Quick Reference

Quickly lookup Java basics, and load code examples directly into editor.

Method Basics

The header of the method declares what the method will return (such as an int), the method name (doStuff), and the method's parameters (such as a and b). The method body follows, where you usually need to write code that returns a value at the end.


//method header: methodName(parameters)
static int doStuff(int a, int b){
  //your code here - erase this comment and write your code!
  return 1;
}

Load Method
Syntax note: Every line of code in Java needs to end with a ; or it will be considered part of the next line. Comments are marked with 2 backslashes // and are ignored by the computer.

Variables and Data types

Variables need to be declared with their type before they can be used. You can declare and assign a variable on one line: int num = 5; Load Code


//variable examples
//type varName = value;
 int num = 5;
 boolean alive = true;
 String word = "hello";
 double pi = 3.14159;
 char c = 'c';

Operations Booleans and Logic

Math operations are performed in the standard order. Note that division of integers will round down to nearest integer. The mod % operator returns the remainder after division.


int product = 4 * (1+2) / 2;   //6.
int roughDivision = 8 / 3;   //2
int remainder = 3 % 2;     //1, the remainder of 3/2

booleans are used to store true/false values, such as the results of comparisons:


boolean check = false; //this will store result of comparison
int five = 5;
//the following are all true
check = (five == 5);    //equals
check = (five != 3);   //not equals
check = (five >= 5);  //greater or equals

The logical operator AND && will return true if multiple conditions are all true, and OR || will return true if at least one condition is true.


//these pointless statements will both set check to true
check = (3>2 && 2>1); //AND
check = (3>2 || 1>3); //OR

if else and while

The if statement will execute code only if a condition is true:

if(condition){
    //doSomething
}

Load If Statement
The else statement will execute code if the condition was false. The while statement will repeatedly execute the same code while a condition is true.


public static void controlFlow(int number) {
    //this if/else block will print out 1 statement for any number
    if(number > 10){
        System.out.println("big number");
    }
    else if(number > 5){
        System.out.println("medium number");
    }
    else{
        System.out.println("small number");
    }

    //this loop will print the numbers from 0 to 9
    int i = 0;
    while(i < 10){
        System.out.println(i);
        i = i + 1;
    }

}

Printing and Arrays

Print a newline
System.out.println(""); Load Print Statement
The above prints text and add a newline afterwards. System.out.print prints without a newline, but you can add in spaces " " to separate text.


System.out.println("hello"); //prints with newline 
String word1 = "hello";
String word2 = "world";
System.out.print(word1 + " " + word2); //prints "hello world"

Arrays are used to store multiple items of one data type together. They are declared with the type they will hold and with brackets [].
This code creates an empty 5-cell array:
int[] ar1 = new int[5]; Load Array Code
This shortcut creates an array with numbers 0 to 4:
int[] nums = {0,1,2,3,4}; Load Code
You can use for-loop to print values of array:

for(int i=0; i < nums.length; i=i+1){
    System.out.print(nums[i]+" "); 
}

Load Code

Math and Comparison Operators

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Basic Math Operators

You've already used + and / to add and divide. Java also includes - and * for subtraction and multiplication. In addition, there's another basic math operator % (it's called mod, and it uses the percent symbol) which is used to get the remainder after division. For example, 5 % 2 will return 1 (the remainder of 5 / 2).

Order of Operations
When you combine multiple operations in one line, Java is similar to the standard order of operations (PEMDAS):

Parentheses
Multiplication and Division and Mod
Addition and Subtraction

Operations on the same level will be evaluated in order from left to right, just like the standard convention.

Comparison Operators

We've seen that = assigns a value and shouldn't be confused with == which checks if two values are equal. These are all the ways to compare values:

Start with int a = 5;

symbol	meaning	true example
==	equal to	a == 5;
!=	not equal to	a != 3;
>	greater than	a > 3;
>=	greater than or equal to	a >= 5;
<	less than	a < 6
<=	less than or equal to	a <= 5;

All Order of Operations

This table summarizes the order of all the operations. The higher an operation appears, the higher its precedence. (You can view the full table on the Java Tutorials.)

Name	Symbol
not	`!`
parenthetical	`( )`
multiplicative	`* / %`
additive	`+ -`
relational	`< > <= >=`
equality	`== !=`
logical AND*	`&&`
logical OR*	`\|\|`
assignment	`=`

*The logical operators will be covered soon.

General Tip
If you need to reference some Java quickly while learning this module, you can click on the button on the sidebar to see sample Java code.

Challenge

Given two numbers a and b, return the remainder when their product is divided by their sum.

For example, if given 2 and 5, return 3 (since their product 10 divided by their sum 7 leaves a remainder of 3).

Please sign in or sign up to submit answers.

Alternatively, you can try out Learneroo before signing up.

Challenge

What is the value of num after the following code?
int num = 15 / (1 + 2) % 3;

Please sign in or sign up to submit answers.

Alternatively, you can try out Learneroo before signing up.

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Comments

Jay Learn
Jul 23, 1:23 AM

I solved this problem with first declaring sum and mul as integers. Then returning their values using modulus.
Ryan
Nov 3, 10:48 AM

the question confused me, when did i have to multiply at any point???
Learneroo
Nov 3, 11:40 PM

@Ryan, the product of a and b means multiply a by b. Would this be clearer:

Multiply a by b and then get the remainder from dividing that product by the sum of a and b ?
Ryan
Nov 4, 8:19 PM

Yeah, thank you. The product made me think addition but i guess that would be the sum. Maybe i should have taken more math classes lol. So far this has been the best teaching program i have used

cont...
online. Some of the ?'s are somewhat difficult but thats what learning is about. Thank you for the email and reply.
Brent Jones
Jan 6, 12:16 AM

Excellent tutorial thus far. It is probably hard to understand the precise syntax that is needed to pose the problems.
Denis Semendyayev
Jan 7, 3:14 PM

i was so confused by the wording of product. to me it processed as the same thing as sum :/
Matt
Apr 27, 11:42 AM

I read the challenge about 5 times and operated exactly what they wrote: return (a*b)%(a+b);

I hope this helps. Wording can be tricky but it's all going back to basic math terminology.
Adhurim Esati
Jul 2, 5:41 AM

The remainder of 5 % 3 could have been -1 too.
ArtTric
Aug 6, 6:50 PM

This one tripped me too. It can be done this way as well:
int product = a * b;
int sum = a + b;
return product % sum;
Mitchell
Mar 20, 6:23 PM

PEMDAS is also known as BEDMAS (brackets instead of parentheses)
thales
Jul 7, 6:59 AM

I am a math teacher so this wasnt to hard for me :-)
Joseph Estridge
Sep 12, 3:13 PM

I have a question... I understand that you can just return the value of "(a * b) % (a + b);" ... But I also tried doing this one by assigning to value to a new variable, int C. I added an int

cont...
C in public static int, and set the equation equal to C, returned C, but the code didn't work. How would I store its value to a new variable?
Eze Sunday Eze
Jan 29, 4:34 PM

i got confused, i got it when i read the question again.
Ahmad Banoori
Jun 29, 5:02 PM

Good way of learning..
Shandeep Kumar
Dec 23, 9:33 AM

that's really awsome man
Roberto Blanco Axelsson
Mar 7, 9:29 AM

My solutions was like ArtTric
Enoch
Mar 31, 7:37 PM

I don't know where to input int product = a*b;
Int sum = a+b;
paul. kirsch
Jun 18, 2:11 AM

I got it correct after 3 tries. I may be wrong but I surmise that '%'is to be put between code that identifies what variables you are working with such as 'sum' and really means: "divide and also give me the remainder".
hunter
Sep 6, 3:15 PM

yeeyee
Kurt
Jan 27, 2:12 PM

something is wrong here because how is 5*1 divided by (5+1) have a remainder of 5?

basic math says its .8 with a remainder of 2. so 5*1 is 5. well 5 divided by 6 equals 0.8 r2
Kurt
Jan 27, 2:14 PM

I get the code, but it has a flaw because 5*1 divided by 6 does not have a remainder of 5. its 2

because basic math says that 5/6 is 0.8 with a remainder of 2.
lucifer
Feb 11, 1:57 AM

guys i did this
//int num
//(1*5)%(6);
return (a*b)%(a+b)
noel
May 1, 6:05 PM

int mul = a * b;
int sum = a+b;
int result = mul%sum;
return result;
Santo Credendino
Aug 9, 9:44 AM

What is going on in this solution:
Applying the rules for Math and Operator Order of Precedence
1. addition is performed first because of ( )
2. multiplication is the next operation
cont...
modulus is the last operation performed before returning the results

solution:
return ( a * b % ( a + b ));
Raj
Apr 27, 3:46 PM

it can be also done by this:
int product = a+b;
return (a*b)%product;
// product is the sum of A and B, i could have named it anything//

All Node Comments