Chapter 6© copyright Janson Industries 20141 Java ▮ Comparing Strings/Objects ▮ Conditional...

Chapter 6 © copyright Janson Industries 2014 1

Java▮ Comparing Strings/Objects

▮ Conditional logic▮ if/else▮ switch

▮ More GUI▮ Check boxes▮ Item listeners▮ Using Event objects

▮ Debugging

▮ Preferences Non-graded Assg


Comparisons

▮ Highlights the difference between primitive and referenced variablesint a = 5; int b = 5

boolean isEqual = (a == b);

String a = new String(“Joe”);

String b = new String(“Joe”);


isEqual is TRUE

isEqual is FALSE


Referenced Variables

▮ Contain the memory address of the value

▮ Defining two strings objects, means memory is allocated like this:

▮ And a = A1, b = B2

A B C

1 Joe

2 Joe

3

a

b


Comparisons

▮ So when we compare

▮ Because address A1 does not equal B2


isEqual is FALSE


String▮ If we change the value of a

▮ The value “Art” is placed in memory

▮ a = C3 and the first Joe is not referenced

a = new String(“Art”);

A B C

1 Joe

2 Joe

3 Art

ab


String▮ However if we define a new string

▮ c = C3 (it points to the object a points to)

▮ c == a is TRUE

String c = a;

A B C

1 Joe

2 Joe

3 Art

abc


Primitive Variables ▮ Defining these two integers:

▮ Results in a memory allocation of:int a = 1; int b = 2;

A B C

1 1

2 2

3


Primitives

▮ If we change the value of a

▮ The value 1 is replaced with 3

a = 3;

A B C

1 3

2 2

3


Comparing Strings

▮ String has the equals method for comparing the text of two StringsString a = new String(“Joe”);

String b = new String(“Joe”);

boolean isEqual = (a.equals(“Joe”));

boolean isEqual = (a.equals(b));

boolean isEqual = (a.equals(“JOE”));

boolean isEqual = (a.equalsIgnoreCase(“JOE”));

isEqual is TRUE

isEqual is FALSE

isEqual is TRUE


IF/ELSE

▮ Syntax of an if/else statement▮ if(boolean expression) { statements to be

executed if true } else { statements to be executed if false }

if (gender.equals(“F”)) {

genderLabel.setText(“Female”);

}

else {

genderLabel.setText(“Male”);

}


Nested IF

if (month == 1) {

monthLabel.setText(“Jan”);}

else {if (month == 2) {

monthLabel.setText(“Feb”);}

else { if (month == 3) {

monthLabel.setText(“Mar”);}

else { if (month == 4) {………

Nested if

▮ Placing an if statement as one of the statements to be executed in an else clause


Try It Exercise

▮ Create a Java class called Month as a subclass of Frame that:

▮ Accepts a number from 1 to 12 (via a text field)

▮ Has button that when clicked displays the associated month name (January, etc.) in a label

▮ Use nested ifs


How to Tackle the Exercise▮ CALTAL!

▮ Using VE create the frame with components, then TEST that it displays correctly

▮ Add code such that when button clicked, data read from textfield and placed in label – TEST IT

▮ Code the first if that checks for value = 1 and puts January in label – TEST IT

▮ Code first nested if for value = 2 and display February – TEST IT

▮ Add code for remaining ifs and TEST IT


Switch

switch (month) {

case 1: monthLabel.setText(“Jan”); break;

case 2: monthLabel.setText(“Feb”); break;

case 3: monthLabel.setText(“Mar”); break;

: : : : :

: : : : :

default: System.out.println(“Not a valid

month!”);

}


Switch

▮ To make the switch work like an if, need the break statements because once the condition is true, all subsequent statements are executed

▮ In the example, if there were no breaks:▮ The label would be set to Dec

▮ The “Not a valid month” message would be displayed


Try It Exercise

▮ Create a Java class called month as a subclass of Frame that:

▮ Accepts a number from 1 to 12 (via a text field)

▮ Has button that when clicked displays the associated month name (January, etc.) in a label

▮ Use a switch


dispose();

▮ System.exit(0) ends the JVM▮ All open Frames are closed

▮ dispose() ▮ In a frame subclass, closes only that

frame▮ dispose() means this.dispose()

▮ When creating an Exit button use System.exit(0)

▮ When window close button clicked, use dispose();


Putting it all to work

▮ When a check box is selected or the Execute button is clicked▮ Calculator will perform a simple arithmetic

calculation between 2 non-negative numbers

▮ Display a message with the result


Check Boxes

▮ Appear as a small, square text field with an attached label

▮ The text field acts a toggle switch. Click it and a check mark will appear.

▮ Click again the check mark is removed


Check Boxes

▮ Check boxes can be grouped

▮ Grouped check boxes are round

▮ Within a grouped check box only one check box can be checked

▮ Checking a box unchecks any other checked box


Check Box & GroupCheckboxGroup arithFunc = new CheckboxGroup(); : : : : : :

Checkbox addCB = new Checkbox();

addCB.setLabel("Add");

addCB.setCheckboxGroup(arithFunc);

addCB.setBounds(new Rectangle(24, 80, 40, 23)); : : : : : :

Checkbox subCB = new Checkbox();

subCB.setLabel("Subtract");

subCB.setCheckboxGroup(arithFunc);

subCB.setBounds(new Rectangle(74, 80, 40, 23)); : : : : : :

this.add(getaddCB(), null);

this.add(getsubCB(), null);

this.add(getmultCB(), null);

this.add(getdivCB(), null);


Check Boxes

▮ To work with check boxes, import the classes and implement ItemListener

▮ Then add listeners to the checkboxes and buttons

public class IOExcept extends Frame implements

WindowListener, ActionListener, ItemListener {

addCB.addItemListener(this);

subCB.addItemListener(this);

multCB.addItemListener(this); divCB.addItemListener(this);

execBtn.addActionListener(this);

exitBtn.addActionListener(this);


Check Boxes

▮ ItemListener requires an itemStateChanged method (like ActionListener requires actionPerformed) that contains the action to take when a box is checked

▮ Within itemStateChanged get the “state” (true or false) of a box to determine which box was checked▮ checkBox.getState(); //returns a boolean value

public void itemStateChanged(ItemEvent choice) { java statements }


Example

▮ Application needs to:▮ Perform the selected arithmetic operation▮ Create message text unique to each

operation ▮ The result of dividing 7 by 2▮ The result of adding 7 to 2

▮ Need to concatenate to the message String based on the selected math function

String msgBegin = new String("The result of ");

String msgPrep = new String(" "); //i.e. "by", "to", "from"

double result, firstNum, secondNum;

firstNum = Double.valueOf(numeratorTF.getText());

secondNum = Double.valueOf(denominatorTF.getText());


if (addCB.getState()) {

msgBegin = msgBegin + "adding ";

msgPrep = msgPrep + "to ";

result = firstNum + secondNum; }

else {

if (subCB.getState()) {

msgBegin = msgBegin + "subtracting ";

msgPrep = msgPrep + "from ";

result = firstNum - secondNum; }

else {

msgPrep = msgPrep + "by ";

if (multCB.getState()) {

msgBegin = msgBegin + "multiplying ";

result = firstNum * secondNum; }

else {

if (divCB.getState()) {

msgBegin = msgBegin + "dividing ";

result = firstNum / secondNum;} } } }


▮ Build and display the message

▮ Assuming you want this to happen when a checkbox is clicked, all the previous code goes into itemStateChanged

Example

resultLbl.setText(msgBegin + firstNum + msgPrep + secondNum + " is " + result);


Run Calculator


Enter dataSelect function

Message oops!


Check Boxes and Buttons

▮ If execute button clicked, calculation should be performed

▮ Problem:

▮ All the code to do the calc is in ItemStateChanged

▮ When button clicked, actionPerformed executed


Check Boxes and Buttons

▮ Move all the ItemStateChange code into separate method (aka doCalc)

▮ Have both itemStateChanged and actionPerformed execute doCalc

▮ Problem is there are two buttons - how do you know which button was clicked?

▮ The event object has which button was clicked


New doCalc called by both

But if you click Exit button, doCalc will be performed


Using Events

▮ .getSource() returns the object’s address that was involved with the event

▮ If an ActionListener is tied to many buttons, the ActionEvent’s getSource() method returns the button object’s address that was clicked

▮ In actionPerformed, use ifs to figure out which button:

if (e.getSource() == exitBtn) {

System.exit(0);

} else {

this.doCalc();}


Now if Exit clicked, application ended


Using Events

▮ Also works for Item Listeners and Item Events

▮ Instead of checking each components state, use the ItemEvent

public void itemStateChanged(ItemEvent e) {

if (e.getSource() == addCB) {…}

else if (e.getSource() == subCB) {…}

else if (e.getSource() == multCB)…


Using Events

▮ Notice that the if compares the (Button or Check box) variable to what is returned by getSource()

▮ So, what is returned by getSource()?

▮ Or, asked another way, what is in the Button variable?


Non-graded Assg

▮ Create Calculator such that it

▮ Performs arithmetic functions and displays correct msgs when checkbox checked or execute button clicked

▮ doCalc checks for which checkbox was clicked

▮ Exit button and Execute button work correctly

▮ Send Calculator.java to [email protected]


Java Error Handling

▮ Syntax/compilation errors

▮ How shown

▮ Help

▮ Run time exceptions

▮ Debug Perspective


Java Error Handling

▮ Some preferences you might want to change

▮ Have line numbers shown

▮ Run time error msgs will often display the line number of the offending statement

▮ Have RAD

▮ Insert tokens automatically

▮ Stop some messages

▮ Format the source code

Chapter 6 © copyright Janson Industries 2014 39Click Window then Preferences


Expand General, Editors, then click on Text Editors

Click the Show line numbers checkbox


Click the Semicolons and Braces checkboxes

Expand Java then Editor and click on Typing


Suppress certain annoying warnings


You can control the content of comments …


and the code that is automatically generated


Open an edit session and click Source, Format (or Ctrl+Shift+F)

You can also have RAD format your source


Like WORD, errors underlined with a red squiggly line in the source code

Errors also indicated by a red circle (with a white X) in the marker bar to the left of the

line number


Display flyover error message by moving cursor over the error icon on left


Light bulb icon (behind error icon) means RAD has some suggestions for fixing the error

Click the light bulb to display suggestions and solutions

Double click a solution and RAD will implement the code in the right hand pane


Have to be a sharpshooter to hit that light bulb icon

Alternative: move the cursor over the red squiggly line to display a list of suggestions

Click a suggestion, RAD will implement the solution code


RAD not perfect!Label problem no longer

flagged New error distracts RAD

(Squirrel!!)


After saving/compiling, errors also indicated in Explorer & Outline views & the Workspace view

tab


Pink rectangles on right show errors in entire program

Clicking on a pink rectangle moves cursor to the error in the source code and shows that line of

code


Lots of Compilation Errors

▮ Errors can cascade, best way to find errors is to code and compile in pieces (caltal)

▮ Instead of entering entire program:▮ Code part▮ Test▮ If errors, fix▮ Repeat until no errors▮ Code next portion▮ Repeat process

▮ Solve errors from top to bottom


Runtime Errors

▮ The most insidious of all errors

▮ Program compiles and executes but generates incorrect results

▮ Time honored (aka cave man) technique is to display as the program executes

▮ I.e. use System.out.println() in source code to display variable values or messages that indicate the logic path


Runtime Errors

▮ RAD provides the common debugger functions of

▮ Setting breakpoints

▮ Stepping through statements

▮ Tracing variables during program execution

▮ Breakpoints are designated lines of code where program execution is halted when the program is run in Debug mode


To define a breakpoint:

Move the cursor to the marker bar directly left of the line number

Double click

A small blue circle will be displayed

To remove: double click blue circle


To run in Debug: Right click fileSelect Debug AsJava Application


Confirm switch to Debug perspective


5 panes - Yikes!

Shows where you are in the source code

Thread shows how you got to the breakpoint•Started in main and got to line 9 •The constructor was invoked (Errors.<init>()) and executed until line 15•The initialize method was invoked and run until line 22


Debugging Functions

▮ Stepping through the program one line at a time

▮ Use the step into for most detail

▮ or F5

▮ But more often you will use the step over

▮ or F6


Step over (F6) causes line 22 to be executed

Notice new variable name and its value is added in Variables view...

And current line changed to 23

Step into (F5) would have sent us into the String class' constructor


Can also display String variable value by moving cursor over variable in source code

Variables view lets you see all object properties assigned to variable


Selecting the variable in the Variables view displays basic info (toString info)


If we step over again (i.e. execute line 23) notice change in label text property


Resumes execution

Ends execution


End Debug by right clicking debug perspective button and choosing Close


Some run time exceptions will result in stack trace displayed in console

RAD added links to particular line numbers


Clicking the line number selects and displays the statement


Another Helpful Feature

▮ In source, clicking to the right of a parenthesis, brace, bracket (left or right)…

▮ Displays the paired parenthesis, brace, bracket (or what the editor thinks is the paired token) with a gray line around it

▮ Invaluable when trying to track down mismatched parentheses

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