c_9_4_new_stream_methods

🧠 9.4 Presenting the New Stream Methods

• Before Java 8, the techniques to perform complex file operations in NIO.2 required alot of code, often requiring its own class.

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🟥 9.4.1 Conceptualising Directory Walking

• A common task in a file system is to iterate over descendants of a particular path
• E.g. you may want to look in a directory and collect all .java files.
• Walking a directory is the process of starting at a parent directory and iterate over all of its descendants until condition is met, or all files are iterated

⭐ Search Strategy ⭐

• There are 2 common strategies for walking a directory tree:

1. Depth-first search will go all the way down to a particular path, once it hits the end, it will back track up a node and traverse the paths its skipped
2. Breadth-first will traverse down 1 depth level, look at all records at that level, and then go down another depth level. Result will be ordered by depth

• For the exam, I need to know that the Streams API uses depth-first search, with a default maximum depth of Integer.MAX_VALUE

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🟥 9.4.2 Walking a Directory

• In chapter 4 we saw the methods which were available to us from the Streams API
• The File.walk(Path) method is a newly added NIO.2 API method as part of Java 8 which aims to reduce the amount of code needed to walk a directory
• This method will traverse in a depth-first and lazy manner - child elements are not loaded unless the target can not be found (improves performance)
• This method has the following signatures:

Stream<Path> walk(Path) throws IOException
Stream<Path> walk(Path, int maxDepth) throws IOException

• Here is code which will print all the files ending with .java in my chapter_9 directory of this repo:

Path chap9 = Paths.get("src//chapter_9");
try {
    Files.walk(chap9)
    .filter(p->p.toString().endsWith(".java"))
    .forEach(System.out::println);
} catch (IOException e) { /* handle */ }

• This printed the following:

src\chapter_9\c_9_1_intro_nio2\javacode\UsingPaths.java
\chapter_9\c_9_3_understanding_file_attributes\java\c_9_3_1\ReadingFileLength.java
src\chapter_9\c_9_3_understanding_file_attributes\java\c_9_3_1\TestingFileAccessibility.java
src\chapter_9\c_9_3_understanding_file_attributes\java\c_9_3_2\ModifyingAttributes.java
src\chapter_9\c_9_3_understanding_file_attributes\java\c_9_3_2\ReadingAttributes.java
src\chapter_9\c_9_4_new_stream_methods\java\WalkingADirectory.java
...

• I could've supplied the max depth argument to this method. If I chose 3 as max depth, I would get the following instead:

src\chapter_9\c_9_1_intro_nio2\javacode\UsingPaths.java
src\chapter_9\c_9_2_interacting_with_paths_files\javacode\CheckingPathType.java
src\chapter_9\c_9_2_interacting_with_paths_files\javacode\CreatingNewPathsWithSubpath.java
src\chapter_9\c_9_2_interacting_with_paths_files\javacode\DuplicatingFileContents.java

⭐ Avoiding Circular Paths ⭐

• By default, symbolic links will not be traversed as this could lead to a cycle where a symbolic link points a to a ancestor and an infinite loop
• You can disable this default behaviour, by overloading an option of FOLLOW_LINKS to the walk() method
• Java can detect if an circular path has occured, by seeing if the path it is traversing has already been traversed
• If it detects a circular path, a FileSystemLoopException is thrown⚠️

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🟥 9.4.3 Searching a Directory

• We have a find() method in the Files class:

Stream<Path> find(Path,int,BiPredicate<Path, BasicFileAttribute>)

• This allows us to to filter based both on the path and it's attributes:

Path chap_9 = Paths.get("src//chapter_9");
try {
    BiPredicate<Path, BasicFileAttributes> biPred =
        (p,basicFileAttr) -> path.toString().endsWith(".java")
            && basicFileAttr.size() > 1234;
    Stream<Path> stream = Files.walk(chap_9, 3, biPred);
    stream.forEach(System.out::println);
} catch (IOException e) { }
/* this prints the following:
src\chapter_9\c_9_2_interacting_with_paths_files\javacode\MakingDirectories.java
src\chapter_9\c_9_2_interacting_with_paths_files\javacode\PathFilePathTest.java
src\chapter_9\c_9_2_interacting_with_paths_files\javacode\UsingDeleteAndDeleteIfExists.java
src\chapter_9\c_9_2_interacting_with_paths_files\javacode\UsingPathObjects.java
src\chapter_9\c_9_4_new_stream_methods\java\WalkingADirectory.java
*/

• This method also supports the FOLLOW_LINKS option

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🟥 9.4.4 Listing Directory Contents

• We have a Files.list(Path) method is equivalent to listFiles() from the File class
• This will return files 1 level deep of a Path, which is also equivalent to using Files.walk() with a maxDepth value of 1
• Example:

Path chap9 = Paths.get("src//chapter_9");
try {
    Stream<Path> stream = Files.list(chap9);
    stream.forEach(System.out::println);
}

• This prints the following:

src\chapter_9\c_9_1_intro_nio2
src\chapter_9\c_9_2_interacting_with_paths_files
src\chapter_9\c_9_3_understanding_file_attributes
src\chapter_9\c_9_4_new_stream_methods
src\chapter_9\README.md

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🟥 9.4.5 Printing File Contents

• We saw the List<String> Files.readAllLines(Path) from before which has the oppurtunity to throw a OutOfMemoryException
• We have a stream equivalent method: Stream<String> Files.lines(Path)

Path  chap9ReadMe = Paths.get("src//chapter_9//README.md");
try {
    List<String> list = Files.readAllLines(chap9ReadMe); 
    Stream<String> stream = Files.lines(chap9ReadMe);
} catch (IOException e) { }