Layouts, Types, & Switches Copyright © 2012 Pearson Education, Inc.

Layouts, Types, & Switches

Copyright © 2012 Pearson Education, Inc.

Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

GUI Principles

• Components: GUI building blocks– Buttons, menus, sliders, etc.

• Layout: arranging components to form a usable GUI– Using layout managers.

• Events: reacting to user input– Button presses, menu selections, etc.


Elements of a frame

Title

Menu bar

Content pane

Window controls


Layout managers

• Manage limited space for competing components (like our content pane)– FlowLayout, BorderLayout, GridLayout, BoxLayout, GridBagLayout.

• Manage Container objects, e.g. a content pane• Each imposes its own style


FlowLayout


BorderLayout


GridLayout


BoxLayout

Note: no componentresizing.

GridBagLayout


http://download.oracle.com/javase/tutorial/uiswing/examples/layout/GridBagLayoutDemoProject/src/layout/GridBagLayoutDemo.java

http://download.oracle.com/javase/tutorial/uiswing/examples/layout/GridBagLayoutDemoProject/src/layout/GridBagLayoutDemo.java

Layout Example: Box vs Border


Layout Example: Box vs Border


http://docs.oracle.com/javase/7/docs/api/javax/swing/BoxLayout.html

Types Revisited


Wrapper Classes• Use wrapper objects in Collections when you can’t

use primitive types

Primitive Type Wrapper Class

byte Byte

short Short

int Integer

long Long

float Float

double Double

char Character

boolean Boolean

Wrapper Classes• For example:

Integer age = new Integer(40);

• Wrapper classes contain static methods to convert between types. For example, to convert a String to an int:

int num = Integer.parseInt(str);

• Wrapper classes also contain constants such as MIN_VALUE and MAX_VALUE in the Integer class, which hold the smallest and largest int values


Enumerated Types• Define a new data type and list all possible values:

enum Season {winter, spring, summer, fall}

• Then the new type can be used to declare variables

Season time;

• And then the variable can be assigned a value:

time = Season.fall;

• The only values this variable can be assigned are the ones from the enum definition


Why would we want to use enums?Why would we want to use enums?


//********************************************************************// Season.java Author: Lewis/Loftus//// Enumerates the values for Season.//********************************************************************

public enum Season{ winter ("December through February"), spring ("March through May"), summer ("June through August"), fall ("September through November");

private String span;

//----------------------------------------------------------------- // Constructor: Sets up each value with an associated string. //----------------------------------------------------------------- Season (String months) { span = months; }

//----------------------------------------------------------------- // Returns the span message for this value. //----------------------------------------------------------------- public String getSpan() { return span; }


//********************************************************************// SeasonTester.java Author: Lewis/Loftus//// Demonstrates the use of a full enumerated type.//********************************************************************

public class SeasonTester{ //----------------------------------------------------------------- // Iterates through the values of the Season enumerated type. //----------------------------------------------------------------- public static void main (String[] args) { for (Season time : Season.values()) System.out.println (time + "\t" + time.getSpan()); }}


//********************************************************************// SeasonTester.java Author: Lewis/Loftus//// Demonstrates the use of a full enumerated type.//********************************************************************

public class SeasonTester{ //----------------------------------------------------------------- // Iterates through the values of the Season enumerated type. //----------------------------------------------------------------- public static void main (String[] args) { for (Season time : Season.values()) System.out.println (time + "\t" + time.getSpan()); }}

Outputwinter December through Februaryspring March through Maysummer June through Augustfall September through November

else-if & switch


Example: else-if


int grade = 85;char letter = ‘’;

if (grade > 89) letter = ‘A’;else if (grade > 79) letter = ‘B’;else if (grade > 69) letter = ‘C’;else if (grade > 59) letter = ‘D’;else letter = ‘F’;



The switch Statement• The switch statement provides another way to

decide which statement to execute next

• The switch statement evaluates an expression, then attempts to match the result to one of several possible cases

• Each case contains a value and a list of statements

• The flow of control transfers to statement associated with the first case value that matches


The switch Statement• The general syntax of a switch statement is:

switch ( expression ){ case value1 : statement-list1 case value2 : statement-list2 case value3 : statement-list3 case ...

}

switchandcaseare

reservedwords

If expressionmatches value2,control jumpsto here


A break statement can be used in a switch to jump to the end of the switch statement

The switch Statement

switch (option){ case 'A': aCount++; break; case 'B': bCount++; break; case 'C': cCount++; break;}

• An example of a switch statement:


Example: else-if vs switch







switch (grade) { case 100: case 99: case 98: … case 90: letter = ‘A’; break; case 89: … default: letter = ‘F’;}


switch (grade) { case 100: case 99: case 98: … case 90: letter = ‘A’; break; case 89: … default: letter = ‘F’;}

If no other case value matches, the optional default case will be executed (if present)

The switch Statement• The type of a switch expression must be integers,

characters, or enumerated types Why?

109.54 == 109.542


• As of Java 7, a switch can also be used with strings

• You cannot use a switch with floating point values

• The implicit boolean condition in a switch statement is equality

• You cannot perform relational checks with a switch statement (i.e., > or <)


//********************************************************************// GradeReport.java Author: Lewis/Loftus//// Demonstrates the use of a switch statement.//********************************************************************

import java.util.Scanner;

public class GradeReport{ //----------------------------------------------------------------- // Reads a grade from the user and prints comments accordingly. //----------------------------------------------------------------- public static void main (String[] args) { int grade, category;

Scanner scan = new Scanner (System.in);

System.out.print ("Enter a numeric grade (0 to 100): "); grade = scan.nextInt();

category = grade / 10;

System.out.print ("That grade is ");

continue


continue

switch (category) { case 10: System.out.println ("a perfect score. Well done."); break; case 9: System.out.println ("well above average. Excellent."); break; case 8: System.out.println ("above average. Nice job."); break; case 7: System.out.println ("average."); break; case 6: System.out.println ("below average. You should see the"); System.out.println ("instructor to clarify the material " + "presented in class."); break; default: System.out.println ("not passing."); } }} Sample Run

Enter a numeric grade (0 to 100): 91That grade is well above average. Excellent.

TemplatesWhile Loop Index Template:

initialize indexwhile (condition){ statements to be repeated update index}

For Loop Template:

for(initialize index; condition; update index){ statements to be repeated}

For Each Loop Template:

for (ElementType elementName : collection){ statements to be repeated}

While Loop Sentinel Template:

get input valuewhile (input != end condition){ statements to be repeated get input value}

Example switch:

switch (option){ case 'A': aCount++; break; case 'B': bCount++; break; case 'C': cCount++; break;}

Create a CeilingFan Class• Step 1: Create the class, fields, & constructors

– Add a Speed enum {off, low, medium, high}– Store the fan’s current speed– Create a default constructor

• Step 2: Create the following methods– changeSpeed that changes the speed w/ no parameters

using a switch statement– changeSpeed with 1 parameter (the number of times to

change it) that uses the first changeSpeed() method & a loop

Layouts, Types, & Switches Copyright © 2012 Pearson Education, Inc.

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