Java Quick Dev Reference

This cheat sheet is a crash course for Java beginners and help review the basic syntax of the Java language.

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 = "ref.softcrony.com";

Primitive Data Types

byte — 1 byte — 0 — -128 to 127

short — 2 byte — 0 — -215 to 215-1

int — 4 byte — 0 — -231 to 231-1

long — 8 byte — 0 — -263 to 263-1

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

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

See:

Loops

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

See:

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:

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:

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("Ref.Softcrony");

┌───┬───┬───┬───┬───┬───┬───┬───┬───┐
| 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("Ref.Softcrony"); 
String s2 = new String("Ref.Softcrony"); 

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

+ - * / % = ++ -- !

== != > >= < <=

&& ||

instanceof

~ << >> >>> & ^ |

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

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

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

public — Y — Y — Y — Y

protected — Y — Y — Y — N

no modifier — Y — Y — N — N

private — Y — N — N — N

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

Math methods

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");
}