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CS 201 Lecture 8:
John Hurley
CS201
Cal State LA
Dialog Boxes
– GUI construction is taught in CS202, but we will
cover a few GUI rudiments in 201
– The first is the JOptionPane class, which
provides pop-up I/O boxes of several kinds
– Need to include this at the very top of your
class:
• import javax.swing.JOptionPane;
– JOptionPane.showMessageDialog()
• displays a dialog box with text you specify
Dialog Boxes
import javax.swing.JOptionPane;
public class DialogBox{
public static void main(String[] args){
for(int i = 0; i < 4; i++)
{
JOptionPane.showMessageDialog(null,
"This is number " + i);
}
}
}
Dialog Boxes
– JOptionPane.showInputDialog() shows a dialog
box that can take input
– The input is a String
– We will learn soon how to parse from String to
other types
Dialog Boxes
import javax.swing.JOptionPane;
public class InputBox{
public static void main(String[] args){
String input = JOptionPane.showInputDialog(null, "Please enter
some input ");
JOptionPane.showMessageDialog(null, "You entered: \"" + input
+ "\"");
String input2 = input.concat(" " +
JOptionPane.showInputDialog(null, "Please enter some more
input "));
JOptionPane.showMessageDialog(null, "You entered: \"" +
input2 + "\"");
}
}
Casting Strings to Numeric Types
• Recall that input from
JOptionPane.showInputDialog is a String
• Cast to integer: Integer.parseInt(intString);
– Example:
– String input =
• int age = Integer.parseInt(JOptionPane.
showInputDialog(null, “Please enter your age”);
• Cast to double:
Double.parseDouble(doubleString);
Casting to Numeric Types
• Note the capitalization in the method names.
Double and Integer are not quite the same as
double and integer.
– You’ll understand when you are a little older…
Parsing to Integer
import javax.swing.JOptionPane;
public class NumericCastDemo{
public static void main(String[] args){
int age =
Integer.parseInt(JOptionPane.showInputDialog(null, "Please enter your
age"));
if(age < 30)
JOptionPane.showMessageDialog(null, age + " is
pretty young.");
else if(age > 100) JOptionPane.showMessageDialog(null, "really?");
else JOptionPane.showMessageDialog(null, "That's OK. Methuseleh lived to
be " + (270 - age) +
" years older than you are now.");
} // end main()
} // end class
Parsing to Double
import javax.swing.JOptionPane;
public class NumericCastDemo{
public static void main(String[] args){
double age = Double.parseDouble(JOptionPane.showInputDialog(null,
"Please enter your age"));
if(age < 30)
JOptionPane.showMessageDialog(null, age + " is pretty young.");
else if(age > 100) JOptionPane.showMessageDialog(null, "really?");
else JOptionPane.showMessageDialog(null, "That's OK. Methuseleh lived to
be " + (270 / age) +
" times as old as you are now.");
} // end main()
} // end class
Imports
– We have already discussed
javax.swing.JOptionPane methods to show input
and message dialogs
– These required the following line at the top of
the class:
• Import javax.swing.JOptionPane;
– If you omit this line, you will get an error
message like this:
SwitchDemo.java: 7: cannot find symbol
Symbol:variable JOptionPane
Imports
–Java classes are organized in packages
• Late in this class or early in CS202 you will
start using packages for multiple classes
–javax.swing is a package of GUI-related
classes that is included with all distributions
of Java
–JOptionPane is a class within this package
–JOptionPane.showMessageDialog() and
JOptionPane.showInputDialog are methods
of the class
Imports
– Including a package in your program adds a
small cost, slowing down the compiler as it
looks through the imported package
– Thus, things like javax.swing are not included
automatically; you must specify that you need
them
– You will eventually be importing your own
packages, too.
– Don’t leave unused imports in your code
Convert Character and Numbers
to Strings
The String class provides several static methods
for converting characters, arrays of characters, and
numeric values to Strings.
These methods are named valueOf() with
different argument types char, char[], double,
long, int, and float.
For example, to convert a double value to a string,
use String.valueOf(5.44). The return value is string
consists of characters ‘5’, ‘.’, ‘4’, and ‘4’.
13
public class ValueOfDemo{
public static void main(String[] args){
double d = 1234.56789;
// this would cause an error: int precision=d.indexOf('.');
String stringD = String.valueOf(d);
int stringLength = stringD.length();
int locationOfDecimalPoint = stringD.indexOf(".");
System.out.println("length of string " + stringD + " is " + stringLength + "
location of decimal point is " + locationOfDecimalPoint);
int precision = stringLength - locationOfDecimalPoint -1;
System.out.println(precision + " digits after the decimal point");
}
}
Validating Data Type
– We have already discussed how to make sure
that numeric input from Scanner is within a
desired range
int age;
do {
System.out.println("How old are you?");
age = sc.nextInt();
} while (age < 0 || age > 100);
Validating Scanner Input
– So far, our programs have crashed if casts to
numeric types failed:
– Try this with input “two point five”:
import java.util.Scanner;
public class ReadDouble2 {
public static void main(String[] args) {
Scanner input = new Scanner(System.in);
Double stuff = 0.0;
do {
System.out.print("Input a double. Enter 0 to
quit:");
stuff = input.nextDouble();
System.out.println("\nYou entered: " + stuff);
} while (stuff != 0.0);
}
Validating Scanner Input
– Here is the simplest way to validate Scanner
input for data type (that is, to make sure you get
input that can be cast to double, integer, etc.)
– Scanner has hasNext…() methods that see if
there is a parseable token
• hasNextInt()
• hasNextDouble()
• hasNextBoolean()
– Also need next() to skip over bad input
Validating Scanner Input
– Try this one with input “nine.three”, then with
input “nine point three:
import java.util.Scanner;
public class ReadDouble3 {
public static void main(String[] args) {
Scanner input = new Scanner(System.in);
Double inpDouble = 10.0;
// bad code ahead!!
do {
System.out.print("Input a double. Enter 0 to quit:");
if(input.hasNextDouble()){
inpDouble = input.nextDouble();
System.out.println("\nYou entered: " + inpDouble);
}
else input.next();
} while (inpDouble != 0.0);
}
}
Validating Scanner Input
– In order to get good output, we need to arrange things in a slightly more
complex way:
import java.util.Scanner;
public class ReadInt{
public static void main(String[] args){
Scanner sc = new Scanner(System.in);
int x;
System.out.println("Enter an integer");
while (!sc.hasNextInt()) {
sc.nextLine();
System.out.println("No, an integer!");
}
x = sc.nextInt();
System.out.println(x);
}
}
Validating Both Type and Value
– In order to get good output, we need to arrange things in a slightly more
complex way:
Scanner sc = new Scanner(System.in);
int value = 0;
do {
System.out.println("Enter an integer greater than 100");
while (!(sc.hasNextInt())) {
sc.nextLine();
System.out.println("no, an integer!");
}
value = sc.nextInt();
if(value <= 100) System.out.println("No, an integer greater than 100");
} while (value <= 100);
System.out.println(value);
Documentation From Oracle
– Java was originally developed by Sun
Microsystems and was closed-source but free for
many years
– Java is now open-source
– Anyone is free to use it or to write compilers,
virtual machines, etc. that implement it
– Oracle bought Sun a couple of years ago and
now is the champion of Java
Documentation From Oracle
– When you need a reference source for
something like Scanner, look it up in Oracle’s
excellent online documentation
– Example: Google +Java +Scanner
• Follow the link to
http://doc.java.sun.com/DocWeb/api/java.util.Scann
er