This document provides an overview of loops in Java, including for, while, do-while loops as well as break and continue statements. It discusses using loops to iterate through arrays or to repeatedly execute blocks of code when the number of iterations is predictable or unpredictable. Examples are provided to calculate factorials using different loop types and to demonstrate the break and continue statements. Additional learning resources on Java programming concepts are also listed.

1. Programming in Java
5-day workshop
Loops
Matt Collison
JP Morgan Chase 2021
PiJ1.4: Loops

2. Session overview
Loops
• for loops
• while loops
break and continue

3. Loops
• A loop can be used to execute a block of statements repeatedly. If the
repeating time is:
• predictable: usually use for loop
• unpredictable: usually use while or do-while loop
for ( initialization; condition; expression ){ ... }
• Example:
for( int i=1 ; i<10 ; i++ ){
...
}

4. Alternatives for loops
• Q1: How about only printing the odd numbers smaller than 10?
for ( initialization; condition; expression ){ ... }
for( int i=1; i<10; i++ ){
if( I % 2 != 0 ){
System.out.println( i );
}
}
• Q2: How about printing the numbers in a descending order?
for( int i=9; i>=1; i--){ ... }

5. for-each loops
for ( Type var : array ) {
...
}
• Example:
int[] numArray = {7,2,6,0}
for ( int i : numArray ) {
System.out.println("number is: " + i);
}
Note: the use of a colon

6. while and do while loops
while(Boolean expression) {
...
}
do {
...
} while ( <Boolean expression> )
• When the condition becomes false, program control passes to the line
immediately following the loop.
• while loop might not ever run.
• do-while loop runs at least once.

7. Examples
Calculate the factorial of n (=1*2*...*n) using the three types of loops.
Option 1: for loop:
int n = 5;
int factorial = 1;
for (int number = 1; number <= n; ++number) {
factorial *= number;
}

8. Examples
Calculate the factorial of n (=1*2*...*n) using the three types of loops.
Option 2: while loop:
int n = 5;
int factorial = 1;
int number = 1;
while (number<=n) {
factorial *= number;
number++; //update
}

9. Examples
Calculate the factorial of n (=1*2*...*n) using the three types of loops.
Option 3: do-while loop:
int n = 5;
int factorial = 1;
int number = 1;
do{
factorial *= number;
number++; //update
} while (number<=n);
Should we print or return
the factorial?

10. break and continue
• The break statement is used to jump out of the loop or switch
statement.
• The continue statement is used to skip to the next iteration of the
loop.

11. break
• Note: Only jump out of the innermost enclosing loop of the statement.
int n = 5;
for ( int i = 1 ; i < n ; i++ ){
for ( int j = 1 ; j < n ; j++ ){
if ( j > i) {
break;
} else {
System.out.print( j + " ");
}
} // break comes here if it runs
System.out.println();
}
What is the output?

12. continue
• The continue statement skips the current iteration of a loop. Only skip the
innermost enclosing loop of the statement.
for (int i=0;i<=10;i++){
if (i==5) {
continue;
}
System.out.println(i);
}
System.out.println("Done!");
1. What is the output?
2. What is the output if replacing continue; with break;?
3. What is the output if replacing continue; with return;?

13. Summary
• Loop control statements
• for loop
• for ( int i=0; i<n; i++) { … }
• while loop and do-while loop
• Both for and while loops might not ever run, do-while loop runs at least once
• break and continue
• break: jump out of the loop or switch statement.
• continue: skips the current iteration of a loop.
• Both only have effect on the innermost enclosing loop.

14. 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

15. 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