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

Logical Operators (and Booleans)

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In previous nodes, we showed how you can use if, while and for statements to execute code only when a certain condition is true. This works fine if you just care about checking one condition, but sometimes you will want to run code when either A OR B is true, or only when A AND B are true.

In such cases, you will want to use the logical operators "And" and "Or". In Java they are written as && and ||. For example:

int a = 5;
int b = 7;
boolean aLargish;
boolean bLarger;            //declare a true/false variable

bLarger =  b > a && a > 0;  //true, since both true.
aLargish =  a > b || a > 0; //true, since a>0 and it's an OR statement
boolean both =  aLargish && bLarger; //true again!

In the above code, the results of the first comparison was stored in the boolean variable bLarger. As in the previous node, this could then be used in a conditional statement :

 if(bLarger){
   //do something...
 }

Alternatively, you can put the expression itself in the if-statement without using a variable:

 if(b > a && a > 0){
   //do something...
 }

Note that:

a && b will return true only when BOTH a AND b are true.
a || b will return true when either a AND/OR b are true.

Challenge
As briefly mentioned in About Programming, computers ultimately calculate just by doing simple logical operation like AND and OR on Logic Gates. While these gates are physical parts of a computer, you can simulate them in software. The NAND gate is a logical gate that returns false only when both its inputs are true. In other cases it returns true. You will simulate a NAND gate in this challenge. Try to do this challenge without using any if statements.

Challenge

You will be given two integer inputs, a and b. Simulate a NAND gate by returning a boolean false only if both inputs are equal to 1, and returning true in all other cases.

Please sign in or sign up to submit answers.

Alternatively, you can try out Learneroo before signing up.

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Comments

Carlos Díaz Ramirez
Jan 1, 10:43 PM

inverse way also works 'a==1 && b ==1'
Carlos Díaz Ramirez
Jan 1, 10:43 PM

'a==1 && b ==1' can work too
Learneroo
Jan 1, 10:47 PM

@Carlos, with an overall not !, yes.
pat
May 21, 9:37 AM

What is the code to solve this?
Vignesh Prabhu S
May 30, 8:27 AM

boolean bLarger = ( a==1 && b==1);
return (!bLarger);
this works
Jesse Li
Aug 7, 8:41 PM

can be done in 1 line of code using sums
'return ((a + b) != 2);'
mistermase
Aug 21, 12:46 PM

boolean gates = a!=1 || b!=1;
Ellis Keith
Dec 8, 7:04 AM

The not so advanced one line solution:

return( ! (a == 1 && b ==1));
joeal
Feb 13, 1:56 AM

return a!=1 || b!=1;
flowra
Feb 13, 4:58 PM

also this solution is true :
return !(a==1 && b==1);
Marlon Buella
Apr 19, 11:33 PM

!((a == 1) && (b == 1)) also works!
Stanixlav
Aug 1, 2:38 PM

This might be a slight cheat, but we know that a and b are both integers, meaning there are no fractions in play. Therefore, the following also works: my code

Asadujjaman Shimul

Dec 3, 2:57 AM

import java.util.Scanner;
public class BooleanExm2
{
public static void main(String[] args)
{
int a;
int b;
Scanner input = new Scanner(System.in);

cont...

    System.out.println ("Enter 1 or 0: ");
    a = input.nextInt();
    System.out.println ("Enter 1 or 0: ");
    b = input.nextInt();
    boolean output = a==1 && b==0|| a==0 && b==1;
    System.out.println (output);
}

}

Gary
Feb 18, 2:19 PM

Lol, my too-many-steps solution:

boolean inputa = a > b || a < b;
boolean inputb = a == 0 && b == 0;
boolean both = inputa || inputb;
return both;

rajni dound

Nov 18, 3:58 PM

import java.util.Scanner;
public class Main(){

public static void main(String[] args){
int a= 4;
int b= 1;

boolean collect=(a==1 && b==1);
boolean recollect=(a==1||b==1);
System.out.println("recollect" + recollect);
System.out.println("collect" + collect);
}

}

Trev
Jun 28, 11:08 PM

thanks guys!