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

Booleans

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Booleans
One of the 4 basic types was the boolean data type, which can store either true or false values. As with ints and Strings, you need to declare the type (boolean) and name, and can assign it a value(true or false). For example:

boolean alive = true;
boolean hungry = false;
boolean haveMoneyLeft = true;

Notice the booleans are not Strings (which would be in quotes), but their own basic values: true and false.

Assigning booleans based on expressions
Often, you don't directly assign true or false to a boolean, but instead provide it with the result of a comparison. For example:

boolean haveMoneyLeft = yourMoney > yourDebts;

yourMoney > yourDebts is a comparison which will return either true or false. That result will then be stored in the boolean variable haveMoneyLeft. This boolean can then be used later.

Using boolean variables in conditional expressions
In previous nodes, we showed how you can use if, while and for statements to execute code only when a certain comparison is true, such as:

if(yourMoney > yourDebts){
    buyStuff();
}

Sometimes you may want to store the results of a comparison to use later. You can use the boolean data type to do so. For example:

boolean haveMoneyLeft = yourMoney > yourDebts;
if(haveMoneyLeft){   //this boolean will be true or false
  buyStuff();       // this will only get executed if haveMoneyLeft is true
}

The boolean haveMoneyLeft can then be used multiple times (so one can check if there's still money without rewriting the whole comparison). However, if 'yourMoney' or 'yourDebts' changes in value, 'haveMoneyLeft' will not automatically check if it should change in value too, so you may need to run the original comparison again then.

Not
If you only wanted to run code if a boolean was false, you could write an equality to check that. For example, to see if haveMoneyLeft was false, you could check
if(haveMoneyLeft == false).
You can also use the NOT operator to get the opposite of a boolean. In Java, an exclamation point is used for NOT: !.
For example:

if( ! haveMoneyLeft){
 declareBankruptcy();
}

Challenge

You will be given two integers a and b. Return true if a is a multiple of b, and false otherwise.

Please sign in or sign up to submit answers.

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Comments

Rod
Nov 5, 3:53 AM

This error was returned. What is the problem with this syntax?
Main.java:6: error: unexpected type
return ((a%b) = 0);
^
required: variable
found: value
1 error
Learneroo
Nov 5, 3:14 PM

= is an assignment, you probability want an equality check ==
Adhurim Esati
Jul 13, 7:49 AM

I didn't finish reading the whole lesson but what if
{
boolean haveMoneyLeft = yourMoney > yourDebts;
}

What if I have a debt of 500gold and yourMoney is 500gold too
this makes:
{
boolean haveMoneyLeft = false;
}
right ?
Bernard Mitchell
May 24, 10:13 PM

Nice this was an easy one. thanks again for the great site.

Asadujjaman Shimul

Dec 3, 2:39 AM

import java.util.Scanner;
public class BooleanExm
{
public static void main(String[] args)
{
double a;
double b;
Scanner input = new Scanner(System.in);

cont...

    System.out.println ("Enter any number: ");
    a = input.nextDouble();
    System.out.println ("Enter any number: ");
    b = input.nextDouble();
    boolean output = a%b==0;
    System.out.println (output);
}

}

rajni dound
Nov 18, 3:17 PM

import java.util.println;
public class Main(){
public static void main(String[] args){
int a = 4;
int b = 2;
if(a%b==0){
System.out.println("true");
}else{
System.out.println("false");
}
rajni dound
Nov 18, 3:19 PM

import java.util.Scanner;
public class Main(){
public static void main(String[] args){
int a = 4;
int b = 2;
if((a%b)==0){
System.out.println("true");
}else{
System.out.println("false");
}
}
}
Mike Grote
Mar 15, 11:41 AM

Keep it simple, the answer is right there.....
boolean x;
if(a%b ==0){
x = true;
}
else{
x=false;
}
return x;
}
Kerry Pollock
Aug 12, 2:17 PM

In my solution, I used only =, not == , and I got a correct response. Can you tell me why that is? Thanks.

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