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

Arrays

Collapse Content Show Content

Strings let you hold characters together into words and do things with them, but what if you want to keep a collection other data? For example, let's say you have a list of numbers, such as scores from a Skeeball tournament. It wouldn't make sense to create a new variable for each number, it would be simpler to keep all the numbers in one structure. Java provides various structures for storing collections of elements, but the simplest one is an array.

An array is a structure of a fixed length for storing items of one type together. It's like a shelf for storing related items. Instead of accessing a specific piece of data by calling its variable (or "container"), you can instead specify its location in the array (or "shelf").

simple empty array

In Java (and other languages), the first item in an array is located at position 0. You can think of the array as a number line, with the first item located in the 0^th slot.

Arrays in Java use a special [brackets] syntax. To declare an array variable, you need to state the data type the array will hold, brackets to show that it's an array, and the array name:

int[] nums;

To create the actual array, you need to use the new keyword, state the type (again), and specify the length of the array:

nums = new int[8];

This will create an empty array that can hold up to 8 integers in index positions from 0-7.

You could have also combined the above steps into one line:

int[] nums = new int[8];

To put items in an array, you specify the position in the array with brackets and assign it to a value:

nums[0] = 7;
nums[3] = 3;

This will put a 7 in the 0^th spot and a 3 in the 3-index spot:

simple empty array

To get an element from an array, you can reverse that process:

 int num = nums[0];

This will access the 0^th position and set num to 7.

When you create an 'empty' integer array, Java sets all the values in it to 0. To quickly create an array with specific values, you can use the following 'curly-braces' shortcut:

int[] numbers = {0, 1, 2, 3, 14, 15, 16, 17};

full-array

Let's say you're given the array numbers but you don't know how long it is. You need to access the property of an array called length. We'll cover Objects and their properties later, but for now, just remember this syntax: arrayName.length

For example, this code would assign the length of numbers to a variable length:

int length = numbers.length;

length will be 8, since that's the number of cells in numbers. The last cell in the array is at position numbers.length -1, since arrays are 0-indexed:

int last = numbers[numbers.length-1];

This will access the last (or 7^th) spot, and set last to 17.

Other Array Examples
The above arrays just stored integers, but you can store other data types in Arrays also. For example, this code will create an array for Strings:

String[] words = new String[10]; 
words[0] = "jim";
String name = words[0];  //name is now "jim"

You can even create an array of arrays, or a 2D array:

 int[][] board = new int[8][8];

This 2D array could be used to store things like game boards.

Summary
Review this example code to make sure you get how arrays work:

Task	Code	Result	Explanation
Declare and create an array	`int[] ar = new int[5];`	{0,0,0,0,0}	Creates an array with default 0 values
Create an array with specific values	`int[] ar2 = {5,4,3,2,1};`	{5,4,3,2,1}	Use Curly braces to create filled-in array
Set the value in an array	`ar[0] = 3;`	{3,0,0,0,0}	Sets the 0^th cell to 3
Get the value from an array	`int num = ar2[4];`	1	Sets `num` to the value of cell 4 in `ar2`
Bonus: get and set the value in an array	`ar[0] = ar2[4];`	{1,0,0,0,0}	Sets cell 0 of `ar` to the value of cell 4 of `ar2`

More Practice
After solving the challenge below, get some more practice with these challenges:

Challenge

You will be given an array of numbers ar. Create and return another array of length 2 that just contains the first and last number in the original array.

Please sign in or sign up to submit answers.

Alternatively, you can try out Learneroo before signing up.

Challenge

In one line of code, declare and initialize an empty array of booleans named array with a length of 6.

Please sign in or sign up to submit answers.

Alternatively, you can try out Learneroo before signing up.

Challenge

We need to get the second-to-last number in an array. In one line of code, declare an int variable n and set its value to equal the second-to-last number in the array numbers.

Please sign in or sign up to submit answers.

Alternatively, you can try out Learneroo before signing up.

Previous Next Node

Comments

niveoserenity
Sep 29, 1:42 PM

I got the answer in the end, but my first attempt was:

int CC = ar[ar.length-1];
int VV = ar[0];
return CC + VV;
Brandon Hall
Dec 24, 7:15 AM

int[] fl = new int[2];
fl[0] = ar[0];
fl[1] = ar[ar.length - 1];
return fl;
Robin Elliott
Feb 3, 4:06 PM

Thanks for the site I am really learning from it.
One thing I'd like to be able to do is practice typing things out as I'm following along with your explanations. I think it would help reinforce

cont...
the information. Could you add blank text entry fields (not compilers) here and there in the lesson to give the user scratch space?
Learneroo
Feb 3, 4:14 PM

@Robin, if you just want to take notes I think it would make sense to do in your own note-taking program, though we can consider adding such a feature. Note that if you click on "split-screen

cont...
view", you can view the code editor and content side-by-side, which may work for your purpose. And if you decide you want to run different code snippets, you can try the visual debugger (launched yesterday).
David
Feb 26, 4:23 PM

Just a funny quote here... On the challenge Second-to-Last i scratched my head for two days trying to think how to declare the variable in a single sentence the reason was because i was understanding: from the second (meaning starting at index 1) to the last, rather than actually thinking about penultimate! Frustrated not being able to declare the variable i decided that i was not understanding the exercise, clear enough google explained me second-to-last=penultimate... OMG LOL

cont...
Pura vida.

-David
Shadov
Mar 23, 2:09 AM

What does "Second to last" mean?
Shadov
Mar 23, 2:16 AM

HOLY MOLY. 2D ARRAYS MADE MY SUDOKU SOLVER EASIER!
Shadov
Mar 23, 2:20 AM

And here is the answer for the challenge:

int [] ars = {ar [0], ar [ar.length -1]};
return ars;
kevintaw
Jun 10, 11:38 AM

these are very good
Gaetano
Oct 24, 8:31 PM

If you get a number from an array, and do not know what slot its in. Is there a code to retrieve the slot?
Example;
I found the largest number in the array and want to know what slot that is in.
Bernard Mitchell
May 26, 9:34 PM

I don't understand why my challenges are wrong:
1. int n = numbers[6];
2. boolean [] frank= new boolean [6];
thales
Jul 7, 2:38 PM

I had the write awnser but it still does not work, what is wrong in this:

int[] ar2 = new int[2];
ar2[0] = ar[0];
ar2[1] = ar[ar.lenght - 1];
return ar2;
thales
Jul 7, 2:40 PM

sorry i solved it lenght mus be length pfffffffffffff :-)
Mauricio Andrian
Aug 14, 8:56 AM

Is the free memebership limit forever? Doesn't it restore after certain time?
Learneroo
Aug 14, 3:45 PM

It doesn't currently restore since it's considered a free trial.
Gary
Feb 19, 12:11 PM

Or declare regular variables:

int a = ar[0];
int b = ar[ar.length -1];
int[] az = {a,b};

return az;
Ghadeer Alali
Jun 25, 7:15 AM

Second-to-last means, the second element in the array starting counting from the last index.
rajni dound
Nov 18, 5:13 PM

import java.util.Scanner;
public class Main(){
public static void main(String[] args){
int n;
int[] numbers= new int[8];
int[] numbers={2,3,4,5,6,7,8,9};
for(int i=0;i<8;i=i+1){
System.out.print(numbers[i]);
if(i==6){
n=numbers[i]
System.out.print(n);
}
}
}
}
Spiro
May 9, 5:57 AM

Do it easy guys ...
int[] res = {ar[0], ar[ar.length -1]};
return res;

All Node Comments