QuickRef.ME

Java

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 methord
  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 = "quickref.me";

#Primitive Data Types

Data Type Size Default Range
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 NA
double 8 byte 0.0d NA
char 2 byte \u0000 0 to 65535
boolean 1 bit false true / false

#Strings

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

See: Strings

#Loops

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

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);

#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

#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);

#Conditionals

#Operators

#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);

#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

#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
HashSet Set N N N N One null
LinkedHashSet Set Y N N N One null
TreeSet Set Y Y N 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)
ArrayDeque Deque Y N N Y N
PriorityQueue Queue Y N N 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

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

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

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");
}