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2d33: Reversed order of input

Write a program that reads a series of lines from standard input, then prints out these lines in reversed order.

You should first implement a version of this program that reads lines of input into an array of strings, making the assumption that input will consist of 100 lines or fewer. You should then implement a more flexible version of this program where the number of input lines is not bounded, by storing input in a Deque object as explained below.

You can read from standard input one line at a time using a BufferedReader object. To create a BufferedReader, add:

import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.io.IOException;

to the top of your Java file, and then write:

BufferedReader br = new BufferedReader(new InputStreamReader(System.in));

in your main method.

To read a line from standard input into a string, line, do:

line = br.readLine();

Because it is possible, in exceptional cases, for input reading to go wrong, the readLine method can throw an exception. We will cover exceptions later in the course. For now, to allow your program to compile, you should declare main as follows:

public static void main(String[] args) throws IOException { ...

This means that if an exception does occur due to readLine, your program will exit and details of the exception will be displayed on the terminal.

The readLine method will return null when the end of input is reached. So, to loop until all input has been consumed, you can write something like:

public static void main(String[] args) throws IOException {

  BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
  String line = br.readLine();
  while (line != null) {
    ... /* Do something with 'line' */
    line = br.readLine();
  }

}

For the second part of the question, you can use the Deque interface from the Java library, which must be imported:

import java.util.Deque;

Deque (pronounced "deck") stands for double-ended queue, and provides the functionality of a stack: push and pop operations, to add/remove an item to/from the front of the queue are supported. Deque is a generic interface: it makes sense to have a deque of integers, a deque of strings, a deque of cars, etc. Deque is implemented by various classes, including ArrayDeque, which you should use for this example. To create an empty deque of strings, you can do:

Deque<String> myDeque = new ArrayDeque<>();

Remember that the <String> part of this definition specifies that the data to be stored in this deque has type String.

To add a string to the top of the deque, call push. E.g.:

myDeque.push("Hello");
String myString = "Another String";
myDeque.push(myString);

To copy the top element of the deque into a variable, and have it removed from the deque, call pop. E.g.:

String myString = myDeque.pop();