Operators and conditional logic in Java

1. Programming in Java
5-day workshop
Operators and
conditionals
Matt Collison
JP Morgan Chase 2021
PiJ1.2: Java Basics

2. Session overview
Operators
• Mathematical operators
• Assignment operators
• Relational and logic operators
Who uses Java?
• popularity TIOBE index
• Java history
Getting started with Java?
• Java download and versions
• The javac Java compiler
• A first Java program - Hello world

3. Operators
• Arithmetic operators (+, -, *, /, %)
double a = 7/2;
• Assignment operators (+=, -=, *=, /=, %=, ...)
x += 2; //equivalent to: x = x + 2;
• Relational operators (==, !=, >=, <=, >, <)
• if (grade != ’A’){...}
• Logical operators (||, &&)
• only on boolean expressions
• if ( (grade != ‘A’) && (year == 2019) ){...}

4. Operator precedence
1. Arithmetic operators – BODMAS or PEMDAS
2. Relational operators – equality testing
3. Logical operators – AND OR
4. Assignment operators
https://docs.oracle.com/javase/tutorial/java/nutsandbolts/operators.html
for full list see docs

5. BODMAS Operators
• The usual arithmetic operators are
available for numeric variables:
+ addition
- subtraction
* multiplication
/ division
% modulus
Math.pow() exponentiation
int a = 2*3 + 4
System.out.println(a)
>> 10
int b = a/2 -1
System.out.println(b)
>> 4
int c = (a + b)*8 – 64
System.out.println(c)
>> 48
double a = 12.0
double aCubed = Math.pow(a,3)
System.out.println(aCubed)
>> 1728.0

6. Incremental and decremental shorthand
• Incremental and decremental operators
• Postfix (x++, x--): The result is the value of x before incremental or
decremental.
• Prefix (++x, --x): The result is the value of x after incremental or decremental.
y=x++; //equivalent to: y=x; x=x+1;
y=++x; //equivalent to: x=x+1; y=x;

7. Incremental and decremental operators class
IncrementalOperators {
public static void main(String[] args) {
float x = 5.2f;
y = x++;
System.out.println("x = " + x + ", y = " + y);
int a = 10;
b = --a;
System.out.println("a = " + a + ", b = " + b);
}
}
Q: What is the output? x = 6.2, y = 5.2 a = 9, b = 9

8. Comparisons and Boolean operators
Comparisons - The result of a
comparison is a Boolean variable
which can have the values true or
false
• < less than
• > greater than
• == equals
• != not equal
• <= less than or equals
• >= greater than or equals
Boolean operators - Boolean variables
can be combined with Boolean
operators. In descending order of
priority:
• ! logical not
• && logical and
• || logical or
Arithmetic operators have higher
precedence than Boolean
comparisons and operators
Arithmetic operations > Boolean comparisons > Boolean operations

9. Order of evaluation
• Precedence: arithmetic operations are evaluated first, then Boolean
comparisons, then Boolean operations.
boolean example = 3*4 + 1 < Math.pow(4,2) -1 && 12 < 7
System.out.println(‘the answer is ‘ + example)
>>false

10. Conditional logic

11. if ( <boolean expression> ) {
block of statements;
} else if ( <boolean expression> ) {
block of statements;
} else if ( <boolean expression> ) {
block of statements;
} else {
block of statements;
}

12. Conditionals
• The block of statements immediately following only the first
condition that evaluates to true is executed
• No other block is executed
• else if and else clauses are optional
• The { … } are an integral part of the syntax although there are
shorthand versions

13. Example
• Example: Determine if an integer is an even or an odd number.
int a = 3;
if ( a%2 == 0 ) {
System.out.println( a + " is an even.” );
} else {
System.out.println( a + " is an odd.” );
}
Remember the mod
operator

14. Find max using for( int item : intArr ) { ... }
1. Set max to negative infinity
2. Iterate over each item in the list
1. If an item value is greater than max:
1. Set max to item value
Why might the this code fail?
// Find the maximum of int array intArr
int max = -999999999
for( int item : intArr ) {
if ( item > max ){
max = item
}
}
System.out.println( max )

15. Nicer max
• Why might the original
code fail?
• All the elements less
than -999999999
• The list were empty
• Where possible
program defensively
• anticipate what might
go wrong and protect
against it.

16. Challenge
• Write a program to decide on grades based on a percentage with the
following boundaries:
• A is 90-100%
• B is 80-89%
• C is 70-79%
• D is 60-69%
• E is 50-59%
• F under 50%

17. switch case syntax
switch(expression) {
case value1:
...
break; // optional
case value2:
...
break; // optional
// You can have any number of case statements.
default: // optional
...
}

18. switch case example
//A demo to output a comment based on the input grade.
public class SwitchApp {
public static void main(String args[]) {
char grade = ’C’;
switch(grade) {
case ’A’:
System.out.println("Excellent!");
break;
case ’B’ :
case ’C’ :
System.out.println("Well done");
break;

19. switch case example ctd.
case ’D’ :
System.out.println("You passed");
break;
case ’E’ :
System.out.println("Better try again");
break;
default :
System.out.println("Invalid grade");
}
System.out.println("Your grade is " + grade);
}
}
• Run through examples in IDE

20. switch case
• The expression can only be an integer (byte, short, int, long, char),
enumerated object, or String object.
• When the expression is equal to a case, the statements following that
case will execute until a break statement is reached.
• No break is needed in the default case.
• Q: In the above example, if the break-statement before the case ’D’: is
removed, what will the program output on the screen?

21. Learning resources
The workshop homepage
https://mcollison.github.io/JPMC-java-intro-2021/
The course materials
https://mcollison.github.io/java-programming-foundations/
• Session worksheets – updated each week

22. Additional resources
• Think Java: How to think like a computer scientist
• Allen B Downey (O’Reilly Press)
• Available under Creative Commons license
• https://greenteapress.com/wp/think-java-2e/
• Oracle central Java Documentation –
https://docs.oracle.com/javase/8/docs/api/
• Other sources:
• W3Schools Java - https://www.w3schools.com/java/
• stack overflow - https://stackoverflow.com/
• Coding bat - https://codingbat.com/java