Java cheatsheet

安装 Chocolatey Chocolatey 安装

$choco install jdk8

安装 HomeBrew HomeBrew 安装

$brew update
$brew cask install java

// 单行注释
 
/*
多行
注释
*/

/**
 * 文档注释
 */

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

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

See: Strings

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

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

// Widening conversion (宽化转型)
// byte<short<int<long<float<double
int i = 10;
long l = i;               // 10

// Narrowing (窄化转型)
double d = 10.02;
long l = (long)d;         // 10
// 截尾和四舍五入
double a = 20.5;
long i1 = (long)a;  // 20 
long i2 = Math.round(a); // 21

String.valueOf(10);       // "10"
Integer.parseInt("10");   // 10
Double.parseDouble("10"); // 10.0

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

int a = 1, b = 2;
System.out.println(a + b); // 3
System.out.println(a - b); // -1
System.out.println(a * b); // 2
System.out.println(a / b); // 0
System.out.println(a % b); // 1

int a = 1, b = 2;
System.out.println(a * (+b));
System.out.println(a * (-b));
// 2
// -2

int a = 1;
System.out.println((a++) + " " + a); // 1 2
System.out.println((++a) + " " + a); // 3 3
System.out.println((a--) + " " + a); // 3 2
System.out.println((--a) + " " + a); // 1 1

Long a = 100L;
System.out.println(Long.toBinaryString(a)); 
// 1100100

Long a = 1_000_000_000L;

double a = 1e-2;
double b = 1E2;
System.out.println(a); // 0.01
System.out.println(b); // 100.0

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

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

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

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

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

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 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 = "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

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;

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[] 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

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

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

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

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

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

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

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

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

// 逗号表达式
for (int i = 0,j = 0; i < 3; i++,j--) {
  System.out.print(j + "|" + i + " ");
}
// Outputs: 0|0 -1|1 -2|2

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

int count = 0;

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

int count = 0;

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

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

int i = 0;
label1:
for (;;) {
    for (; i < 10; i++) {
        System.out.print(i);
        if (i == 3) {
            i += 2;
            continue label1;
        }
    }
    break;
}
// 012356789

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

int i = 0;
label1:
for (;;) {
    for (; i < 10; i++) {
        System.out.print(i);
        if (i == 3) {
            i += 2;
            break label1;
        }
    }
    break;
}
// 0123

class TestCase {
    /**
     * 无参构造器
     */
    TestCase() {
        System.out.println("TestCase");
    }

    /**
     * 带有参数的构造器
     */
    TestCase(String str) {
        System.out.println(str);
    }
}
TestCase testCase = new TestCase();
TestCase testCase2 = new TestCase("Hello");
// TestCase
// Hello

class TestCase {

    private int value = 10;

    public void printValue(int value) {
        System.out.println(value);
        System.out.println(this.value);
    }
}
TestCase testCase = new TestCase();
testCase.printValue(1);
// 1
// 10

// 定义包
// 定义包
package com.ans20xx;
// 导入包
import java.util.Date;

public class App {
    public static void main(String[] args) {
        System.out.println(new Date());
        // 指定要导入的类位置
        System.out.println(
          new java.util.Date()
        );
    }
}
// Mon Jan 02 19:56:51 CST 2023
// Mon Jan 02 19:56:51 CST 2023

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);
}

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);
});

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);
}

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());

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

• 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

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