Control Structures

1. Make a Decision: If-Statement

Patrick 🐥 is a very polite duck who likes to greet everyone he meets with “Good Morning,” “Good Afternoon,” or “Good Night” depends on the current time.

animation of duck Patrick

(image by National Geographic:

As we learned before, we can print these statements as below:

System.out.println("Good Morning");   // say it between 0:00 - 11:59
System.out.println("Good Afternoon"); // say it between 12:00 - 19:59
System.out.println("Good Night");     // say it between 20:00 - 23:59

However, we also know Patrick 🐥 should only decide to say one of these statements depends on the time.

In other words, we need to write code that makes a decision on what to print depend on a situation.

In Java, we make a decision by using if statement or if…else statement. Read the notes below:

If Statement

if(boolean expression){

An if statement follows the structure shown above. The statement in the block will only be executed if the boolean expression is true. Otherwise, the program will skip to the statement after the block.

If-Else Statement

if(boolean expression){
    statement(s) A
    statement(s) B

An if-else statement follows the structure shown above. If the boolean expression is true, the program will execute the if-block with statement(s) A. Otherwise, the program executes only the else-block with statement(s) B.


boolean likeMusic = true;
if(likeMusic == true){
    System.out.println("I like Music");
    System.out.println("I don't like Music");

The code above will print the statement I like Music because the boolean expression, likeMusic == true, is true.

Let’s revisit the greating example above, and make decision for Patrick on whether to say “Good Morning,” “Good Afternoon,” “Good Night”.

Patrick should say “Good Morning” between 0:00 - 11:59, “Good Afternoon” between 12:00 - 19:59, and “Good Night” between 20:00 - 23:59.

  1. We have a variable currentHour that stores the current hour.
  2. Fill out the correct boolean expression A and boolean expression B to complete the program.
  3. Test your result by assigning the variable currentHour to numbers from 0 to 23:

2. Repeat It Again: For-Loop, While-Loop

In math class, Patrick’s 🐥 teacher ask the class to solve a math challenge, and he might need some help.

The teacher asks them to find the sum of 1, 2, 3, …., 100. That is what is 1 + 2 + 3 + 4 … + 99 + 100?

While we can plug this in to a calculator, that is gonna take a while. Thankfully, in Java, you can easily calculate this in 3 lines with the help of a for loop or while loop.

For loop and While loop are control structures in Java that allow you to execute blocks of code multiples times.

While Loop

while(boolean expression){

statement(s) in the while block will be executed as long as the boolean expression is true.

The following is an example that prints out 1 through 10:

int number = 1;
while(number <= 10){
    number = number + 1;        // number is incremented by 1

In the while loop above, the program will print out the value of number and increment number. This step will be performed as long as number <= 10, and in this case the loop terminates when number is 11.

For Loop

for(initialization; termination condition; update statement){

In a for loop, there are three parts the in condition block: initialization, termination condition, and update statement.

  1. When executing a for loop, the program will execute the initialization part. An example is, int number = 1.
  2. Next, the program checks for termination condition. An example is number <= 10.
  3. If it is true, execute the statement(s) and the update statement (an example is number = number + 1) then repeat step 2. If it is false, exit the for loop.

The following is an example that prints out 1 through 10:

for(int number = 1; number <= 10; number = number + 1){

After learning about for loop and while loop, let’s see how do we help Patrick 🐥 and calculate 1 + 2 + 3 + 4 … + 99 + 100 easily?

Let’s try to do it in a while loop:

  1. We have a variable called total that tracks the total addition sum we get so far.
  2. We have a variable called num that tracks the next number to be added to total.
  3. Write out the appropriate boolean expression and write out the while loop.

By following those step we will get a while loop like this:

int total = 0;
int num = 1;
while(num <= 100){
    total = total + num;
    num = num + 1;
System.out.println("Answer calculated in a while loop: " + total);

Test this out below by clicking Run and write a for loop version that calculates the same thing! You should get the same answer!