Java Cheat Sheet

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

• 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

// I am a single line comment!

/\*
And I am a 
multi-line comment!
\*/
/\*\*
 \* This 
 \* is 
 \* documentation 
 \* comment 
 \*/

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

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());

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);
}

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);
});

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);
}

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

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

int count = 0;
do {
  System.out.print(count);
  count++;
} while (count < 5);
// Outputs: 01234

int count = 0;
while (count < 5) {
  System.out.print(count);
  count++;
}
// Outputs: 01234

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

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

int a = 10;
int b = 20;
int max = (a > b) ? a : b;
// Outputs: 20
System.out.println(max);

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);

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

• +
• -
• *
• /
• %
• =
• ++
• --

* !

• ==
• !=
• >
• >=
• <

* <=

• &&
• ||

* ?:

* instanceof

• ~
• <<
• >>
• >>>
• &
• ^
• |

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

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 

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

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

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

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;

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

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

String str = "Abcd";
str.toUpperCase();     // ABCD
str.toLowerCase();     // abcd
str.concat("#");       // Abcd#
str.replace("b", "-"); // A-cd
" abc ".trim();       // abc
"ab".toCharArray();    // {'a', 'b'}

String s1 = new String("QuickRef"); 
String s2 = new String("QuickRef"); 
s1 == s2          // false
s1.equals(s2)     // true
"AB".equalsIgnoreCase("ab")  // true

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

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

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

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

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

// 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

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

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

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

See: Loops

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

See: Strings

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

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!