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

Data Types

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Basic Types

As mentioned in a previous node, when you declare a variable in Java, you need to also state what type it is, i.e. what kind of data it can hold. There are 8 primitive data types in Java, but these are the main 4:

type	definition	example operation //result
`int`	This holds integers with values from about -2 billion to +2 billion. This is what we've been dealing with until now.	`int sum = 999 + 1;` //1000
`double`	This holds decimal numbers (real numbers) with a wide range, like `-2.1` or `123.456789`.	`double sum = 1+0.23` //1.23
- `boolean`	This can only hold two values: `true` or `false`.	`boolean greater = (7 > 10)` //false
`char`	This holds single characters, like `'c'` or `'ß'` or `'1'` in character form. To create a character you need to use single quotes.	`char letter = 'z';`

String

In addition, there's the important String type, which is a little different than the above types, but is also built-into the language. A String holds any number of characters together, so it is used to store any text, like words and sentences. You can create a String by using double-quotes: String str ="hello";. Note that it is written as String (with a capital "S") unlike the simple types above. This is some sample code with String:

String name = "Jim"; Simply store the string "Jim" in the variable name.
String num = 23; ERROR! You cannot store a number in a String!
String num = "23" ...But you can store a String representation of a number.

In addition, you can concatenate characters with Strings or String with each other to make larger Strings.* For example:

String alpha = "ab" + 'c'; combines the character with a String to make a larger String, "abc".
String sentence = "Pack " + "my " + "box."; will set sentence to "Pack my box."

You can even concatenate numbers with Strings, and they will automatically be turned into Strings:

int n = 5;
String message = "Pick a number from 1 to " + n;
//message is now:  "Pick a number from 1 to 5"

Soon you'll see how to print Strings. Meanwhile, see if you can handle different types in the following problem:

Challenge

You will be given an int a and a double b as input. You need to calculate their sum and then return a String message which says:
"The sum is [sum here]"
(Place the correct sum in the string instead of the brackets.)

* However, don't use + to combine characters together with each other, since they'll 'turn into' numbers. This will be covered in a later node.

Challenge

Return an exact sentence with the sum, as specified.

Please sign in or sign up to submit answers.

Alternatively, you can try out Learneroo before signing up.

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Comments

William Kennedy

Oct 12, 8:35 AM

It doesn't seem to compile my answer but it runs on Eclipse just fine.
Here is my code

public class DataTypes {

public static void main(String[] args) {

        int a = 2;
        double b = 7.5;
        double sum = a + b;
        System.out.println("The sum is" +  sum);

}

}

Learneroo
Oct 13, 4:14 PM

@William the only class that can be public is Main. So either remove 'public' or rename the class to 'Main'.
Honzis
Dec 7, 12:26 PM

First getting an error while compiling about return statement missing, wheb added about incompatible types - required String, found double
Here the code:

cont...
public static String doStuff(int a, double b){
double sum = a+b;
return sum;
String message = "The sum is "+ sum;
}

Hint and cheat missing, what's wrong in my code? Thx
Learneroo
Dec 7, 10:24 PM

@Honzis, you need to return a String that contains the correct message. So remove return sum and return message instead.
Honzis
Dec 8, 4:49 AM

Thanks for the quick answer, got it now. :)
pat
May 19, 9:43 AM

What is the code to solve this
Learneroo
May 21, 9:41 AM

@pat, I added an explanation, which you can 'cheat' to view.
Cameron Bee
Mar 12, 5:21 PM

Don't system.out.print, return.
sanka
Jun 12, 6:37 AM

@William, in this question You should implement code in doStuff() method and return the message. my code
sanka
Jun 12, 6:39 AM

@Honzis , You should return only a string type in doStuff() method. because return type of doStuff method is string, check my code :) my code
Alexander
Aug 20, 11:38 AM

Or you could cast a double on the sum my code
Tatyana Royer
Oct 7, 2:50 PM

Just FYI, I had originally put my string as "The sum is: " + sum. It said it was incorrect because I put the : in. So, even though it ran fine, gave the correct answer, etc., it showed incorrect

cont...
due to that. Just for any others who may have missed it for something that tiny and couldn't figure out why what they were doing was wrong.

Asadujjaman Shimul

Dec 3, 1:44 AM

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

cont...

    System.out.println ("Enter an integer number: ");
    a = input.nextInt();
    System.out.println ("Enter an double number: ");
    b = input.nextDouble();
    double sum = a+b;
    String result = "Your sum is " + sum;
    System.out.println (result);
}

}

Mike Grote
Mar 15, 11:29 AM

The exercise is to get you to understand how to attach a variable into a string, over complicating the output or process only clouds the issue. keep it simple like this...

String answer = "The sum is " + (a+b);
return answer;

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