In.java

Motivation

In.java lets us read information stored in different files. As long as we know the format of a file, we can parse its contents and programmatically interact with it. The first example we will see of In.java is with hw02, Personality Quiz, where you will determine a users personality based upon user input and information stored in a separate file containing quiz questions, answer options, a personality scoring guide and details about each personality.

Setting up a File Reader

The standard library provides the class In.java to support accessing information from a file. Study StudentsFileProcessor.java, which is contained in the nbody_data.zip you downloaded in Part 0. This file provides an example of reading information from a file. Compile StudentsFileProcessor.java, and run it from the Codio Terminal with the argument students.txt:

java StudentsFileProcessor students.txt

In your program, declare and initialize a variable, inStream, as below:

In inStream = new In(filename); // creates a variable inStream of type In to read from the file

inStream is just a variable name. You could name this variable anything, but for convenience we are giving you the name “inStream” for your file reader variable.

Reading values from a file

Now that inStream is initialized, you can access/read information from it using the following function calls. These functions behave identically to those in StdIn. Below, you will see that each value read from the file (accessed via inStream) is immediately stored in a variable (i.e. b, i, d, s).

boolean b = inStream.isEmpty();     // boolean value that is true if there are no more values, false otherwise
int     i = inStream.readInt();     // reads in an int from inStream
double  d = inStream.readDouble();  // reads in a double from inStream
boolean b = inStream.readBoolean(); // reads in a boolean from inStream
String  s = inStream.readString();  // reads in a string from inStream
String  s = inStream.readLine();    // reads in an entire line from inStream
String  s = inStream.readAll();     // reads in the entire file from inStream

inStream will start reading from the beginning of the file (top left). Each time a function like readDouble() is called, inStream attempts to read the next (unread) number as a double (an error will occur if it cannot be parsed to a double). The next time a read function is called, inStream moves to the next item in the file.

For example, say that a file, sample.txt, is as follows:

4 5

The code snippet

In inStream = new In("sample.txt");
int x = inStream.readInt();
double y = inStream.readDouble();

will set variable x to 4 and variable y to 5.0.