How to Read Code in Java

Once you recognize the frequent symbols used in Java, you'll be a pro at understanding what's happening in the code!

Java Symbols to Know 🏆

• ==  compares the value on the left and the value on the right
• % gives you the remainder when the value on the left is divided by the right
• ! means "not", so != means "not equal to"
• i++ is the same as i += 1 or i = i+1
• i-- is the same as i -=1 or i = i-1
• i= 2* is the same as i = i2*

Java Examples

Example #1 🤔

for (int i=0; i<5; i++)
{
   System.out.print(i);
}

​ What is printed as a result of executing this code?

• The value of i starts off at 0 when the for loop begins, so when i is printed, the computer prints "0"
• Since the loop increments by i++, the value of i is now 1, and the computer prints "1"
• i increases in value until i equals 4 and the computer prints "4"
• After that, i increases in value to 5, but 5 is not less than 5, so the computer exits the for loop and does not print anything else
• Since the code uses "print", all of the numbers are printed on the same line Your output is:  01234

Example #2 😃

for (int i=0; i<5; i+=1)
{
   if (i%2 == 0)
   {
      System.out.print(i);
   }
​
   else if (i!=3)
   {
      System.out.println(i*2);
   }
 
   else
   {
      System.out.print("Three");
   }
}

​ What is printed as a result of executing this code?

• The value of i starts off at 0 when the for loop begins
• Since 0%2 equals 0 (since the remainder when 0 is divided by 2 is 0), the first if statement is true and the code inside executes, printing "0"
• The value of i then increases by 1 to become 1
• Since 1%2 is not equal to 0, the computer checks the else-if statement
• 1 is not equal to 3, so the computer prints 1*2, or "2"
• BE CAREFUL!!! Since the code says "println", the computer prints the "2" next to the "0", then starts a new line!
• If you continue tracing the code, when i becomes 2, the computer prints "2"
• When i equals 3, it fails the if statement and the else-if statement, which means the code in the else statement runs, printing "Three"
• Continue tracing the code for i equals 4 and the computer prints "4" Your output is: 02 2Three4

Example #3 😎

int count = 1;
while (count<5)
{
   count*=2;
}
System.out.println(count);

​ What is printed as a result of executing this code?

• When the code begins, count equals 1
• Since 1 is less than 5, the computer enters the while loop and count is doubled, making count equal to 2
• Since 2 is still less than 5, the computer re-enters the while loop and count is doubled again, making count equal to 4
• Since 4 is still less than 5, the computer re-enters the while loop and count is doubled and now equals 8
• Now that 8 is not less than 5, the computer moves on and prints count, or "8" Your output is: 8

Example #4: 👍

public int addNums (int x, int y)
{
   return x+y;
}

​ What does the method return with after the method call addNums (5,10)?

The method returns the integer value 15. (Since 5+10 = 15.) ​

Example #5 🤓

for (int y=5; y>0; y--)
{
   for (int x=0; x<5; x++)
   {
      System.out.print(y);
   }
   System.out.println();
}

​ What is the output when you execute this code?

First of all, don't panic when you see this snippet, it's easy to solve if you follow it step-by-step and write every step down. 🙌

Start when y equals 5:

• Now that you're on the inside of the double for loop, just focus on the inner for loop
• The inner for loop prints y, which is currently 5, five times
• Now the the inner for loop is finished, the computer exits that loop and moves on to "System.out.println()", which causes the computer to skip a line
• Since there's nothing inside the parentheses, nothing is printed The value of y then decreases by 1, so y now equals 4. Repeat the same process as above for y equals 4, then 3, then 2, then 1. The code stops running after that because y ends up being 0 and does not meet the condition that y is greater than 0, causing the computer to exit all of the loops.

Your output is: 55555 44444 33333 22222 11111

Summary 🎉✨😎

• As you trace the code, write down what variables the computer has stored and what is being printed (and how it is printed)
• Understand the common symbols used in Java
• Pay careful attention to what is happening to the variables (and write down any changes) because a wrong value will lead you to the wrong answer
• Pay attention to "print" vs. "println" because they lead to very different outputs