Create functional_iterface.md

Author: janbodnar

functional/functional_iterface.md

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+# @FunctionalInterface
+
+## How to throw exception in lambda
+
+Create a custom `Consumer` that throws an exception.  
+
+```java
+import java.io.IOException;
+import java.nio.file.FileStore;
+import java.nio.file.FileSystem;
+import java.nio.file.FileSystems;
+import java.util.function.Consumer;
+
+@FunctionalInterface
+public interface ThrowingConsumer<T> extends Consumer<T> {
+
+    @Override
+    default void accept(final T elem) {
+        try {
+            acceptThrows(elem);
+        } catch (final Exception e) {
+            // Implement your own exception handling logic here..
+            // For example:
+//            System.out.println("handling an exception...");
+            // Or ...
+            throw new RuntimeException(e);
+        }
+    }
+
+    void acceptThrows(T elem) throws Exception;
+
+}
+
+void main(String[] args) throws IOException {
+
+    FileSystem fileSystem = FileSystems.getDefault();
+
+    System.out.printf("%30s | %10s | %23s | %20s \n", "", "Type", "Total space", "Free space");
+    System.out.println("-------------------------------------------------"
+            + "----------------------------------------------------------");
+
+    Iterable<FileStore> fileStores = fileSystem.getFileStores();
+
+//    for (var fileStore : fileStores) {
+//        System.out.printf("%30s | %10s | %20s GB | %20s GB\n", fileStore, fileStore.type(),
+//                (fileStore.getTotalSpace() / 1073741824f),
+//                (fileStore.getUsableSpace() / 1073741824f));
+//    }
+
+    fileStores.forEach((ThrowingConsumer<FileStore>) fileStore -> {
+
+            System.out.printf("%30s | %10s | %20s GB | %20s GB\n", fileStore, fileStore.type(),
+                    (fileStore.getTotalSpace() / 1073741824f),
+                    (fileStore.getUsableSpace() / 1073741824f));
+        });
+}
+```