1.5 Input and Output

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Standard input makes it convenient for us to write programs that process arbitrary amounts of input and to interact with our programs; standard draw makes it possible for us to work with graphics; and standard audio adds sound.

A Java program takes input values from the command line and prints a string of characters as output. By default, both command-line arguments and standard output are associated with an application that takes commands, which we refer to as the terminal window.

Here are continuing on binary input and output in java instructions for using the command line on your system. All of our classes have a main method that takes a String array args[] as argument.

That array is the sequence of command-line arguments that we type. If we intend for an argument to be a number, we must use a method such as Integer.

To print output values in our programs, we have been using System. Java sends the results to an abstract stream of characters known as standard output. By default, the operating system connects standard output to the terminal window. All of the output in our continuing on binary input and output in java so far has been appearing in the terminal window. It takes a command-line argument n and prints to standard output a sequence of n random numbers between 0 and 1.

To complete our programming model, we add the following libraries: Read numbers and strings from the user. Nevertheless, to treat standard input and standard output in a uniform manner and to provide a few technical improvementswe use similar methods that are defined in our StdOut library: Java's print and println methods are the ones that you have been using. The printf method gives us more control over the appearance of the output. In its simplest form, printf takes two arguments. The first argument is called the format string.

It contains a conversion specification that describes how the second argument is to be converted to a string for output. The following table summarizes the most frequently used codes: The format string can contain characters in addition to those for the conversion specification. The conversion specification is replaced by the argument value converted to a string as specified and all remaining characters are passed through to the output. The printf function can take more than two arguments.

In this case, the format string will have an additional conversion specification for each additional argument. Here is more documentation on printf format string syntax. Our StdIn library takes data from a standard input stream that contains a sequence of values separated by whitespace. Each value is a string or a value from one of Java's primitive types. One of the key features of the standard input stream is that your program consumes values when it reads them.

Once your program has read a value, it continuing on binary input and output in java back up and read it again. The library is defined by the following API: We now consider several examples in detail.

When you use the java command to invoke a Java program from the command line, you actually are doing three things: The string of characters that you type in the terminal window after the command line is the standard input stream.

If you type abc or StdIn treats strings of consecutive whitespace characters as identical to one space and allows continuing on binary input and output in java to delimit your numbers with such strings.

The program generates a random integer and then gives clues to a user trying to guess the number. The fundamental difference between this program and others that we have written is that the user has the ability to change the control flow while the program is executing.

Processing an arbitrary-size input stream. Typically, input streams are finite: But there is no restriction of the size of the input stream. For many applications, typing input data as a standard input stream from the terminal window is untenable because doing so limits our program's processing power by the amount of data that we can type. Similarly, we often want to save the information printed on the standard output stream for later use. We can use operating system mechanisms to address both issues.

Redirecting standard output to a file. By adding a simple directive to the command that invokes a program, we can redirect its standard output to a file, either for permanent storage or for input to some other program at a later time.

For example, continuing on binary input and output in java command specifies that the standard output stream is not to be printed in the terminal window, but instead is to be written to a text file named data. Each call to StdOut. In this example, the end result is a file that contains 1, random values. Redirecting standard output from a file. Similarly, we can redirect standard input so that StdIn reads data from a file instead of the terminal window.

For example, the command reads a sequence of numbers from the file data. When the program calls StdIn. Continuing on binary input and output in java facility to redirect standard input from a file enables us to process huge amounts of data from any source with our programs, limited only by the size of the files that we can store. The most flexible way to implement the standard input and standard output abstractions is to specify that they are implemented by our own programs!

This mechanism is called piping. For example, the following command specifies that the standard output for RandomSeq and the standard input stream for Average are the same stream.

For many common tasks, it is convenient to think of each program as a filter that converts a standard input stream to a standard output stream in some way, RangeFilter. Your operating system also provides a number of filters.

For example, the sort filter puts the lines on standard input in sorted order: Now we introduce a simple abstraction for producing drawings as output. We imagine an abstract drawing device capable of drawing lines and continuing on binary input and output in java on a two-dimensional canvas. The device continuing on binary input and output in java capable of responding to the commands that our programs issue in the form of calls to static methods in StdDraw.

The primary interface consists of two kinds of methods: We first consider the drawing commands: These methods are nearly self-documenting: The default coordinate scale is the unit square all x - and y -coordinates between 0 and 1. The standard implementation displays the canvas in a window on your computer's screen, with black lines and points on a white background. The HelloWorld for graphics programming with StdDraw is to draw a triangle with a point inside.

The default canvas size is by pixels and the default coordinate system is the unit square, but we often want to draw plots at different scales. Also, we often want to draw line segments of different thickness or points of different size from the standard.

To accommodate these needs, StdDraw has the following methods: For example, the two-call sequence StdDraw. Filtering data to a standard drawing. It adopts the convention that the first four numbers on standard input specify the bounding box, so that it can scale the plot.

There are an infinite number of points in the interval, so we have to make do with evaluating the function at a finite number of points within the interval. We sample the function by choosing a set of x -values, then computing y -values by evaluating the function at each x -value. Plotting the function by connecting successive points with lines produces what is known as a piecewise linear approximation.

Outline and filled shapes. StdDraw also includes methods to draw circles, rectangles, and arbitrary polygons. Each shape defines an outline. When the method name is just the shape name, that continuing on binary input and output in java is traced by the drawing pen.

When the method name begins with filledthe named shape is instead filled solid, not traced. The arguments for circle define a circle of radius r; the arguments for square define a square of side length 2r centered on the given point; and the arguments for polygon define a sequence of points that we connect by lines, including one from the last point to the first point.

To annotate or highlight various elements in your drawings, StdDraw includes methods for drawing text, setting the font, and setting the the ink in the pen. In this code, java. Color are abstractions that are implemented with non-primitive types that you will learn about in Section 3.

Until then, we leave the details to StdDraw. The default ink color is black; the default font is a point plain Serif font. StdDraw supports a powerful computer graphics feature known as double buffering. When double buffering is enabled by calling enableDoubleBufferingall drawing takes place on the offscreen canvas. The offscreen canvas is not displayed; it exists only in computer memory. Only when you call show does your drawing get copied from the offscreen canvas to the onscreen canvaswhere it is displayed in the standard drawing window.

You can think of double buffering as collecting all of the lines, points, shapes, and text that you tell it to draw, and then drawing them all simultaneously, upon request. One reason to use double buffering is for efficiency when performing a large number of drawing commands. Our most important use of double buffering is to produce computer animationswhere we create the illusion of motion by rapidly displaying static drawings.

We can produce animations by repeating the following four steps: Clear the offscreen canvas. Draw objects on the offscreen Copy the offscreen canvas to the onscreen canvas.

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You could do it by looking at a few samples. Scanner and Formatter , and C-like printf and format methods for formatted output using format specifiers. It also introduces a new try -with-resources syntax to simplify the coding of close method. File can represent either a file or a directory. Path , which overcomes many limitations of java. A path string is used to locate a file or a directory. Unfortunately, path strings are system dependent, e.

A path could be absolute beginning from the root or relative which is relative to a reference directory. File class maintains these system-dependent properties, for you to write programs that are portable:. You can construct a File instance with a path string or URI, as follows. A file URL takes the form of file: For applications that you intend to distribute as JAR files, you should use the URL class to reference the resources, as it can reference disk files as well as JAR'ed files , for example,.

The following program recursively lists the contents of a given directory similar to Unix's "ls -r" command. You can apply a filter to list and listFiles , to list only files that meet a certain criteria.

FilenameFilter declares one abstract method:. You can program your filtering criteria in accept. The following program lists only files that meet a certain filtering criteria. Programs read inputs from data sources e. A stream is a sequential and contiguous one-way flow of data just like water or oil flows through the pipe. It is important to mention that Java does not differentiate between the various types of data sources or sinks e. They are all treated as a sequential flow of data.

The Java program receives data from a source by opening an input stream, and sends data to a sink by opening an output stream.

If your program needs to perform both input and output, you have to open two streams - an input stream and an output stream.

Java internally stores characters char type in bit UCS-2 character set. All the byte streams are derived from the abstract superclasses InputStream and OutputStream , as illustrated in the class diagram. The abstract superclass InputStream declares an abstract method read to read one data-byte from the input source:.

The read method returns an int instead of a byte , because it uses -1 to indicate end-of-stream. The term " block " means that the method and the program will be suspended. The program will resume only when the method returns.

Two variations of read methods are implemented in the InputStream for reading a block of bytes into a byte-array. It returns the number of bytes read, or -1 if "end-of-stream" encounters. Similar to the input counterpart, the abstract superclass OutputStream declares an abstract method write to write a data-byte to the output sink.

The least-significant byte of the int argument is written out; the upper 3 bytes are discarded. Similar to the read , two variations of the write method to write a block of bytes from a byte-array are implemented:. Both the InputStream and the OutputStream provides a close method to close the stream, which performs the necessary clean-up operations to free up the system resources.

This could prevent serious resource leaks. Unfortunately, the close method also throws a IOException , and needs to be enclosed in a nested try-catch statement, as follows.

This makes the codes somehow ugly. This produces much neater codes. In addition, the OutputStream provides a flush method to flush the remaining bytes from the output buffer. InputStream and OutputStream are abstract classes that cannot be instantiated. You need to choose an appropriate concrete subclass to establish a connection to a physical device.

For example, we can layer a BufferedInputStream to a FileInputStream for buffered input, and stack a DataInputStream in front for formatted data input using primitives such as int , double , as illustrated in the following diagrams.

This is grossly inefficient, as each call is handled by the underlying operating system which may trigger a disk access, or other expensive operations.

To chain the streams together, simply pass an instance of one stream into the constructor of another stream. This example copies a file by reading a byte from the input file and writing it to the output file. The method close is programmed inside the finally clause. It is guaranteed to be run after try or catch.

However, method close also throws an IOException , and therefore must be enclosed inside a nested try-catch block, which makes the codes a little ugly. The output shows that it took about 4 seconds to copy a KB file. As mentioned, JDK 1. For example, the above example can be re-written in a much neater manner as follow:. This program took only 3 millisecond - a more than times speed-up compared with the previous example. However, there is a trade-off between speed-up the the memory usage.

For file copying, a large buffer is certainly recommended. But for reading just a few bytes from a file, large buffer simply wastes the memory. I re-write the program using JDK 1. The JRE decides on the buffer size. The program took 62 milliseconds, about 60 times speed-up compared with example 1, but slower than the programmer-managed buffer.

To use DataInputStream for formatted input, you can chain up the input streams as follows:. DataInputStream implements DataInput interface, which provides methods to read formatted primitive data and String , such as:. DataOutputStream implements DataOutput interface, which provides methods to write formatted primitive data and String. The following program writes some primitives to a disk file. It then reads the raw bytes to check how the primitives were stored.

Finally, it reads the data as primitives. The data stored in the disk are exactly in the same form as in the Java program internally e. The byte-order is big-endian big byte first, or most significant byte in lowest address. If this character is to be written to a file uses UTF-8, the character stream needs to translate " 60 A8 " to " E6 82 A8 ". The reserve takes place in a reading operation. This is because some charsets use fixed-length of 8-bit e.

When a character stream is used to read an 8-bit ASCII file, an 8-bit data is read from the file and put into the bit char location of the Java program.

The abstract superclass Reader operates on char. It declares an abstract method read to read one character from the input source. There are also two variations of read to read a block of characters into char -array. The abstract superclass Writer declares an abstract method write , which writes a character to the output sink.

The lower 2 bytes of the int argument is written out; while the upper 2 bytes are discarded. The default charset is kept in the JVM's system property " file. You can get the default charset via static method java. BufferedReader provides a new method readLine , which reads a line and returns a String without the line delimiter. The main class java. Charset provides static methods for testing whether a particular charset is supported, locating charset instances by name, and listing all the available charsets and the default charset.

The default charset for file encoding is kept in the system property " file. For example, the following command run the program with default charset of UTF The following example encodes some Unicode texts in various encoding scheme, and display the Hex codes of the encoded byte sequences. As mentioned, Java internally stores characters char type in bit UCS-2 character set. You can list the available charsets via static method java.

The commonly-used Charset names supported by Java are:. The following program writes Unicode texts to a disk file using various charsets for file encoding. It then reads the file byte-by-byte via a byte-based input stream to check the encoded characters in the various charsets. Finally, it reads the file using the character-based reader. Primitives are converted to their string representation for printing.

The printf and format were introduced in JDK 1. Instead, it sets an internal flag which can be checked via the checkError method. A PrintStream can also be created to flush the output automatically. The standard output and error streams System. All characters printed by a PrintStream are converted into bytes using the default character encoding. The PrintWriter class should be used in situations that require writing characters rather than bytes. The character-stream PrintWriter is similar to PrintStream , except that it write in characters instead of bytes.

The PrintWriter also supports all the convenient printing methods print , println , printf and format.