Chapter 14 I/O (Notes) Flashcards

Question

Consider the following method, which prints various Path elements: ``` public void printPathInformation(Path path) { System.out.println("Filename is: " + path.getFileName()); System.out.println(" Root is: " + path.getRoot()); Path currentParent = path; while((currentParent = currentParent.getParent()) != null) System.out.println(" Current parent is: " + currentParent); System.out.println(); } ```

Answer 1

The while loop in the printPathInformation() method continues until getParent() returns null. We apply this method to the following three paths: ``` printPathInformation(Path.of("zoo")); printPathInformation(Path.of("/zoo/armadillo/shells.txt")); printPathInformation(Path.of("./armadillo/../shells.txt")); ``` This sample application produces the following output: ``` Filename is: zoo Root is: null Filename is: shells.txt Root is: / Current parent is: /zoo/armadillo Current parent is: /zoo Current parent is: / Filename is: shells.txt Root is: null Current parent is: ./armadillo/.. Current parent is: ./armadillo Current parent is: . ```

Answer 2

* **On the exam, when you see ***`resolve()`***, think *`concatenation`*.** * The ***`resolve()`*** method provides ***`overloaded`*** versions that let you pass either a ***`Path`* or *`String`* parameter**. * ***`public Path resolve(String p)`*** * ***`public Path resolve(Path p)`*** * **The object on which the resolve() method is invoked becomes the basis of the new Path object, with the input argument being appended onto the Path.** **Apply resolve() to an absolute path and a relative path:** ``` Path path1 = Path.of("/cats/../panther"); Path path2 = Path.of("food"); System.out.println(path1.resolve(path2)); ``` **output :** ***`/cats/../panther/food`*** For the exam, you should be cognizant of * **mixing *`absolute`* and *`relative` paths* with the *`resolve()`* method.** * **If an *`absolute`* path is provided as input to the method, that is the value returned.** * Simply put, you **cannot combine two absolute paths using resolve()**. ``` Path path3 = Path.of("/turkey/food"); System.out.println(path3.resolve("/tiger/cage")); ``` **output :** ***`/tiger/cage`***

Answer 3

The Path interface includes a ***`relativize()`*** method for **constructing the relative path from one Path to another, often using path symbols.** * **If both path values are relative, the relativize() method computes the paths as if they are in the same current working directory.** * Alternatively, **if both path values are absolute, the method computes the relative path from one absolute location to another, regardless of the current working directory.** * The **relativize()** method **requires both paths to be absolute or relative** and throws an exception if the types are mixed. * **On Windows-based systems, it also requires that if absolute paths are used, both paths must have the same root directory or drive letter**

Answer 4

**The examples print the following:** ``` ../friendly/birds.txt ../../fish.txt ``` The idea is this: if you are pointed at a path in the file system, what steps would you need to take to reach the other path? For example, to get to fish.txt from friendly/birds.txt, you need to go up two levels **(the file itself counts as one level)** and then select fish.txt.

Answer 5

This code snippet produces the following **output**: ``` ..\sanctuary\raven\poe.txt ..\..\..\habitat ```

Answer 6

The **relativize()** method **requires both paths to be absolute or relative** and throws an exception if the types are mixed. **On Windows-based systems, it also requires that if absolute paths are used, both paths must have the same root directory or drive letter**.

Answer 7

**The *`normalize()`* method removes any redundant elements, which includes any *`"." or "directory/.."`* occurrences.** ``` var p1 = Path.of("./armadillo/../shells.txt"); System.out.println(p1.normalize()); // shells.txt var p2 = Path.of("/cats/../panther/food"); System.out.println(p2.normalize()); // /panther/food ``` **The *`normalize()`* method does not remove all of the path symbols, only the ones that can be reduced.** ``` var p3 = Path.of("../../fish.txt"); System.out.println(p3.normalize()); // ../../fish.txt ```

Answer 8

* **The *`normalize()`* method also allows us to compare equivalent paths.** * **The equals() method returns true if two paths represent the same value.** * In the first comparison, the path values are different. * In the second comparison, the path values have both been reduced to the same normalized value, ***`/weather.txt`***. * This is the primary function of the normalize() method: **to allow us to better compare different paths.**

Answer 9

* **verify that the path exists** within the file system using ***`toRealPath()`***. * This method is **similar to *`normalize()`*** in that it **eliminates any redundant path symbols**. * It is also similar to ***`toAbsolutePath()`***, in that it will join the path with the current working directory if the path is relative. * Unlike those two methods, though, **toRealPath() will throw an exception if the path does not exist.** * In addition, it will **follow symbolic links**, with an optional LinkOption varargs parameter to ignore them.

Answer 10

The **output** of both lines is the following: `/horse/food.txt` In this example, the absolute and relative paths both resolve to the same absolute file, as the symbolic link points to a real file within the file system. **We can also use the toRealPath() method to gain access to the current working directory as a Path object.** `System.out.println(Paths.get(".").toRealPath());`

Answer 11

**TABLE 14.6 Path APIs** * **File path as string** - public String **toString**() * **Single segment** - public Path **getName**(int index) * **Number of segments** - public int **getNameCount**() * **Segments in range** - public Path **subpath**(int beginIndex, int endIndex) * **Final segment** - public Path **getFileName**() * **Immediate parent** - public Path **getParent**() * **Top-level segment** - public Path **getRoot**() * **Concatenate paths** - public Path **resolve**(String p), public Path **resolve**(Path p) * **Construct path to one provided** - public Path **relativize**(Path p) * **Remove redundant parts of path** - public Path **normalize**() * **Follow symbolic links to find path on file system** - public Path **toRealPath**()

Answer 12

* ***Making Directories*** **Both of these methods also accept an optional list of *FileAttribute``* values to apply to the newly created directory or directories.** * **public static Path *`createDirectory`*(Path dir, FileAttribute``… attrs) throws IOException** * **create a directory** * **throw an exception if it already exists** or **if the paths leading up to the directory do not exist** * **public static Path *`createDirectories`*(Path dir, FileAttribute``… attrs) throws IOException** * **Creates the target directory along with any nonexistent parent directories leading up to the path** * **Do nothing If all of the directories already exist** * **Use to ensure a directory exists and create it if it does not.** * **Copying Files** **The Files class provides a method for copying files and directories within the file system.** **When directories are copied, the copy is *`shallow`*.** * **public static Path copy(Path source, Path target, CopyOption… options) throws IOException** * **Copying Files with I/O Streams** * **public static long copy(*`InputStream`* in, Path target, CopyOption… options) throws IOException** * **public static long copy(Path source, *`OutputStream`* out) throws IOException** * **Moving or Renaming Paths with move()** * **public static Path move(Path source, Path target, CopyOption… options) throws IOException** * **Deleting a File with delete() and deleteIfExists()** * **public static void delete(Path path) throws IOException** * **public static boolean deleteIfExists(Path path) throws IOException**

Answer 13

* The first example creates a new directory, field, in the directory /bison, assuming /bison exists; otherwise, an exception is thrown. * Contrast this with the second example, which creates the directory green along with any of the following parent directories if they do not already exist, including bison, field, and pasture.

Answer 14

**When directories are copied, the copy is shallow.**

Answer 15

The method first copies the path, whether a file or a directory. If it is a directory, only a shallow copy is performed. Next, it checks whether the path is a directory and, if it is, performs a recursive copy of each of its elements. What if the method comes across a symbolic link? Don't worry: the JVM will not follow symbolic links when using the list() method.

Answer 16

* **By default, if the target already exists, the copy() method will throw an exception.** * You can change this behavior by providing the **StandardCopyOption** enum value **`REPLACE_EXISTING`** to the method. * The following method call will overwrite the movie.txt file if it already exists: ``` Files.copy(Paths.get("book.txt"), Paths.get("movie.txt"), StandardCopyOption.REPLACE_EXISTING); ``` **For the exam, you need to know that without the *`REPLACE_EXISTING`* option, this method will throw an exception if the file already exists.**

Answer 17

The Files class includes two copy() methods that operate with I/O streams. ``` public static long copy(InputStream in, Path target, CopyOption… options) throws IOException public static long copy(Path source, OutputStream out) throws IOException ``` The first method reads the contents of an I/O stream and writes the output to a file. The second method reads the contents of a file and writes the output to an I/O stream. These methods are quite convenient if you need to quickly read/write data from/to disk.

Answer 18

While we used FileInputStream in the first example, the I/O stream could have been any valid I/O stream including website connections, in-memory stream resources, and so forth. The second example prints the contents of a file directly to the System.out stream.

Answer 19

For the exam, it is important that you understand how the **copy() method operates on both files and directories.**

Answer 20

If you said it would create a new file at /enclosure/food.txt, you're way off. It throws an exception. **The command tries to create a new file named /enclosure. Since the path /enclosure already exists, an exception is thrown at runtime.** On the other hand, if the directory did not exist, the process would create a new file with the contents of food.txt, but the file would be called `/enclosure`. **Remember, we said files may not need to have extensions, and in this example, it matters.**

Answer 21

``` var file = Paths.get("food.txt"); var directory = Paths.get("/enclosure/food.txt"); Files.copy(file, directory); ```

Answer 22

The Files class provides a useful method for moving or renaming files and directories. ``` public static Path move(Path source, Path target, CopyOption… options) throws IOException ``` The following sample code uses the move() method: ``` Files.move(Path.of("C:\\zoo"), Path.of("C:\\zoo-new")); Files.move(Path.of("C:\\user\\addresses.txt"), Path.of("C:\\zoo-new\\addresses2.txt")); ``` * The first example renames the zoo directory to a zoo-new directory, keeping all of the original contents from the source directory. * The second example moves the addresses.txt file from the directory user to the directory zoo-new and renames it addresses2.txt.

Answer 23

* Like **copy(), move() requires `REPLACE_EXISTING` to overwrite the target if it exists; otherwise, it will throw an exception.** * Also like **copy(), move() will not put a file in a directory if the source is a file and the target is a directory. Instead, it will create a new file with the name of the directory.**

Answer 24

Another enum value that you need to know for the exam when working with the move() method is the **StandardCopyOption** value **`ATOMIC_MOVE`**. ``` Files.move(Path.of("mouse.txt"), Path.of("gerbil.txt"), StandardCopyOption.ATOMIC_MOVE); ``` You may remember the atomic property from Chapter 13, “Concurrency,” and the principle of an atomic move is similar. **An atomic move is one in which a file is moved within the file system as a single indivisible operation.** Put another way, any process monitoring the file system never sees an incomplete or partially written file. **If the file system does not support this feature, an *`AtomicMoveNotSupportedException`* will be thrown.** Note that while `ATOMIC_MOVE` is available as a member of the StandardCopyOption type, it will likely throw an exception if passed to a copy() method.

Answer 25

The Files class includes two methods that delete a file or empty directory within the file system. ``` public static void delete(Path path) throws IOException public static boolean deleteIfExists(Path path) throws IOException ``` * **To delete a directory, it must be empty. Both of these methods throw an exception if operated on a nonempty directory.** * In addition, **if the path is a symbolic link, the symbolic link will be deleted, not the path that the symbolic link points to.** * The methods differ on how they handle a path that does not exist. * The delete() method throws an exception if the path does not exist, * while the deleteIfExists() method returns true if the delete was successful or false otherwise. * Similar to createDirectories(), deleteIfExists() is useful in situations where you **want to ensure that a path does not exist and delete it if it does**. Here we provide sample code that performs delete() operations: ``` Files.delete(Paths.get("/vulture/feathers.txt")); Files.deleteIfExists(Paths.get("/pigeon")); ``` * The first example deletes the feathers.txt file in the vulture directory, and it throws a NoSuchFileException if the file or directory does not exist. * The second example deletes the pigeon directory, assuming it is empty. If the pigeon directory does not exist, the second line will not throw an exception.

Answer 26

**Since a path may include path symbols and symbolic links within a file system, the *`equals()` method can't be relied* on to know if two Path instances refer to the same file.** Luckily, there is the **isSameFile()** method. * This method takes **two Path objects as input**, * **resolves all path symbols**, * and **follows symbolic links**. * Despite the name, the method can also be used to determine whether two Path objects refer to the **same directory**. * While most uses of **`isSameFile()` will trigger an exception if the paths do not exist**, there is a special case in which it does not. * **If the two path objects are equal in terms of *`equals()`*, the method will just return true without checking whether the file exists.**

Answer 27

Since snake is a symbolic link to cobra, the first example outputs **true**. In the second example, the paths refer to different files, so **false** is printed.

Answer 28

Sometimes you want to **compare the contents of the file rather than whether it is physically the same file.** For example, we could have two files with text hello. * The mismatch() method was introduced in Java 12 to help us out here. * It takes two Path objects as input. * The method returns -1 if the files are the same; * otherwise, it returns the index of the first position in the file that differs. ``` System.out.println(Files.mismatch( Path.of("/animals/monkey.txt"), Path.of("/animals/wolf.txt"))); ``` Suppose monkey.txt contains the name Harold and wolf.txt contains the name Howler. The previous code prints 1 in that case because the second position is different, and we use zero-based indexing in Java. Given those values, what do you think this code prints? ``` System.out.println(Files.mismatch( Path.of("/animals/wolf.txt"), Path.of("/animals/monkey.txt"))); ``` The answer is the same as the previous example. The code prints 1 again. The mismatch() method is symmetric and returns the same result regardless of the order of the parameters.

Answer 29

* **The contents of a file may be *accessed* or *written* via an *I/O stream***, * **which is a *list of data elements presented sequentially*.** * **Each type of *I/O stream segments data into a wave or block* in a particular way.** * On top of that, some **I/O stream classes read or write larger groups of *bytes* or *characters* at a time**, specifically those with the word **Buffered** in their name.

Answer 30

**Higher-level I/O streams exist for *`convenience`* as well as *`performance`*.**

Answer 31

When writing code where you don't know what the I/O stream size will be at runtime, it may be helpful to visualize an I/O stream as being so large that all of the data contained in it could not possibly fit into memory. For example, a 1 TB file could not be stored entirely in memory by most computer systems (at the time this book is being written). The file can still be read and written by a program with very little memory, since the I/O stream allows the application to focus on only a small portion of the overall I/O stream at any given time.

Answer 32

The java.io API provides numerous classes for creating, accessing, and manipulating I/O streams`—`so many that it tends to overwhelm many new Java developers. Stay calm! **We review the major differences between each I/O stream class and show you how to distinguish between them.** **Even if you come across a particular I/O stream on the exam that you do not recognize, the name of the I/O stream often gives you enough information to understand exactly what it does.** The goal of this section is to familiarize you with common terminology and naming conventions used with I/O streams. Don't worry if you don't recognize the particular stream class names used in this section or their function; we cover how to use them in detail in this chapter.

Answer 33

Data is stored in a file system (and memory) as a 0 or 1, called a bit. Since it's really hard for humans to read/write data that is just 0s and 1s, they are grouped into a set of 8 bits, called a byte.

Answer 34

**The java.io API defines *`two` sets of I/O stream classes* for reading and writing I/O streams:** * ***`byte`* I/O streams** * **read/write *`binary data (0s and 1s)`*** * **class names that end in `InputStream` or *`OutputStream`*** * and ***`character`* I/O streams**. * **read/write *`text`* data** * **class names that end in *`Reader`* or *`Writer`*** **The API frequently includes *`similar classes for both byte and character I/O streams`*, such as *`FileInputStream`* and *`FileReader`*.**

Answer 35

* **The *`character encoding`* determines how characters are encoded and stored in bytes in an I/O stream and later read back or decoded as characters.** * Although this may sound simple, Java supports a wide variety of character encodings, ranging from ones that may use one byte for Latin characters, **UTF-8** and **ASCII** for example, to using two or more bytes per character, such as **UTF-16**. * For the exam, you don't need to memorize the character encodings, but you should be familiar with the names. **In Java, the character encoding can be specified using the *`Charset class`* by passing a name value to the *`static Charset.forName() method`***, such as in the following examples: ``` Charset usAsciiCharset = Charset.forName("US-ASCII"); Charset utf8Charset = Charset.forName("UTF-8"); Charset utf16Charset = Charset.forName("UTF-16"); ``` Java supports numerous character encodings, each specified by a different standard name value.

Answer 36

* **Most *`InputStream`* classes have a corresponding *`OutputStream`* class**, and vice versa. * For example, the ***`FileOutputStream`*** class writes data that can be read by a ***`FileInputStream`***. * There are **exceptions to this rule**. For the exam, you should know that * ***`PrintWriter`* has no accompanying PrintReader class.** * Likewise, the ***`PrintStream`* is an OutputStream that has no corresponding InputStream class**. It also does not have Output in its name.

Answer 37

* Another way that you can familiarize yourself with the java.io API is by **segmenting I/O streams into** * **low-level** * and **high-level streams**. * **A *`low-level stream`* connects directly with the source of the data, such as a *`file`*, an *`array`*, or a *`String`***. * ***`Low-level I/O streams`* process the raw data or resource and are accessed in a direct and unfiltered manner. For example, a *`FileInputStream`* is a class that reads file data one byte at a time.** * Alternatively, a ***`high-level stream`* is built on top of another I/O stream using *`wrapping`*.** * ***`Wrapping`* is the process by which an instance is passed to the *`constructor`* of another class, and operations on the resulting instance are filtered and applied to the original instance.** * For **example**, take a look at the FileReader and BufferedReader objects in the following sample code: ``` try (var br = new BufferedReader(new FileReader("zoo-data.txt"))) { System.out.println(br.readLine()); } ``` * In this example, **FileReader is the low-level I/O stream**, whereas **BufferedReader is the high-level I/O stream that takes a FileReader as input**. * **Many operations on the high-level I/O stream pass through as operations to the underlying low-level I/O stream**, such as ***`read()`*** or ***`close()`***. * Other operations override or add new functionality to the low-level I/O stream methods. * The ***`high-level I/O stream`* may add new methods, such as *`readLine()`*, as well as *`performance enhancements`* for reading and filtering the low-level data.** * **High-level I/O streams can also take other high-level I/O streams as input.** For example, although the following code might seem a little odd at first, the style of **wrapping an I/O stream is quite common in practice:** ``` try (var ois = new ObjectInputStream(new BufferedInputStream(new FileInputStream("zoo-data.txt")))) { System.out.print(ois.readObject()); } ``` In this example, the low-level FileInputStream interacts directly with the file, which is wrapped by a high-level BufferedInputStream to improve performance. Finally, the entire object is wrapped by another high-level ObjectInputStream, which allows us to interpret the data as a Java object. For the exam, the only low-level stream classes you need to be familiar with are the ones that operate on files. The rest of the nonabstract stream classes are all high-level streams.

Answer 38

**The java.io library defines *`four abstract classes`* that are the *`parents of all I/O stream classes`* defined within the API:** 1. **InputStream**, 2. **OutputStream**, 3. **Reader**, 4. and **Writer**. **The constructors of high-level I/O streams often take a reference to the abstract class.** For example, **BufferedWriter takes a Writer object as input, which allows it to take any subclass of Writer.**

Answer 39

* The first two examples **do not compile because they mix Reader/Writer classes with InputStream/OutputStream classes**, respectively. * The third example **does not compile because we are mixing an OutputStream with an InputStream**. Although it is possible to read data from an InputStream and write it to an OutputStream, wrapping the I/O stream is not the way to do so. As you see later in this chapter, the data must be copied over. * Finally, the last example **does not compile because InputStream is an abstract class, and therefore you cannot create an instance of it.**

Answer 40

**TABLE 14.7 The java.io abstract stream base classes** * **InputStream** - Abstract class for all input byte streams * **OutputStream** - Abstract class for all output byte streams * **Reader** - Abstract class for all input character streams * **Writer** - Abstract class for all output character streams **TABLE 14.8 The java.io concrete I/O stream classes** * **FileInputStream** - **Low** - Reads file data as bytes * **FileOutputStream** - **Low** - Writes file data as bytes * **FileReader** - **Low** - Reads file data as characters * **FileWriter** - **Low** - Writes file data as characters * **BufferedInputStream**- **High**- Reads byte data from existing InputStream in buffered manner, which improves efficiency and performance * **BufferedOutputStream**- High- Writes byte data to existing OutputStream in buffered manner, which improves efficiency and performance * **BufferedReader**- **High**- Reads character data from existing Reader in buffered manner, which improves efficiency and performance * **BufferedWriter**- **High**- Writes character data to existing Writer in buffered manner, which improves efficiency and performance * **ObjectInputStream**- **High**- Deserializes primitive Java data types and graphs of Java objects from existing bInputStream * **ObjectOutputStream**- **High**- Serializes primitive Java data types and graphs of Java objects to existing OutputStream * **PrintStream**- **High**- Writes formatted representations of Java objects to binary stream * **PrintWriter**- **High**- Writes formatted representations of Java objects to character stream

Answer 41

* **Both *`InputStream`* and *`Reader`* declare a *`read()`* method to read byte data from an I/O stream.** * Likewise, ***`OutputStream`* and *`Writer`* both define a *`write()`* method to write a byte to the stream**: In both examples, ***`-1`* is used to indicate the end of the stream.** ``` void copyStream(InputStream in, OutputStream out) throws IOException { int b; while ((b = in.read()) != -1) { out.write(b); } } void copyStream(Reader in, Writer out) throws IOException { int b; while ((b = in.read()) != -1) { out.write(b); } } ``` **Why do the methods use *`int`* instead of *`byte`*?** * the byte data type has a range of 256 characters. * They needed an extra value to indicate the end of an I/O stream. * The authors of Java decided to use a larger data type, int, so that special values like -1 would indicate the end of an I/O stream. * The output stream classes use int as well, to be consistent with the input stream classes. * Reading and writing one byte at a time isn't a particularly efficient way of doing this. * Luckily, there are overloaded methods for reading and writing multiple bytes at a time. * The offset and length values are applied to the array itself. * For example, an offset of 3 and length of 5 indicates that the stream should read up to five bytes/characters of data and put them into the array starting with position 3.

Answer 42

* Instead of reading the data one byte at a time, we read and write up to 1024 bytes at a time on line 14. * The return value lengthRead is critical for determining whether we are at the end of the stream and knowing how many bytes we should write into our output stream. * Unless our file happens to be a multiple of 1024 bytes, the last iteration of the while loop will write some value less than 1024 bytes. * For example, if the buffer size is 1,024 bytes and the file size is 1,054 bytes, the last read will be only 30 bytes. If we ignored this return value and instead wrote 1,024 bytes, 994 bytes from the previous loop would be written to the end of the file. * We also added a flush() method on line 16 to reduce the amount of data lost if the application terminates unexpectedly. When data is written to an output stream, the underlying operating system does not guarantee that the data will make it to the file system immediately. The flush() method requests that all accumulated data be written immediately to disk. It is not without cost, though. Each time it is used, it may cause a noticeable delay in the application, especially for large files. Unless the data that you are writing is extremely critical, the flush() method should be used only intermittently. For example, it should not necessarily be called after every write, as it is in this example.

Answer 43

* The key is to choose the most useful high-level classes. * In this case, we are dealing with a File, so we want to use a FileReader and FileWriter. * Both classes have constructors that can take either a String representing the location or a File directly. * If the source file does not exist, a FileNotFoundException, which inherits IOException, will be thrown. * If the destination file already exists, this implementation will overwrite it. We can pass an optional boolean second parameter to FileWriter for an append flag if we want to change this behavior.

Answer 44

PrintStream

Answer 45

The NIO.2 APIs provide even easier ways to read and write a file using the Files class. Let's start by looking at three ways of copying a file by reading in the data and writing it back: ``` private void copyPathAsString(Path input, Path output) throws IOException { String string = Files.readString(input); Files.writeString(output, string); } private void copyPathAsBytes(Path input, Path output) throws IOException { byte[] bytes = Files.readAllBytes(input); Files.write(output, bytes); } private void copyPathAsLines(Path input, Path output) throws IOException { List lines = Files.readAllLines(input); Files.write(output, lines); } ``` That's pretty concise! You can read a Path as a String, a byte array, or a List. Be aware that the entire file is read at once for all three of these, thereby storing all of the contents of the file in memory at the same time. If the file is significantly large, you may trigger an OutOfMemoryError when trying to load all of it into memory. Luckily, there is an alternative. This time, we print out the file as we read it. ``` private void readLazily(Path path) throws IOException { try (Stream s = Files.lines(path)) { s.forEach(System.out::println); } } ``` Now the contents of the file are read and processed lazily, which means that only a small portion of the file is stored in memory at any given time. Taking things one step further, we can leverage other stream methods for a more powerful example. ``` try (var s = Files.lines(path)) { s.filter(f -> f.startsWith("WARN:")) .map(f -> f.substring(5)) .forEach(System.out::println); } ``` This sample code searches a log for lines that start with WARN:, outputting the text that follows. Assuming that the input file sharks.log is as follows: ``` INFO:Server starting DEBUG:Processes available = 10 WARN:No database could be detected DEBUG:Processes available reset to 0 WARN:Performing manual recovery INFO:Server successfully started ``` Then the sample output would be the following: ``` No database could be detected Performing manual recovery ``` As you can see, we have the ability to manipulate files in complex ways, often with only a few short expressions.

Answer 46

For the exam, you need to know the difference between readAllLines() and lines(). Both of these examples compile and run: ``` Files.readAllLines(Paths.get("birds.txt")).forEach(System.out::println); Files.lines(Paths.get("birds.txt")).forEach(System.out::println); ``` The first line reads the entire file into memory and performs a print operation on the result, while the second line lazily processes each line and prints it as it is read. The advantage of the second code snippet is that it does not require the entire file to be stored in memory at any time. You should also be aware of when they are mixing incompatible types on the exam. Do you see why the following does not compile? ``` Files.readAllLines(Paths.get("birds.txt")) .filter(s -> s.length()> 2) .forEach(System.out::println); ``` The readAllLines() method returns a List, not a Stream, so the filter() method is not available.

Answer 47

Sometimes you need to mix I/O streams and NIO.2. Conveniently, Files includes two convenience methods for getting I/O streams. ``` private void copyPath(Path input, Path output) throws IOException { try (var reader = Files.newBufferedReader(input); var writer = Files.newBufferedWriter(output)) { String line = null; while ((line = reader.readLine()) != null) writer.write(line); writer.newLine(); } } } ``` You can wrap I/O stream constructors to produce the same effect, although it's a lot easier to use the factory method. The first method, newBufferedReader(), reads the file specified at the Path location using a BufferedReader object.

Answer 48

TABLE 14.9 Common I/O read and write methods TABLE 14.10 Common Files NIO.2 read and write methods

Answer 49

* Serialization is the process of converting an in-memory object to a byte stream. * deserialization is the process of converting from a byte stream into an object. * Serialization often involves writing an object to a stored or transmittable format, while deserialization is the reciprocal process. * Java provides built-in mechanisms for serializing and deserializing I/O streams of objects directly to and from disk, respectively.

Answer 50

* To serialize an object using the I/O API, the object **must implement the java.io.Serializable interface.** * The **Serializable interface is a marker interface**, which means it **does not have any methods.** * Any class can implement the Serializable interface since there are no required methods to implement.

Answer 51

In this example, the Gorilla class contains three instance members (name, age, friendly) that will be saved to an I/O stream if the class is serialized. Note that since Serializable is not part of the java.lang package, it must be imported or referenced with the package name. What about the favoriteFood field that is marked transient? Any field that is marked transient will not be saved to an I/O stream when the class is serialized. We discuss that in more detail next.

Answer 52

It's a good practice to declare a static serialVersionUID variable in every class that implements Serializable. The version is stored with each object as part of serialization. Then, every time the class structure changes, this value is updated or incremented. Perhaps our Gorilla class receives a new instance member Double banana, or maybe the age field is renamed. The idea is a class could have been serialized with an older version of the class and deserialized with a newer version of the class. The serialVersionUID helps inform the JVM that the stored data may not match the new class definition. If an older version of the class is encountered during deserialization, a java.io.InvalidClassException may be thrown. Alternatively, some APIs support converting data between versions.

Answer 53

The transient modifier can be used for sensitive data of the class, like a password. There are other objects it does not make sense to serialize, like the state of an in-memory Thread. If the object is part of a serializable object, we just mark it transient to ignore these select instance members. What happens to data marked transient on deserialization? It reverts to its default Java values, such as 0.0 for double, or null for an object. You see examples of this shortly when we present the object stream classes.

Answer 54

Since Serializable is a marker interface, you might think there are no rules to using it. Not quite! Any process attempting to serialize an object will throw a NotSerializableException if the class does not implement the Serializable interface properly.

Answer 55

* The class must be marked Serializable. * Every instance member of the class must be * serializable, * marked transient, * or have a null value at the time of serialization. Be careful with the second rule. For a class to be serializable, we must apply the second rule recursively. Do you see why the following Cat class is not serializable? ``` public class Cat implements Serializable { private Tail tail = new Tail(); } public class Tail implements Serializable { private Fur fur = new Fur(); } public class Fur {} ``` Cat contains an instance of Tail, and both of those classes are marked Serializable, so no problems there. Unfortunately, Tail contains an instance of Fur that is not marked Serializable. Either of the following changes fixes the problem and allows Cat to be serialized: ``` public class Tail implements Serializable { private transient Fur fur = new Fur(); } public class Fur implements Serializable {} ``` We could also make our tail or fur instance members null, although this would make Cat serializable only for particular instances, rather than all instances.

Answer 56

Do you think this record is serializable? `record Record(String name) {}` It is not serializable because it does not implement Serializable. A record follows the same rules as other types of classes with respect to whether it can be serialized. Therefore, this one can be: ` record Record(String name) implements Serializable {}`

Answer 57

The ObjectInputStream class is used to deserialize an object, while the ObjectOutputStream is used to serialize an object. They are high-level streams that operate on existing I/O streams. While both of these classes contain a number of methods for built-in data types like primitives, the two methods you need to know for the exam are the ones related to working with objects. // ObjectInputStream public Object readObject() throws IOException, ClassNotFoundException // ObjectOutputStream public void writeObject(Object obj) throws IOException Note the parameters, return types, and exceptions thrown. We now provide a sample method that serializes a List of Gorilla objects to a file: void saveToFile(List gorillas, File dataFile) throws IOException { try (var out = new ObjectOutputStream( new BufferedOutputStream( new FileOutputStream(dataFile)))) { for (Gorilla gorilla : gorillas) out.writeObject(gorilla); } } Pretty easy, right? Notice that we start with a file stream, wrap it in a buffered I/O stream to improve performance, and then wrap that with an object stream. Serializing the data is as simple as passing it to writeObject(). Once the data is stored in a file, we can deserialize it by using the following method: List readFromFile(File dataFile) throws IOException, ClassNotFoundException { var gorillas = new ArrayList(); try (var in = new ObjectInputStream( new BufferedInputStream( new FileInputStream(dataFile)))) { while (true) { var object = in.readObject(); if (object instanceof Gorilla g) gorillas.add(g); } } catch (EOFException e) { // File end reached } return gorillas; } Ah, not as simple as our save method, was it? When calling readObject(), null and -1 do not have any special meaning, as someone might have serialized objects with those values. Unlike our earlier techniques for reading methods from an input stream, we need to use an infinite loop to process the data, which throws an EOFException when the end of the I/O stream is reached.

Chapter 14 I/O (Notes) Flashcards

(119 cards)