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

Variables in Programming and Algebra

Collapse Content Show Content

Now that you know what a variable is, we will clarify a common area of confusion between algebra and programming.

In algebra, you can have a statement like this:

x = 2 + 3

This is a "truth-statement" that the variable x is equal to 2+3 or 5.

Similarly, in programming, you can write int x = 2 + 3 and the variable x will end up as the integer 5. However, there's a big difference between the two statements, as will now be seen.

What if you faced the following two statement together?
x = 2 + 3
x = 11

In algebra, you would immediately recognize that this is impossible. x cannot be both 2+3 and 11 since they're not equal. However, in programming, = is not a statement of truth, it's an action that assigns a value to a variable.
So you can assign 2+3 to x and then afterwards assign 11 to x, and x will now be set to 11 instead of 5.

Equality Check
If you wanted to check if something was equal in programming, you use ==. For example:

int x = 2 + 3; //assignment
x == 11; //this would be false

Review this:

= assignment
== equality check

Challenge

After the following code runs, what number will sum equal?

int x = 2;
int y = 5;
x = 100;
int sum = x+y;

Please sign in or sign up to submit answers.

Alternatively, you can try out Learneroo before signing up.

Previous Next Node

Comments

Matthew
Oct 17, 4:26 PM

Easy I'm liking this website.
Ricky
Dec 22, 1:02 AM

Enter Integer or Decimal Answer: I expect int sum == answer
Jamison Norwood
Dec 26, 2:03 AM

I love this site. Now every other book is looking like a reference sheet.
Brent Jones
Jan 6, 12:05 AM

You say "In the following code, what number will sum equal?"
It would be better to say "In the following code, what number will sum become?"
Also, you should give an extra example of multi-line assignment of variables before testing student understanding

cont...
int x = 1;
int y = 333;
int total = x+y \ total is a container containing the number 334
int x = 5; \ x is now assigned the value of the number 5
int total = x+y \ total is a container which has been changed to contain the number 338

Perhaps using sum or total as variable names is not a good idea. Maybe int z = x + y

For beginners, operators can be easily confused with variables
heya
Mar 4, 1:14 PM

This is amazing thank you so much
Kenneth Au
Mar 27, 8:20 AM

Thanks to this website, I dont feel like a dumb as.. anymore!
xiaochayou
Mar 31, 1:39 AM

so good, I like it so much!
May 5, 8:37 PM

Good site so far :)
Santineida Oliveira Ribeiro
May 7, 11:45 AM

I'm not able to answer.
Derpybunneh
Sep 1, 7:33 PM

It's easy. Forget the int x = 2 and go to x = 100 and add the y to the x.
Paul Lee
Mar 10, 4:16 PM

I have increased my programming knowledge by 500% in the last 10 minutes.
Gomati Damle
Jul 5, 3:30 PM

In the code why it is written
int x = 2;
int y = 5;
x = 100; .Can anyone tell me why data type int is not written befor x=100;
Learneroo
Jul 5, 11:06 PM

As mentioned on the previous page, you need to declare a variable's type the first time, but afterwards you can use it without re-mentioning what type it is.
thales
Jul 7, 6:54 AM

Yes, but i want to learn programming. And isnt this a bit to easy to really learn programming?
Learneroo
Jul 7, 9:26 AM

You can start off easy. There's lot's of challenging material later if you want it!
thales
Jul 7, 11:02 AM

ok i want that challenging stuff :-)
Learneroo
Jul 7, 11:04 AM

When you're ready, check out Algorithms
thales
Jul 7, 12:11 PM

we will, but we are not ready yet :-)
Heckingson Jumbo
Sep 18, 2:41 PM

this is just like js and c
Dion Matthew Levy
Sep 30, 12:43 AM

This was my hardest challenge yet...How does 100 + 5 = 105 if x=2???? Computers are DU|\/|@$$3$ !!!! If X(2)=100 And X(1)=2 and Y(5) then wouldn't x+x+y =107? the only place xxy ever made sense was in Klinefelter syndrome! :p
Gomati Damle
Jun 6, 7:57 PM

Thanks.
chima oscar
Jul 26, 8:11 AM

Got confused here since x = 100; wasn't given any data type.
Ershad
Sep 24, 8:39 AM

Great Website! Thanks.
paul. kirsch
Jun 18, 12:25 AM

This website makes a lot of sense, because a lot of people are not employed or partially employed and can't afford the high monthly costs of other training programs. Also, the questions are simple

cont...
enough to build confidence rather than destroy confidence as many other training sites, (including free ones) do. Learneroo, you have my loyalty.
COD
Feb 16, 8:02 AM

Am so great-full for the amazing challenges on this website

All Node Comments