Standard Input and Exceptions

Up to this point we have only introduced you to command line arguments as a source of application input.
In this lesson you will learn how to accept lines of input from the standard input stream.

Here is the example:
import*; class MathStandardInput { public static void main(String args[]) { try { BufferedReader br=new BufferedReader(new InputStreamReader(; String strNextOperation=""; String strOperand=""; String strOperand0=""; double dblOperand=0.0d; double dblOperand0=0.0d; double dblOperated=0.0d; do { System.out.println("Type in \"addition\" or \"subtraction\" or \"multiplication\" or \"division\" or \"remainder\" or \"exponent\" or \"exit\"\n"); strNextOperation=br.readLine(); if(strNextOperation.equals("exit")) break; System.out.println("Type in first operand"); strOperand=br.readLine(); System.out.println("Type in second operand"); strOperand0=br.readLine(); dblOperand=Double.valueOf(strOperand).doubleValue(); dblOperand0=Double.valueOf(strOperand0).doubleValue(); if(strNextOperation.equals("addition")) { dblOperated=dblOperand+dblOperand0; } else if(strNextOperation.equals("subtraction")) { dblOperated=dblOperand-dblOperand0; } else if(strNextOperation.equals("multiplication")) { dblOperated=dblOperand*dblOperand0; } else if(strNextOperation.equals("division")) { dblOperated=dblOperand/dblOperand0; } else if(strNextOperation.equals("remainder")) { dblOperated=dblOperand%dblOperand0; } else if(strNextOperation.equals("exponent")) { dblOperated=Math.pow(dblOperand, dblOperand0); } else { System.out.println("\nUnrecognized operation.\n\n"); continue; } System.out.println("\nThe numbers computed to a result of "+dblOperated+"\n\n"); } while(true); } catch(Exception ex) { ex.printStackTrace(); } } }
The first thing that must done is import the input/output libraries necessary for reading standard input. They are and . Rather than typing in two import statements, we use "import*;" which imports all input/output classes. The first line that appears in the "main" function is "try {". "try" is a keyword that helps the programmer with exception handling. I will cover exception handling later in this lesson. For the purposes of this console application, "try" is necessary because the "readLine" function of BufferedReader throws an IOException . Next, we declare a BufferedReader variable and use the constructors for BufferedReader and InputStreamReader with the standard input stream "". We can't simply use BufferedReader with "" because the constructors for BufferedReader only accept a Reader object as a parameter, and isn't a Reader, but InputStreamReader is a Reader. Next, the variables we use in the "do while loop" are declared. Notice that the keyword "do" is followed by a curly brace "{" which is done because everything in between the curly braces will be executed repeatedly until either the loop is broken using the "break" keyword or until the condition inside the "while" evaluates to false. So, the loop will execute every instruction in between the curly braces in sequence from top to bottom one after the other until it reaches the "while" keyword at which time the execution returns back to the instruction directly following the "do" keyword. The line containing "br.readLine()" is the first instruction where we receive user input. The "readLine" function is a blocking function which means that program execution halts until the "readLine" function returns. The "readLine" function will not return until the user presses the Enter key on his/her keyboard. The "println" function immediately preceding "readLine" tells the user the acceptable input text for the "readLine" function. If the user types in "exit" and presses the Enter key then the conditional if statement will make the program "break" the loop allowing the program to end its execution(just a quick comment about console applications, the user can terminate execution of any program at any time by using the keyboard keys Ctrl and c pressed at the same time). The rest of the application executes in a fashion similar to from Lesson 4. At the end of the "main" function notice that after the curly brace "}" for the "try" block there is a catch clause with a variable "Exception ex". "ex" is a variable that will contain exception info if an exception occurs within the "try" block. "ex" is most often used with its function "printStackTrace" which prints out a stack trace containing the classes, functions, and lines where the exception occured with the most recently entered function appearing first. You should attempt to cause an exception yourself by typing in "notanumber" where either the first operand or second operand is expected as user input. A NumberFormatException will be generated after both the operands are entered. Scan down the stack until you see MathStandardInput.main . That line is telling you where in your code the exception was generated: the "main" function in the source file MathStandardInput at line number 34. If you open up your source file and scroll down to line 34 you will find the instruction "dblOperand=Double.valueOf(strOperand).doubleValue();" in which a String is converted to a double. However, the String must be a digits only real number in order for the "valueOf" function to be able to read the String. As a programmer, it is your decision what to do with exceptions. Should the NumberFormatException force a program aborting operation? If you don't think it should then you could write another "try" block specifically for the "valueOf" functions. Replace the code:
dblOperand=Double.valueOf(strOperand).doubleValue(); dblOperand0=Double.valueOf(strOperand0).doubleValue();
try { dblOperand=Double.valueOf(strOperand).doubleValue(); dblOperand0=Double.valueOf(strOperand0).doubleValue(); } catch(Exception ex) { System.out.println(""); continue; }
Now instead of exiting the loop due to the exception handling "try" block, if a NumberFormatException is generated then the output stream is advanced by a "println" new line and the loop is continued. The "continue" keyword tells program execution to immediately move to the first instruction at the top of the "do while loop". If you ever find yourself writing code and when you compile the source code you see an error that states: unreported exception Exception; must be caught or declared to be thrown. You now know that you must enclose the code in question with a "try catch" exception handling block. In this lesson you learned another method for receiving input in the form of standard input stream. You also learned about exception handling and controlling program flow.