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How to Read Code in Java

4 min readdecember 17, 2021

Milo Chang

Milo Chang


AP Computer Science A 💻

130 resources
See Units

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 = i*2

 

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 is now 1, and the computer prints "1"
  • 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 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 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
Browse Study Guides By Unit
Unit 1 – Primitive Types
📱Unit 2 – Using Objects
🖥Unit 3 – Boolean Expressions & if Statements
🕹Unit 4 – Iteration
⚙️Unit 5 – Writing Classes
⌚️Unit 6 – Array
💾Unit 7 – ArrayList
💻Unit 8 – 2D Array
🖲Unit 9 – Inheritance
🖱Unit 10 – Recursion

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