java.md

title	Java
date	2021-03-10 11:50:01 -0800
icon	icon-java
background	bg-red-700
tags	object-oriented class
categories	Programming
intro	This cheat sheet is a crash course for Java beginners and help review the basic syntax of the Java language.
plugins	tooltip

Getting started {.cols-3}

Hello.java {.row-span-2}

publicclassHello {
// main methordpublicstaticvoidmain(String[] args)
{
// Output: Hello, world!System.out.println("Hello, world!");
}
}

Compiling and running

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

Variables

intnum = 5;
floatfloatNum = 5.99f;
charletter = 'D';
booleanbool = true;
Stringsite = "quickref.me";

Primitive Data Types {.row-span-2}

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

Stringfirst = "John";
Stringlast = "Doe";
Stringname = first + " " + last;
System.out.println(name);

See: Strings

Loops

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

See: Loops

Arrays

char[] chars = newchar[10];
chars[0] = 'a'chars[1] = 'b'String[] letters = {"A", "B", "C"};
int[] mylist = {100, 200};
boolean[] answers = {true, false};

See: Arrays

Swap

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

Type Casting

// Widening// byte<short<int<long<float<doubleinti = 10;
longl = i; // 10// Narrowing doubled = 10.02;
longl = (long)d; // 10String.valueOf(10); // "10"Integer.parseInt("10"); // 10Double.parseDouble("10"); // 10.0

Conditionals

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

See: Conditionals

User Input

Scannerin = newScanner(System.in);
Stringstr = in.nextLine();
System.out.println(str);
intnum = in.nextInt();
System.out.println(num);

Java Strings {.cols-3}

Basic

Stringstr1 = "value"; Stringstr2 = newString("value");
Stringstr3 = String.valueOf(123);

Concatenation

Strings = 3 + "str" + 3; // 3str3Strings = 3 + 3 + "str"; // 6strStrings = "3" + 3 + "str"; // 33strStrings = "3" + "3" + "23"; // 3323Strings = "" + 3 + 3 + "23"; // 3323Strings = 3 + 3 + 23; // 29

StringBuilder {.row-span-3}

StringBuilder sb = new StringBuilder(10);

┌───┬───┬───┬───┬───┬───┬───┬───┬───┐
| | | | | | | | | |
└───┴───┴───┴───┴───┴───┴───┴───┴───┘
0123456789

sb.append("QuickRef");

┌───┬───┬───┬───┬───┬───┬───┬───┬───┐
| Q | u | i | c | k | R | e | f | |
└───┴───┴───┴───┴───┴───┴───┴───┴───┘
0123456789

sb.delete(5, 9);

┌───┬───┬───┬───┬───┬───┬───┬───┬───┐
| Q | u | i | c | k | | | | |
└───┴───┴───┴───┴───┴───┴───┴───┴───┘
0123456789

sb.insert(0, "My ");

┌───┬───┬───┬───┬───┬───┬───┬───┬───┐
| M | y | | Q | u | i | c | k | |
└───┴───┴───┴───┴───┴───┴───┴───┴───┘
0123456789

sb.append("!");

┌───┬───┬───┬───┬───┬───┬───┬───┬───┐
| M | y | | Q | u | i | c | k | ! |
└───┴───┴───┴───┴───┴───┴───┴───┴───┘
0123456789

Comparison

Strings1 = newString("QuickRef"); Strings2 = newString("QuickRef"); s1 == s2// falses1.equals(s2) // true"AB".equalsIgnoreCase("ab") // true

Manipulation

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

Information

Stringstr = "abcd";
str.charAt(2); // cstr.indexOf("a") // 0str.indexOf("z") // -1str.length(); // 4str.toString(); // abcdstr.substring(2); // cdstr.substring(2,3); // cstr.contains("c"); // truestr.endsWith("d"); // truestr.startsWith("a"); // truestr.isEmpty(); // false

Immutable

Stringstr = "hello";
str.concat("world");
// Outputs: helloSystem.out.println(str);

---

Stringstr = "hello";
Stringconcat = str.concat("world");
// Outputs: helloworldSystem.out.println(concat);

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

Java Arrays {.cols-3}

Declare

int[] a1;
int[] a2 = {1, 2, 3};
int[] a3 = newint[]{1, 2, 3};
int[] a4 = newint[3];
a4[0] = 1;
a4[2] = 2;
a4[3] = 3;

Modify

int[] a = {1, 2, 3};
System.out.println(a[0]); // 1a[0] = 9;
System.out.println(a[0]); // 9System.out.println(a.length); // 3

Loop (Read & Modify)

int[] arr = {1, 2, 3};
for (inti=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 (inta: arr) {
System.out.print(a + " ");
}
// Outputs: a b c 

Multidimensional Arrays

int[][] matrix = { {1, 2, 3}, {4, 5} };
intx = matrix[1][0]; // 4// [[1, 2, 3], [4, 5]]Arrays.deepToString(matrix)
for (inti = 0; i < a.length; ++i) {
for(intj = 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 {.cols-3}

Operators {.row-span-2}

• +
• -
• *
• /
• %
• =
• ++
• --
• ! {.style-none .cols-4}

---

• ==
• !=
• >
• >=
• <
• <= {.style-none .cols-4}

---

• &&
• ||
• ?:{data-tooltip="Ternary (shorthand for if-then-else statement)"} {.style-none .cols-4}

---

• instanceof {.style-none}

---

• ~
• <<
• >>
• >>>
• &
• ^
• | {.style-none .cols-4}

If else

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

Switch {.row-span-2}

intmonth = 3;
Stringstr;
switch (month) {
case1:
str = "January";
break;
case2:
str = "February";
break;
case3:
str = "March";
break;
default:
str = "Some other month";
break;
}
// Outputs: Result MarchSystem.out.println("Result " + str);

Ternary operator

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

Java Loops {.cols-3}

For Loop

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

---

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

Used to loop around array's or List's

While Loop

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

Do While Loop

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

Continue Statement

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

Break Statement

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

Java Collections Framework {.cols-3}

Java Collections {.col-span-2}

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
{.show-header .left-text}

ArrayList

List<Integer> nums = newArrayList<>();
// Addingnums.add(2);
nums.add(5);
nums.add(8);
// RetrievingSystem.out.println(nums.get(0));
// Indexed for loop iterationfor (inti = 0; i < nums.size(); i++) {
System.out.println(nums.get(i));
}
nums.remove(nums.size() - 1);
nums.remove(0); // VERY slowfor (Integervalue : nums) {
System.out.println(value);
}

HashMap

Map<Integer, String> m = newHashMap<>();
m.put(5, "Five");
m.put(8, "Eight");
m.put(6, "Six");
m.put(4, "Four");
m.put(2, "Two");
// RetrievingSystem.out.println(m.get(6));
// Lambda forEachm.forEach((key, value) -> {
Stringmsg = key + ": " + value;
System.out.println(msg);
});

HashSet

Set<String> set = newHashSet<>();
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 (Stringelement : set) {
System.out.println(element);
}

ArrayDeque

Deque<String> a = newArrayDeque<>();
// Using add()a.add("Dog");
// Using addFirst()a.addFirst("Cat");
// Using addLast()a.addLast("Horse");
// [Cat, Dog, Horse]System.out.println(a);
// Access elementSystem.out.println(a.peek());
// Remove elementSystem.out.println(a.pop());

Misc {.cols-3}

Access Modifiers {.col-span-2}

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

Stringtext = "I am learning Java";
// Removing All Whitespacetext.replaceAll("\\s+", "");
// Splitting a Stringtext.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 {.col-span-2}

• 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 {.style-none .cols-7}

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 (Exceptione) {
e.printStackTrace();
} finally {
System.out.println("always printed");
}