How does this work? I can't understand ... P - java help on CodeGym

OK, so I copied the code from Johannes in hopes that I would be able to understand it and rewrite it myself, from memory. But no matter how I tried, I couldn't figure out the logic. Let me explain what puzzles me: In line 10 we are attributing out a new stream. But that stream isn't configured above.. What does it do? Then, in line 11 it calls the printSomething method. This in turn calls the system.out in lines 25-29. But how does it know what to do, when the logic for it is declared in lines 12 and 16-19 and even after out is reatributted the initial value in line 14. What kind of mystical sorcery time traveling law physics bending magic spell is this?

public class Solution {
    public static TestString testString = new TestString();

    public static void main(String[] args) {

        PrintStream consoleS = System.out;
        ByteArrayOutputStream baos = new ByteArrayOutputStream();
        PrintStream newStream = new PrintStream(baos);

        System.setOut(newStream);
        testString.printSomething();
        String[] str  = baos.toString().split("\n");

        System.setOut(consoleS);

        for (int i=0;i< str.length; i++) {
            System.out.println(str[i]);
            if (i%2 == 1)
                System.out.println("CodeGym - online Java courses");
        }
    }

    public static class TestString {
        public void printSomething() {
            System.out.println("first");
            System.out.println("second");
            System.out.println("third");
            System.out.println("fourth");
            System.out.println("fifth");
        }
    }
}

public static void main(String[] args) throws Exception{ ByteArrayInputStream bais = new ByteArrayInputStream("1\n2\n3\n4\n5\n6\n7\n8\n9\n".getBytes()); BufferedReader reader = new BufferedReader(new InputStreamReader(bais)); int sum = 0; while(reader.ready()) sum += Integer.parseInt(reader.readLine()); System.out.println(sum); }

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Guadalupe Gagnon

Level 37 , Tampa, United States

22 February 2021, 14:37

solution

You know what a FileOutputStream is, correct? A PrintStream is similar. The System.out.println() method writes everything to the current PrintStream, which is saved as to the static variable System.out. By default this would be for the console (and System.in by default is reading from the console). In this code it (by line #): 6) first saves the current System.out to a PrintStream 7-8) Creates a new PrintStream from a ByteArrayOutputStream 10) Sets that object from line 8 to the System.out, which means now everything that would normally be written to the console gets written to the ByteArrayOutputStream 11) call the printSomething() method, which on lines 25-29 print stuff 12) Gets the String of what was written to the ByteArrayOutputStream, splits it by line break to an Array 14) sets the System.out back to the original console output (saved from line 6) 16-20) Outputs according to the task requirements

Andrei

Level 41

22 February 2021, 14:46

ohhhhhhhhhhhhhhhhh ok, so that's what I was missing, it actually writes everything in the array, then you revert to the original out and then you can manipulate the array to print in the desired way. Thank you!

22 February 2021, 14:50

Probably the first few lines are the parts you haven't seen. You can change the System.in and System.out to whatever you wish. I use this a lot for testing purposes. The ByteArrayOutputStream stores everything internally as a byte[]. Think of like reading from a file where you use a byte[] to store the data. You can return the byte[] or you can get the actual String. Getting the String saves you a step of having to convert the String manually. For testing purposes you can use a ByteArrayInputStream to replace all user input. Lets say you have a task that asks for you to accept 9 numbers and output the sum. You could write code, then hit the run button and enter 9 numbers every time that you want to test.... or you could do this:

Normally, line 4 would have System.in instead of the ByteArrayInputStream, but by using this every time that I hit run it sum. Makes debugging your code WAY faster when you don't have to enter new values by hand. You could change that input and test different scenarios and fix any bugs you find much faster this way. Anywhere you could use an OutputStream, you can use a ByteArrayOutputStream; and anywhere you can use an InputStream you can use a ByteArrayinputStream. Actually the InputStream and OutputStream part tell you a lot on how to use the object. InputStreamReaders take any type of InputStream, and BufferedReaders take any type of readers. You can pass OutputStreams to BufferedOutputStreams.

Anthony Chalk

Level 30 , London, United Kingdom

22 February 2021, 14:36

useful

ByteArrayOutputStream is essentially a buffer where you can store the bytes of information you are writing (think of a BufferedWriter). Generally, when you use System.out.println(), you print the information to the "monitor". On line 10, we're configuring the system so that "out" points to the Buffer created on lines 7 and 8. Now, whenever you System.out.println() anything (as you do 5 times by calling the printSomething() method), instead of printing the information on your screen, it prints it into the buffer (where it is stored for use, such as on line 12)

Thanks! :)

How does this work? I can't understand ... Please explain the logic