Common Code Snippets

!

!true       // false

!false      // true

!!true      // true

!!!true     // false

!=

2 != 2;     // false

3 != 4;     // true

&&

true && false; // false

true && true; // true

<

2 < 5; // true

2 > 5; // false

2 > 2; // false

2 < 2; // false

<=

2 <= 2;     // true

3 <= 2;     // false

2 <= 3;     // true

==

2 == 2;     // true

2 == 4;     // false

>

2 > 1;      // true

2 > 4;      // false

>=

5 >= 5;       // true

10 >= 5;      // true

5 >= 10;      // false

Absolute value

Math.abs(-16);      // returns 16

Math.abs(98.8);     // returns 98.8

Addition

2 + 2;              // 4

"Hi" + " there";   // "Hi there"

alist.add("Add me");

ArrayList ourList = new ArrayList();

ourList.add("Hello");

alist.clear();

ArrayList ourList = new ArrayList();

ourList.add("Hello");

ourList.add("Allo");

ourList.add("Hi there!");

ourList.clear();

alist.get(0);

ArrayList ourList = new ArrayList();

ourList.add("Hello");

ourList.get(0);         // returns "Hello"

alist.insert(1, "Insert me");

ArrayList ourList = new ArrayList();

ourList.add("Hello");

ourList.insert(0, "Goodbye");

alist.isEmpty();

ArrayList ourList = new ArrayList();

ourList.isEmpty();              // returns true

ourList.add("Hello");

ourList.isEmpty();              // returns false

ourList.clear();

ourList.isEmpty();              // returns true

alist.remove(0);

ArrayList ourList = new ArrayList();

ourList.add("Hello");

ourList.add("Goodbye");

ourList.remove(1); //Removes and returns the "Goodbye" String

alist.set(0, "Set me");

ArrayList ourList = new ArrayList();

ourList.set(0, "Hello");

alist.size();

ArrayList ourList = new ArrayList();

ourList.size(); //Returns 0

ourList.add("Hello");

ourList.size(); //Returns 1

arraylist

ArrayList ourList = new ArrayList();

boolean

true

false

2 == 2                  // evaluates to true

2 != 2                  // evaluates to false

4 != 4.90               // evaluates to true

boolean willRain = false;

boolean[]

boolean[] options = new boolean[2];

options[0] = true;

options[1] = false;

// Alternatively:

boolean[] options = {true, false};

// And then ...

System.out.println(options[0]);         // prints `true`

System.out.println(options[1]);         // prints `false`

casting

(int) 11.0           // = 11

(int) 11.84          // = 11

(double) 11          // = 11.0

(double) 11/2        // = 5.5

char

char myCharacter = '!';

charAt (Strings)

String name = "Mike";

name.charAt(0);   // = 'M'

name.charAt(1);   // = 'i'

name.charAt(2);   // = 'k'

name.charAt(3);   // = 'e'

class

class Book

{

String title;

String author;

Book(String titleFromuser, authorFromuser)

{

     this.title = titleFromuser;

     this.author = authorFromuser;

}

public static void main(String[] args)

{

//Make a book!

     Book b = new Book("Alice in Wonderland", "Lewis Carroll");

}

}

Concatenate

"Hello" + "there!";         // "Hellothere!"

"Hello" + " there!";        // "Hello there!"

constructor

class Book

{

    String title;

    String author;

    public Book()

    {

        this.title = "Alice in Wonderland";

        this.author = "Lewis Carroll";

    }

}

Decrementor

// These code snippets are equivalent

n--;

n -= 1;

n = n - 1;

double

-4.5

9.4949

double d = 500.0;

double[]

double[] coins = new double[4];

coins[0] = 0.01;

coins[1] = 0.05;

coins[2] = 0.10;

coins[3] = 0.25;

// Alternatively:

double[] coins = {0.01, 0.05, 0.10, 0.25};

// And then ...

System.out.println(coins[0]);         // prints `0.01`

System.out.println(coins[1]);         // prints `0.05`

e, the base of natural logarithms

Math.E;   // 2.71828

final

final double PI = 3.14159;

PI = 4.5 // illegal!

Find max of two numbers

Math.max(-2.01,-5.4);   // returns -2.01

Math.max(4, 2);         // returns 4

Find min of two numbers

Math.min(-2.01,-5.4);   // returns -5.4

Math.min(4, 2);         // returns 2

function - static

Math.abs(-4);

StringUtils.isEmpty(String text);

String.valueOf(boolean b);

for loop

for(int i = 0; i < 5; i++)

{

System.out.println("i is " + i);

}

/* Prints:

   i is 0

   i is 1

   i is 2

   i is 3

   i is 4

*/

function

int addTwo(int x)

{

return x + 2;

}

int y = addTwo(4);

System.out.println(y)       // Prints 6

If statement

if(2 == 2)
{

System.out.println("I'll print!");

}

else

{

System.out.println("I won't print!");

}

if(2 != 2)

{

System.out.println("I won't print!");

}

else

{

System.out.println("I'll print!");

}

Incrementor

// These code snippets are equivalent

n++;

n += 1;

n = n + 1;

            


indexOf (String)

String str = "Hello world!";


str.indexOf("World");                   // -1

str.indexOf("world");                   // 7

str.indexOf("world", 10)                // -1

str.indexOf("hi");                      // -1

str.indexOf("Hello world!");            // 0

str.indexOf("!", 10)                    // 12

            


instance

int bills[] = {1, 2, 5, 10, 20, 50, 100};

bills.length;     // returns 7

            


Instance variables

class Book{

    String title;

    String author;


    Book(String titleFromUser, String authorFromUser){

        this.title = titleFromUser;

        this.author = authorFromUser;

    }

}

            


int

49

0

-1045

int i = 3;

            


int[]

int[] grades = new int[3];

grades[0] = 97;

grades[1] = 84;

grades[2] = 92;


// Alternatively:

int[] grades = {97, 84, 92};


// And then ...

System.out.println(grades[0]);         // prints `97`

System.out.println(grades[1]);         // prints `84`


            


Integer division

4 / 2; // = 2

5 / 2; // = 2

6 / 2; // = 4

7 / 2; // = 3

            


lowercase

"HI THERE".toLowerCase();           // "hi there"

"Hello".toLowerCase();              // "hello"

            


main function

public static void main(String[] args){

//Your program here

}

            


Modulus

0 % 3;      // returns 0

1 % 3;      // returns 1

2 % 3;      // returns 2

3 % 3;      // returns 0

4 % 3;      // returns 1

5 % 3;      // returns 2

6 % 3;      // returns 0

7 % 3;      // returns 1

8 % 3;      // returns 2

9 % 3;      // returns 0

10 % 3;      // returns 1

            


Multiplication

2 * 2;              // returns 4

10.0  * 0.25;       // returns 2.5

            


New line

String s = "First line!\nSecond line!"

System.out.println(s);


First line!

Second line!

            


Next random double

Random gen = new Random();

int d = gen.nextDouble();


// d will be a double in the range 0 <= d < 1

            


Next random integer

Random gen = new Random();

int chosen = gen.nextInt(5);

// chosen will be 0, 1, 2, 3 or 4


Random dice = new Random();

int roll = dice.nextInt(6) + 1;

// roll is a whole number between 1 and 6, inclusive.

            


null

String empty;

empty == null;              // returns true


String blank = "";

blank == null;              // returns false


Object alsoEmpty;

alsoEmpty == null;          // returns true

            


object

class Book{

String title;

String author;


Book(String titleFromuser, authorFromuser){

this.title = titleFromuser;

this.author = authorFromuser;

}


public static void main(String[] args){

//Make a book - that's your object

Book b = new Book("Alice in Wonderland", "Lewis Carroll");

}

}

            


Parentheses

(5 + 5) / 2.0;      // returns 10.0

5 + 5/2.0;          // returns 7.5

            


pi, the ratio of a circle's circumfrence to its diameter

Math.PI;   // 3.141592653589793

            


Powers

Math.pow(2, 4);   // returns 16

            


Prefix incrementor

int x = 1; 

int y = ++x;


System.out.println(y);          // prints 2

System.out.println(x);          // prints 2


// Versus ...

int x = 1;

int y = x++;


System.out.println(y);          // prints 1

System.out.println(x);          // prints 2



            


print

System.out.println("Yo!");              // prints "Yo!"

System.out.println(1 + 3);              // prints 4

System.out.println(4 == 2 + 2);         // prints true

            


replace (Strings)

"Hello".replace("ll", "mm")         // "Hemmo"

            


return

return;     // ends the method

return 5;   // ends the method, sends back the value 5

return x;   // ends method, sends back the variable x

            


Round a number

Math.round(4.222);    // returns 4

Math.round(-2.444);   // returns -2

            


Round a number down

Math.floor(4.722);    // returns 4

Math.floor(-2.444);   // returns -3

            


Round a number up

Math.ceil(4.222);    // returns 5

Math.ceil(-2.444);   // returns -2

            


Square roots

Math.sqrt(16);   // returns 4

Math.sqrt(66);   // returns 6

Math.sqrt(81);   // returns 9

            


String

String company = "Google";

String state = new String("California");

            


String comparisons

"Jessica".compareTo("Jessica");

0


"Jessica".compareTo("jessica");

-32


"jessica".compareTo("Jessica");

32


"Amy".compareTo("Jessica");

-9


"Amy".compareTo("jessica");

-41


"Stephanie".compareTo("Jessica");

9


"Stephanie".compareTo("jessica");

-23

            


String length

"Measure me".length(); // 10

"Hello".lenght();       // 5

            


String[]

String[] smalltalk = new String[10];

smalltalk[0] = "Hi!";

smalltalk[1] = "How's it going?";

smalltalk[2] = "Have a good day.";


// Alternatively:

String[] smalltalk = {"Hi!", "How's it going?", "Have a good day."};


// And then ...

System.out.println(smalltalk[0]);         // prints `Hi!`

System.out.println(smalltalk[1]);         // prints `How's it going?`


            


substring

"Hello".substring(0);               // "Hello"

"Hello".substring(1);               // "ello"

"Hello".substring(2);               // "llo"

"Hello".substring(3);               // "lo"

"Hello".substring(4);               // "o"

"Hello".substring(5);               // ""

"Hello".substring(6);               // StringIndexOutOfBoundsError



"Hello".substring(0, 5);            // "Hello"

"Hello".substring(0, 4);            // "Hell"

"Hello".substring(0, 3);            // "Hel"

"Hello".substring(0, 2);            // "He"

"Hello".substring(0, 1);            // "H"

"Hello".substring(0, 0);            // ""

            


Subtraction

2 - 2;          // returns 0

10.5 - 0.25;   // returns 10.25

            


uppercase

"hi there".toUpperCase();           // "HI THERE"

"Hello".toUpperCase();              // "HELLO"

            


value

4

"Hello"

false


            


variable

x is the variable in int x = 2; 2 is the value.

            


Variable scope

//The variable 'even' is scoped to the 'isEven' function

boolean isEven(int num){

boolean even = num % 2 == 0;

return even;

}

            


while loop

/* Prints

 Spent another $25!

 Spent another $25!

 Spent another $25!

 Spent another $25!

 No money left!

 */

int wallet = 100;

while(wallet > 0){

    wallet = wallet - 25;

    System.out.println("Spent another $25!");

}

System.out.println("No money left!");



/* Prints

 Spent another $25! Now my wallet has $75

 Spent another $25! Now my wallet has $50

 Spent another $25! Now my wallet has $25

 Spent another $25! Now my wallet has $0

 No money left!

 */

int wallet = 100;

while(wallet > 0){

    wallet = wallet - 25;

    System.out.println("Spent another $25! Now my wallet has $" + wallet);

}

System.out.println("No money left!");


/* Prints

 My wallet has $100

 Spent another $25! 

 My wallet has $75

 Spent another $25! 

 My wallet has $50

 Spent another $25!

 My wallet has $25

 Spent another $25!

 No money left!

 */

int wallet = 100;

while(wallet > 0){

    System.out.println("My wallet has $" + wallet);

    wallet = wallet - 25;

    System.out.println("Spent another $25!");

}

System.out.println("No money left!");

            


||

true || true;        // true

true || false;       // true

false || true;       // true

false || false;      // false

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