Lesson 5: Input and Output

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Scanner Basics:

• Scanner is an API added in Java 1.5

• Used to manage input

• Need to import before use

• First step is to create a Scanner object that will manage all inputs:

public class FahrenheitToCelsius {
    public static void main(String[] args) {
        Scanner input = new Scanner(System.in); //read keyboard input
    }
}

• Can read data from multiple sources

• Stream of data connects program to keyboard

  • Input is System.in/standard in

  • Then we can use Scanner to read each chunk, or token of data.

    • To split the input into tokens, a delimiter is used

      • Default delimiter is whitespace, but can be changed to anything

    • Different token types for different variable types:

      • nextInt ⇒ int
      • nextDouble ⇒ double
      • next ⇒ String
      • nextLine ⇒ String until newline character is reached

Scanner Errors:

• Error if token type is not in stream

• can append ‘has’ to any normal scanner method

  • Checks to see whether token type is in stream

Multiple Prompts and Tokens:

• After inputting user input, a newline character is leftover

  • Only execption is nextLine, which removes it

  • This is inputted to the next user input prompt

    • Ex. if user inputs 5 then 9, Scanner sees ‘5’, ‘/n9/n’

    • This is not a problem unless nextLine is being used

      • Add extra nextLine prompt before the actual one

Multiple Tokens Per Line:

• You can input mutiple tokens in a single call

  • Any leftover data is left in the stream

• Can get data for two next methods from one input:

  • Ex.

    • Input = 78 Monday

    • nextInt = 78, leaves behind ’ Monday/n’

    • next = Monday

      • Doesn’t wait for user input as data is already in stream

      • nextLine would’ve also taken the space before Monday

• We can remove any spaces using the ‘.trim()’ method

Packages:

• Classes can be grouped together based on the functions they provide

  • These are called packages and each have a name

• Built-in Java classes are part of a package called java.lang

• To use classes not in the java.lang package, you must import them:

import java.util.Scanner; //imports just scanner
import java.util.*; //imports all util packages

• Dot seperation is like hierarchy/directories

  • Packages can house subpackages

• Using wildcard(*) to import multiple classes can cause problems

  • It’s possible for two packages to have the same name

    • To fix, only import the packages you need (don’t use *)

  • Can create larger compile times

    • Generated bytecode is the same either way

Formatting with printf:

• Example format:

System.out.printf(formatString, values); //Format
//Actual Example:
System.out.println(day + " Celsius: " + celsius);//Normal
System.out.printf("%s Celsius: %f\n", day, celsius);//Printf
System.out.printf("%s Celsius: %f\n", celsius, day);//This would result in an error

• In this example, the fixed part is “Celsius” and variables are day and celsius

• %f and %s are format placeholders

  • Format = %[flag][width][.precision]type

  • Format placeholders must start with % to show they aren’t part of the String

  • Some format placeholder types:

    • Decimal ⇒ %d
    • float ⇒ %f
    • String ⇒ %s

  • Must specify the actual paramters in the same order as the placeholders (Ex above)

  • Reduce decimal places:

    • Add the decimal place in front of the ”%”

System.out.printf("%s Celsius: %0.1f\n", day, celsius);//Print value to 1 decimal

• Change width/alignment of values:

//Will print var celcius with a width of 10 (extra space will be whitespace)
System.out.printf("%10s Celsius: %f\n", day, celsius); //Padding before string
System.out.printf("%-10s Celsius: %f\n", day, celsius); //Padding after string

• Add thousand’s seperator:

//would print for example 100,543 instead of 100543
System.out.printf("%s Celsius: %,f\n", day, celsius);//comma in front of f

String.format:

• Same as printf but values can be stored to a String instead of printing

NumberFormat (currency formatting):

• Import the package, then create NumberFormat object, then print using one:

//Import
import java.text.NumberFormat;
//Create number
int total = 5;
//Create object
NumberFormat currencyFmt = NumberFormat.getCurrencyInstance();//Local currency
NumberFormat currencyFmt = NumberFormat.getCurrencyInstance(Locale.FRANCE);//Specific Locale(France)
//Print
System.out.println ("Formatted Total: " + currencyFmt.format(total));

DecimalFormat:

import java.text.DecimalFormat;
 
public class DecimalFormatDemo {
    public static void main(String[] args) {
        DecimalFormat formatter1 = new DecimalFormat("0.0");
        DecimalFormat formatter2 = new DecimalFormat("00.00");
        DecimalFormat formatter3 = new DecimalFormat(".00");
        DecimalFormat formatter4 = new DecimalFormat("0.00%");
 
        System.out.println("0.0: " + formatter1.format(.8675309));
        System.out.println("00.00: " + formatter2.format(.8675309));
        System.out.println(".00: " + formatter3.format(.8675309));
        System.out.println("0.00%: " + formatter4.format(.8675309));
        System.out.println(".00: " + formatter3.format(8675309));
    }
}