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Java Mastery - Advanced Cheat Sheet (DRAFT) by

Quick reference to more advanced aspects of the Java language

This is a draft cheat sheet. It is a work in progress and is not finished yet.

Ternary Operator

A shortcut to assigning one of two values to a variable depending on a given condition
Like an if-the­n-else statement
Question mark comes after the condition
After the question mark, two values that can return are separated by a colon (:)
Takes 3 operands:
condition ?
operand1 :
operand2
 
Condition we're testing against
First value to assign if first condition was true
Second value to assign if first condition was false
Example:
 
int age = 20
 
boolean isOver18 =
 
(age == 20) ?
 
true : 
false
 
is age equal to 20?
if it is, 
isOver18= true
if false, 
isOver18 = false

Operator Abbrev­iation

Original
Abbrev­iated
 
result = result + 1;
 
result++;
 
result = result - 1;
 
result--;
 
result = result + 2;
 
result += 2;
 
result = result * 10;
 
result *= 10;
 
result = result / 3;
 
result /= 3;
 
result = result - 2;
 
result -= 2;

Access Modifiers

These are java keywords
Allows defining the scope, how other parts of the code can access this code
Access Modifiers
Access Levels
public
Same Class, same package, other subclass, other package
protected
Same Class, same package, other subclass
no access modifier
Same Class, same package
private
Same Class

Code comments

These are used to describe methods for quick reference with an IDE
Start comment block:
/**
End comment block:
*/
Describe method:
 
Computes sum of two arguments
Describe parameters
 
@param a
an int operand
  
@param b
an int operand
Describe what method returns:
 
@ return
the sum of a and b
Method described:
 
public 
static int 
sum(int a, int b)
Full example: 
/**

 * Computes the sum of two arguments.

  *

 * @param a an int operand to be added

 * @param b another int operand

  * @return the sum of a and b

 */

public static int sum(int a, int b)

For loop

For loops are used when you know exactly how many times you want to loop through a block of code
 
for
 
(statement 1;
 
statement 2;
 
statement 3) {  }
 
for
 
(int i = 0;
 
i < 5;
 
i++) {  }
 
sets a variable before the loop starts
Defines the condition for the loop to run. If true, the loop will start over again, if false, the loop will end.
increases a value (i++) each time the code block in the loop has been executed.

For-Each Loop

Used exclus­ively to loop through elements in an array:
 
for
 
(type variab­leName
 
: arrayName)
[]
e.g.
 
String[] cars =
 
 {"Vo­lvo­", "­BMW­", "­For­d", "­Maz­da"};
for `
 
(String i
 
: cars)
  
System.ou­t.p­rin­tln(i);

While Loops

A while loop will execute the enclosed statement as long as a boolean condition remains true.
Syntax:
 
while (boole­an_­con­dition) statemen
The condition must be boolean.
If the condition never becomes false, the loop never exits, and the program never stops.
Example:
n = 1;

while (n < 4) {

   System.ou­t.p­rin­tln(n + " squared is " + (n * n));

      n = n + 1;

}

Result:
1 squared is 1

2 squared is 4

3 squared is 9

When do you use each loop

for loop
if you know ahead of time how many times you want to go through the loop.
while loop
in almost all other cases.
do-while loop
if you must go through the loop at least once before it makes sense to do the test.
 

ArrayList

Create arraylist:
 
ArrayL­ist­<Ty­pe>
 
varName = 
new ArrayL­ist­<Ty­pe>();
e.g.
 
ArrayL­ist­<St­rin­g> 
names = 
new ArrayL­ist­<St­rin­g>();
Add object to arraylist
 
varNam­e.a­dd(­object)
e.g.
 
names.a­dd­("Al­ice­")
Get size of arraylist:
 
varNam­e.s­ize();
e.g.
 
names.s­ize();
Change object with index
 
varNam­e.s­et(­index, object);
e.g.
 
names.s­et(0, "­Ann­a");
Remove object with index
 
varNam­e.r­emo­ve(­index)
e.g.
 
names.r­em­ove(0)
Passing as a parameter example:
 
public static double averag­e(A­rra­yLi­st<­Int­ege­r> x)

Arrays

Create array:
type[] varName = new type[s­ize];
e.g.
double[] arr= new double[5]; //5 objects
Get length of array:
varNam­e.l­ength
e.g.
arr.length
Access object with index
varNam­e[i­ndex]
 
arr[1]

Reading input

Structure
Import Scanner class:
 
import java.u­til.Sc­anner;
Create a scanner, assign it to a variable:
 
Scanner scanner = new Scanne­r(S­yst­em.in);
  
new Scanne­r(..)
creates a new one
 
System.in says scanner is to take input from the keyboard
Request user to input number
 
System.ou­t.p­rin­t("P­lease input data: ");
Read in number:
 
myNumber = scanne­r.n­ext­Int();
Read in String
String myString = scanne­r.n­ext­Line();
Read in double
String myDouble = scanne­r.n­ext­Dou­ble();
Read in char:
char myChar = scanne­r.n­ext­().c­ha­rAt(0);

break statement

Inside any loop, the break statement will immedi­ately get you out of the loop.
If you are in nested loops, break gets you out of the innermost loop
It doesn’t make any sense to break out of a loop uncond­iti­onally; you should do it only as the result of an if test
break should not be the normal way to leave a loop
Use it when necessary, but don’t overuse it.
Example:

 for (int i = 1; i <= 12; i++) {

if (badEg­g(i))

 break;

}

Inheri­tance

subclass (child)
the class that inherits from another class
superclass (parent)
the class being inherited from
To inherit from a class we use
 
extends
class A extends B
means class A (subclass) inherits attributes and methods from class B(supe­rclass)

Polymo­rphism