Java -

Getting Started

Hello.java

public class Hello {
  // main method
  public static void main(String[] args)
  {
    // Output: Hello, world!
    System.out.println("Hello, world!");
  }
}

Compiling and running

$ javac Hello.java
$ java Hello
Hello, world!

Variables

int num = 5;
float floatNum = 5.99f;
char letter = 'D';
boolean bool = true;
String site = "cheatsheets.zip";

Primitive Data Types

Data Type	Size	Default	Range
`byte`	1 byte	0	-128 ^to^ 127
`short`	2 byte	0	-2^15^ ^to^ 2^15^-1
`int`	4 byte	0	-2^31^ ^to^ 2^31^-1
`long`	8 byte	0	-2^63^ ^to^ 2^63^-1
`float`	4 byte	0.0f	N/A
`double`	8 byte	0.0d	N/A
`char`	2 byte	\u0000	0 ^to^ 65535
`boolean`	N/A	false	true / false
{.show-header}

Strings

String first = "John";
String last = "Doe";
String name = first + " " + last;
System.out.println(name);

See: Strings

Loops

String word = "CheatSheets";
for (char c: word.toCharArray()) {
  System.out.print(c + "-");
}
// Outputs: C-h-e-a-t-S-h-e-e-t-s-

See: Loops

Arrays

char[] chars = new char[10];
chars[0] = 'a'
chars[1] = 'b'

String[] letters = {"A", "B", "C"};
int[] mylist = {100, 200};
boolean[] answers = {true, false};

See: Arrays

Swap

int a = 1;
int b = 2;
System.out.println(a + " " + b); // 1 2

int temp = a;
a = b;
b = temp;
System.out.println(a + " " + b); // 2 1

Type Casting

// Widening
// byte<short<int<long<float<double
int i = 10;
long l = i;               // 10

// Narrowing 
double d = 10.02;
long l = (long)d;         // 10

String.valueOf(10);       // "10"
Integer.parseInt("10");   // 10
Double.parseDouble("10"); // 10.0

Conditionals

int j = 10;

if (j == 10) {
  System.out.println("I get printed");
} else if (j > 10) {
  System.out.println("I don't");
} else {
  System.out.println("I also don't");
}

See: Conditionals

User Input

Scanner in = new Scanner(System.in);
String str = in.nextLine();
System.out.println(str);

int num = in.nextInt();
System.out.println(num);

Java Strings

Basic

String str1 = "value"; 
String str2 = new String("value");
String str3 = String.valueOf(123);

Concatenation

String s = 3 + "str" + 3;     // 3str3
String s = 3 + 3 + "str";     // 6str
String s = "3" + 3 + "str";   // 33str
String s = "3" + "3" + "23";  // 3323
String s = "" + 3 + 3 + "23"; // 3323
String s = 3 + 3 + 23;        // 29

StringBuilder

StringBuilder sb = new StringBuilder(10);

┌───┬───┬───┬───┬───┬───┬───┬───┬───┐
|   |   |   |   |   |   |   |   |   |
└───┴───┴───┴───┴───┴───┴───┴───┴───┘
0   1   2   3   4   5   6   7   8   9

sb.append(“QuickRef”);

┌───┬───┬───┬───┬───┬───┬───┬───┬───┐
| Q | u | i | c | k | R | e | f |   |
└───┴───┴───┴───┴───┴───┴───┴───┴───┘
0   1   2   3   4   5   6   7   8   9

sb.delete(5, 9);

┌───┬───┬───┬───┬───┬───┬───┬───┬───┐
| Q | u | i | c | k |   |   |   |   |
└───┴───┴───┴───┴───┴───┴───┴───┴───┘
0   1   2   3   4   5   6   7   8   9

sb.insert(0, “My “);

┌───┬───┬───┬───┬───┬───┬───┬───┬───┐
| M | y |   | Q | u | i | c | k |   |
└───┴───┴───┴───┴───┴───┴───┴───┴───┘
0   1   2   3   4   5   6   7   8   9

sb.append(”!”);

┌───┬───┬───┬───┬───┬───┬───┬───┬───┐
| M | y |   | Q | u | i | c | k | ! |
└───┴───┴───┴───┴───┴───┴───┴───┴───┘
0   1   2   3   4   5   6   7   8   9

Comparison

String s1 = new String("QuickRef"); 
String s2 = new String("QuickRef"); 

s1 == s2          // false
s1.equals(s2)     // true

"AB".equalsIgnoreCase("ab")  // true

Manipulation

String str = "Abcd";

str.toUpperCase();     // ABCD
str.toLowerCase();     // abcd
str.concat("#");       // Abcd#
str.replace("b", "-"); // A-cd

"  abc ".trim();       // abc
"ab".toCharArray();    // {'a', 'b'}

Information

String str = "abcd";

str.charAt(2);       // c
str.indexOf("a")     // 0
str.indexOf("z")     // -1
str.length();        // 4
str.toString();      // abcd
str.substring(2);    // cd
str.substring(2,3);  // c
str.contains("c");   // true
str.endsWith("d");   // true
str.startsWith("a"); // true
str.isEmpty();       // false

Immutable

String str = "hello";
str.concat("world");

// Outputs: hello
System.out.println(str);

String str = "hello";
String concat = str.concat("world");

// Outputs: helloworld
System.out.println(concat);

Once created cannot be modified, any modification creates a new String

Java Arrays

Declare

int[] a1;
int[] a2 = {1, 2, 3};
int[] a3 = new int[]{1, 2, 3};

int[] a4 = new int[3];
a4[0] = 1;
a4[2] = 2;
a4[3] = 3;

Modify

int[] a = {1, 2, 3};
System.out.println(a[0]); // 1

a[0] = 9;
System.out.println(a[0]); // 9

System.out.println(a.length); // 3

Loop (Read & Modify)

int[] arr = {1, 2, 3};
for (int i=0; i < arr.length; i++) {
    arr[i] = arr[i] * 2;
    System.out.print(arr[i] + " ");
}
// Outputs: 2 4 6

Loop (Read)

String[] arr = {"a", "b", "c"};
for (int a: arr) {
    System.out.print(a + " ");
}
// Outputs: a b c

Multidimensional Arrays

int[][] matrix = { {1, 2, 3}, {4, 5} };

int x = matrix[1][0];  // 4
// [[1, 2, 3], [4, 5]]
Arrays.deepToString(matrix)

for (int i = 0; i < a.length; ++i) {
  for(int j = 0; j < a[i].length; ++j) {
    System.out.println(a[i][j]);
  }
}
// Outputs: 1 2 3 4 5 6 7

Sort

char[] chars = {'b', 'a', 'c'};
Arrays.sort(chars);

// [a, b, c]
Arrays.toString(chars);

Java Conditionals

Operators

+
-
*
/
%
=
++
–
! {.marker-none .cols-4}

==
!=
>
>=
«/a>
<= {.marker-none .cols-4}

&&
||
?:{data-tooltip=“Ternary (shorthand for if-then-else statement)”} {.marker-none .cols-4}

instanceof {.marker-none}

~
«
»
»>
&
^
| {.marker-none .cols-4}

If else

int k = 15;
if (k > 20) {
  System.out.println(1);
} else if (k > 10) {
  System.out.println(2);
} else {
  System.out.println(3);
}

Switch

int month = 3;
String str;
switch (month) {
  case 1:
    str = "January";
    break;
  case 2:
    str = "February";
    break;
  case 3:
    str = "March";
    break;
  default:
    str = "Some other month";
    break;
}

// Outputs: Result March
System.out.println("Result " + str);

Ternary operator

int a = 10;
int b = 20;
int max = (a > b) ? a : b;

// Outputs: 20
System.out.println(max);

Java Loops

For Loop

for (int i = 0; i < 10; i++) {
  System.out.print(i);
}
// Outputs: 0123456789

for (int i = 0,j = 0; i < 3; i++,j--) {
  System.out.print(j + "|" + i + " ");
}
// Outputs: 0|0 -1|1 -2|2

Enhanced For Loop

int[] numbers = {1,2,3,4,5};

for (int number: numbers) {
  System.out.print(number);
}
// Outputs: 12345

Used to loop around array’s or List’s

While Loop

int count = 0;

while (count < 5) {
  System.out.print(count);
  count++;
}
// Outputs: 01234

Do While Loop

int count = 0;

do {
  System.out.print(count);
  count++;
} while (count < 5);
// Outputs: 01234

Continue Statement

for (int i = 0; i < 5; i++) {
  if (i == 3) {
    continue;
  }
  System.out.print(i);
}
// Outputs: 01245

Break Statement

for (int i = 0; i < 5; i++) {
  System.out.print(i);
  if (i == 3) {
    break;
  }
}
// Outputs: 0123

Java Collections Framework

Java Collections

Collection	Interface	Ordered	Sorted	Thread safe	Duplicate	Nullable
ArrayList	List	Y	N	N	Y	Y
Vector	List	Y	N	Y	Y	Y
LinkedList	List, Deque	Y	N	N	Y	Y
CopyOnWriteArrayList	List	Y	N	Y	Y	Y
HashSet	Set	N	N	N	N	One `null`
LinkedHashSet	Set	Y	N	N	N	One `null`
TreeSet	Set	Y	Y	N	N	N
CopyOnWriteArraySet	Set	Y	N	Y	N	One `null`
ConcurrentSkipListSet	Set	Y	Y	Y	N	N
HashMap	Map	N	N	N	N (key)	One `null` (key)
HashTable	Map	N	N	Y	N (key)	N (key)
LinkedHashMap	Map	Y	N	N	N (key)	One `null` (key)
TreeMap	Map	Y	Y	N	N (key)	N (key)
ConcurrentHashMap	Map	N	N	Y	N (key)	N
ConcurrentSkipListMap	Map	Y	Y	Y	N (key)	N
ArrayDeque	Deque	Y	N	N	Y	N
PriorityQueue	Queue	Y	N	N	Y	N
ConcurrentLinkedQueue	Queue	Y	N	Y	Y	N
ConcurrentLinkedDeque	Deque	Y	N	Y	Y	N
ArrayBlockingQueue	Queue	Y	N	Y	Y	N
LinkedBlockingDeque	Deque	Y	N	Y	Y	N
PriorityBlockingQueue	Queue	Y	N	Y	Y	N
{.show-header .left-text}

ArrayList

List<Integer> nums = new ArrayList<>();

// Adding
nums.add(2);
nums.add(5);
nums.add(8);

// Retrieving
System.out.println(nums.get(0));

// Indexed for loop iteration
for (int i = 0; i < nums.size(); i++) {
    System.out.println(nums.get(i));
}

nums.remove(nums.size() - 1);
nums.remove(0); // VERY slow

for (Integer value : nums) {
    System.out.println(value);
}

HashMap

Map<Integer, String> m = new HashMap<>();
m.put(5, "Five");
m.put(8, "Eight");
m.put(6, "Six");
m.put(4, "Four");
m.put(2, "Two");

// Retrieving
System.out.println(m.get(6));

// Lambda forEach
m.forEach((key, value) -> {
    String msg = key + ": " + value;
    System.out.println(msg);
});

HashSet

Set<String> set = new HashSet<>();
if (set.isEmpty()) {
    System.out.println("Empty!");
}

set.add("dog");
set.add("cat");
set.add("mouse");
set.add("snake");
set.add("bear");

if (set.contains("cat")) {
    System.out.println("Contains cat");
}

set.remove("cat");
for (String element : set) {
    System.out.println(element);
}

ArrayDeque

Deque<String> a = new ArrayDeque<>();

// Using add()
a.add("Dog");

// Using addFirst()
a.addFirst("Cat");

// Using addLast()
a.addLast("Horse");

// [Cat, Dog, Horse]
System.out.println(a);

// Access element
System.out.println(a.peek());

// Remove element
System.out.println(a.pop());

Misc

Access Modifiers

Modifier	Class	Package	Subclass	World
public	Y	Y	Y	Y
protected	Y	Y	Y	N
no modifier	Y	Y	N	N
private	Y	N	N	N
{.show-header .left-text}

Regular expressions

String text = "I am learning Java";
// Removing All Whitespace
text.replaceAll("\\s+", "");

// Splitting a String
text.split("\\|");
text.split(Pattern.quote("|"));

See: Regex in java

Comment

// I am a single line comment!
 
/*
And I am a 
multi-line comment!
*/

/**
 * This  
 * is  
 * documentation  
 * comment 
 */

Keywords

abstract
continue
for
new
switch
assert
default
goto
package
synchronized
boolean
do
if
private
this
break
double
implements
protected
throw
byte
else
import
public
throws
case
enum
instanceof
return
transient
catch
extends
int
short
try
char
final
interface
static
void
class
finally
long
strictfp
volatile
const
float
native
super
while {.marker-none .cols-6}

Math methods

Method	Description
`Math.max(a,b)`	Maximum of a and b
`Math.min(a,b)`	Minimum of a and b
`Math.abs(a)`	Absolute value a
`Math.sqrt(a)`	Square-root of a
`Math.pow(a,b)`	Power of b
`Math.round(a)`	Closest integer
`Math.sin(ang)`	Sine of ang
`Math.cos(ang)`	Cosine of ang
`Math.tan(ang)`	Tangent of ang
`Math.asin(ang)`	Inverse sine of ang
`Math.log(a)`	Natural logarithm of a
`Math.toDegrees(rad)`	Angle rad in degrees
`Math.toRadians(deg)`	Angle deg in radians

Try/Catch/Finally

try {
  // something
} catch (Exception e) {
  e.printStackTrace();
} finally {
  System.out.println("always printed");
}

object-oriented class