CIS3931 – Intro to JAVA

1. CIS3931 – Intro to JAVA Lecture Note Set 3 19-May-05

2. Compound Assignment Operators • Way of shortening code. • Assume, c = 3. • c = c + 7; c += 7; //same as above

3. Compound Operators

4. More Shorthand • You can also use ++ and -- if you wish to only change the integer by 1. • For example, d++ is the same as writing d=d+1. • It is also the same as writing d += 1 • The same holds true for the --.

5. Prefix vs. Postfix • Prefix is ++b while postfix is b++. • Prefix and postfix operators are used mainly in loops. • Whether you use prefix or postfix will determine the outcome of your program.

6. Prefix Increment • Example: ++a • It would increment by 1, then use the new value of a in the expression in which a resides. • int a = 5; b = ++a; • What is b? What is a?

7. Postfix Increment • Example: a++ • It uses the current value of a in the expression in which it resides, then increments by 1. • int a = 5; b = a++; • What is b? What is a?

8. Decrement • The same holds true for the postfix and prefix decrement operators. • Depending on whether or not it is pre or post depends on when it gets computed.

9. Ternary Operator • Takes in 3 operands. • The first leftmost operand is a boolean expression (it can only return true or false) • The second (between ? and :) is what is done if it is true. • The third is what is done if it returns false. • Shorthand for if/else statements.

10. Ternary Operator 2 • An example: System.out.println( grade>=60 ? “Passed” : “Failed” ); • If the grade is >= 60, it prints out Passed. • Else it prints out Failed. ( grade >= 60 ? “Passed” : “Failed”)

11. Precedence • Some operators execute before others. • This is because they have a higher precedence. • Useful if you have more than one mathematical operation taking place on one line.

12. Precedence Chart

13. Counter-Controlled Repetition Counter-controlled repetition requires: • A control variable • Initial Value • The increment (or decrement) • Loop-continuation condition

14. Counting a while loop int counter = 1; while (counter <=10 ) { System.out.println(counter); counter++; } //prints out integers 1 through 10 on a // separate line

15. The for statement for( int count = 1; count <=10; count++) { System.out.println(count); } //same output as the previous while loop

16. For Brackets • Once again, the brackets are not required if there is just one line of code below the for statement. • If there is more than one line, they are required.

17. For loops • Is this legal? for( ; ; ) • If so, what does it do? • What does it not do?

18. General For format for ( initialization ; loopContinuationCondition ; increment) //should be 1 line { statement/algorithm }

19. Common errors • When a for statement’s control variable is declared in the initialization section of the for loop, editing that same variable in the for body will produce an error. • In other words, don’t alter the variable inside the body of the for loop. • To sidestep this problem, use temp variables and set them equal if need be.

20. Finding Sum int total = 0; for( int i=2; i<=20 ; i+2 ) total += i; System.out.println(“Sum is “ + total ); /* Above finds the sum of all even numbers 2 through 20 and prints it out */

21. Do…while statement int counter = 1; do { System.out.println(counter); counter++; } while ( counter <= 10 ); //notice the ; // Same as while statement a few slides ago

22. Braces on do…while • Although you do not need braces on a do while statement if it is only 1 line, it is common to add them to help readability and to separate between the while and the do.

23. Do…While • Every do while statement can be written as a for statement or a while statement. • Most people opt for the for loop instead of writing a do…while loop.

24. Switch statement • Can only compare integer values • Useful if there are several different conditions. • See the SampleSwitch.java code

25. switch switch (integer value) case 1: case 2: do something here break; default: do something here break;

26. Switch Defaults • Goes to default if it doesn’t match anything else. • You should always have a default after the last case. • The last case in a switch statement doesn’t need a default, although most programmers add it for clarity.

27. Break • A break statement will “break” out of the loop it was in. • for( int i=1; I <=10; i++) { if (i == 5) //if i == 5, break; // break out of for loop System.out.println( i ); }

28. Continue • Continue statements can be avoided by using structured programming. • Therefore, they will not be covered in this course.

29. Assignment #2 • Tax withholding calculator • Due Thursday May 26th • Description available online at course website http://www.cs.fsu.edu/~cis3931

30. Questions…