Java | Research Asia Labs

Getting Started

Hello.java

java

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

Compiling and running

shell

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

Variables

java

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

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

Strings

java

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

See: Strings

Loops

java

String word = "QuickRef";
for (char c: word.toCharArray()) {
  System.out.print(c + "-");
}
// Outputs: Q-u-i-c-k-R-e-f-

See: Loops

Arrays

java

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

java

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

java

// 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

java

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

java

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

java

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

Concatenation

java

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);

java

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

sb.append("QuickRef");

java

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

sb.delete(5, 9);

java

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

sb.insert(0, "My ");

java

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

sb.append("!");

java

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

Comparison

java

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

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

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

Manipulation

java

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

java

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

java

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

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

java

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

java

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

java

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)

java

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)

java

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

Multidimensional Arrays

java

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

java

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

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

Java Conditionals

Operators

instanceof

If else

java

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

Switch

java

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

java

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

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

Java Loops

For Loop

java

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

java

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

java

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

java

int count = 0;

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

Do While Loop

java

int count = 0;

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

Continue Statement

java

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

Break Statement

java

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

ArrayList

java

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

java

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

java

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

java

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

Regular expressions

java

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

java

// 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

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

java

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

Getting Started ​

Hello.java ​

Variables ​

Primitive Data Types ​

Strings ​

Loops ​

Arrays ​

Swap ​

Type Casting ​

Conditionals ​

User Input ​

Java Strings ​

Basic ​

Concatenation ​

StringBuilder ​

Comparison ​

Manipulation ​

Information ​

Immutable ​

Java Arrays ​

Declare ​

Modify ​

Loop (Read & Modify) ​

Loop (Read) ​

Multidimensional Arrays ​

Sort ​

Java Conditionals ​

Operators ​

If else ​

Switch ​

Ternary operator ​

Java Loops ​

For Loop ​

Enhanced For Loop ​

While Loop ​

Do While Loop ​

Continue Statement ​

Break Statement ​

Java Collections Framework ​

Java Collections ​

ArrayList ​

HashMap ​

HashSet ​

ArrayDeque ​

Misc ​

Access Modifiers ​

Regular expressions ​

Comment ​

Keywords ​

Math methods ​

Try/Catch/Finally ​

Getting Started

Hello.java

Variables

Primitive Data Types

Strings

Loops

Arrays

Swap

Type Casting

Conditionals

User Input

Java Strings

Basic

Concatenation

StringBuilder

Comparison

Manipulation

Information

Immutable

Java Arrays

Declare

Modify

Loop (Read & Modify)

Loop (Read)

Multidimensional Arrays

Sort

Java Conditionals

Operators

If else

Switch

Ternary operator

Java Loops

For Loop

Enhanced For Loop

While Loop

Do While Loop

Continue Statement

Break Statement

Java Collections Framework

Java Collections

ArrayList

HashMap

HashSet

ArrayDeque

Misc

Access Modifiers

Regular expressions

Comment

Keywords

Math methods

Try/Catch/Finally