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Java

Java Cheat Sheet - 快速参考指南，收录常用语法、命令与实践。

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Getting Started

Hello.java

JAVA

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

Compiling and running

BASH

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

Variables

JAVA

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

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

{.show-header}

Strings

JAVA

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

See: Strings

Loops

JAVA

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String word = "CheatSheets";
for (char c: word.toCharArray()) {
  System.out.print(c + "-");
}
// Outputs: C-h-e-a-t-S-h-e-e-t-s-

See: Loops

Arrays

JAVA

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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

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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

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// 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

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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

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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

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

Concatenation

JAVA

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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;        // Incompatible types

StringBuilder

StringBuilder sb = new StringBuilder(10);

JAVA

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

sb.append("QuickRef");

JAVA

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

sb.delete(5, 9);

JAVA

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

sb.insert(0, "My ");

JAVA

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

sb.append("!");

JAVA

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

Comparison

JAVA

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String s1 = new String("cheatsheets.zip");
String s2 = new String("cheatsheets.zip");

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

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

Manipulation

JAVA

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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

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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

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

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

JAVA

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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

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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

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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

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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

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

Multidimensional Arrays

JAVA

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int[][] matrix = { {1, 2, 3}, {4, 5} };

int x = matrix[1][0];  // 4
// [[1, 2, 3], [4, 5]]
Arrays.deepToString(matrix);

int[][] a = 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

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

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

Java Conditionals

Operators

<a data-tooltip="Additive operator (also used for String concatenation)">+</a>
<a data-tooltip="Subtraction operator">-</a>
<a data-tooltip="Multiplication operator">*</a>
<a data-tooltip="Division operator">/</a>
<a data-tooltip="Remainder operator">%</a>
<a data-tooltip="Simple assignment operator">=</a>
<a data-tooltip="Increment operator; increments a value by 1">++</a>
<a data-tooltip="Decrement operator; decrements a value by 1">--</a>
<a data-tooltip="Logical complement operator; inverts the value of a boolean">!</a>

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<a data-tooltip="Equal to">==</a>
<a data-tooltip="Not equal to">!=</a>
<a data-tooltip="Greater than">></a>
<a data-tooltip="Greater than or equal to">>=</a>
<a data-tooltip="Less than"><</a>
<a data-tooltip="Less than or equal to"><=</a>

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<a data-tooltip="Conditional-AND">&&</a>
<a data-tooltip="Conditional-OR">||</a>
?:{data-tooltip="Ternary (shorthand for if-then-else statement)"}

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<a data-tooltip="Compares an object to a specified type">instanceof</a>

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<a data-tooltip="Unary bitwise complement">~</a>
<a data-tooltip="Signed left shift"><<</a>
<a data-tooltip="Signed right shift">>></a>
<a data-tooltip="Unsigned right shift">>>></a>
<a data-tooltip="Bitwise AND">&</a>
<a data-tooltip="Bitwise exclusive OR">^</a>
<a data-tooltip="Bitwise inclusive OR">|</a>

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If else

JAVA

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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

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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

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

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

Java Loops

For Loop

JAVA

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

JAVA

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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

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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

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int count = 0;

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

Do While Loop

JAVA

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int count = 0;

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

Continue Statement

JAVA

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for (int i = 0; i < 5; i++) {
  if (i == 3) {
    continue;
  }
  System.out.print(i);
}
// Outputs: 0124

Break Statement

JAVA

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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

{.show-header .left-text}

ArrayList

JAVA

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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

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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

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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

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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

{.show-header .left-text}

Regular expressions

JAVA

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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

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// 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

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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

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

Java

Getting Started

Java Strings

Java Arrays

Java Conditionals

Java Loops

Java Collections Framework

Misc

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