", names.get(1));
+        assertEquals("MEGAN", names.get(2));
+
+    }
+
+    @Test
+    public void whenUsingBinaryOperatorWithStreamReduce_thenResultIsSumOfValues() {
+
+        List<Integer> values = Arrays.asList(3, 5, 8, 9, 12);
+
+        int sum = values.stream()
+                .reduce((acc, value) -> acc + value)
+                .get();
+
+        assertEquals(37, sum);
+
+    }
+
+    @Test
+    public void whenComposingTwoFunctions_thenFunctionsExecuteSequentially() {
+
+        Function<Integer, String> intToString = Object::toString;
+        Function<String, String> quote = s -> "'" + s + "'";
+
+        Function<Integer, String> quoteIntToString = quote.compose(intToString);
+
+        assertEquals("'5'", quoteIntToString.apply(5));
+
+    }
+
+
+}

core-java-8/src/test/java/com/baeldung/functionalinterface/ShortToByteFunction.java

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+package com.baeldung.functionalinterface;
+
+@FunctionalInterface
+public interface ShortToByteFunction {
+
+    byte applyAsByte(short s);
+
+}