1. The most important decision statement
The Java IF Statement
The most important decision statement
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2. Flow of Control in Java Recall the flow statements in Java:
Decisions If
Switch Case
Loops
for
while do
Break
Here we consider the if
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3. Why? There are too many times when we need a conditional statement
One set of statements is executed in one set of circumstances and another otherwise
We also want our programs to be robust
Unlikely to abnormally terminate regardless of what the user does
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4. Example Suppose the following statement: avg = total/count;
If count is zero the program will abnormally terminate with a divide by zero exception
The solution is nesting this in an if: if(count > 0) avg = total/count;
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5. Example Revisited The basic form is: if (cond) statement;
The if is a reserved word
The parentheses are required
They show the limit of the condition
Only a single statement is allowed
A compound statement may be used
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6. A Condition Any expression of type boolean Usually a comparison
May include boolean methods
May include boolean operators
Unlike C/C++ must be a real boolean
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7. Two forms of if statement
if (boolean) statement
if (boolean) statement else statement
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8. The Else There is no THEN like in Pascal or BASIC The else is optional
The statement to do if true immediately follows the parenthesized condition
The else is optional
Else is a reserved word and signals the start of the statement to execute if the condition is false
The if without an else means else do nothing
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9. Compound statements
Either the then statement or the else statement may be replaced with a compound statement
In any combination
Even the compound statement of an if may create local variables
Their scope is confined to the compound statement
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10. Example Program Modify a picture by increasing contrast
Make the brighter pixels brighter still
Dim the pixels already less than average brightness
This requires an if because pixel processing is not uniform
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11. Basic Program is Same public static void main(String [] a){
String fileName;
FileChooser.setMediaPath (“…/mediasources");
fileName=FileChooser.pickAFile();
Picture p1=new Picture(fileName);
p1.explore();
process(p1); }
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12. Process Method Beginning
static void process(Picture p){
for(int y=0;y<p.getHeight();y++)
for(int x=0;x<p.getWidth();x++){
Pixel px = p.getPixel(x,y);
int red = px.getRed();
int green = px.getGreen();
int blue = px.getBlue();
// Now what?
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13. Process Method End if(red + green + blue > 128*3){ // brighten
red = red*3/2;
green = green*3/2;
blue = blue*3/2; }
else { // Dim
red = red*2/3;
green = green*2/3;
blue = blue*2/3;
px.setRed(red);
px.setGreen(green);
px.setBlue(blue);
} // end of fors
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14. Barbara
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