Basic Language Structure¶

前置知识¶

无。

Comments 注释¶

// This is a one-line comment

/*
 * This is a multi-line comment
 */

/**
 * This is JavaDoc
 */

Variable Declare & Assignment / Data types 变量声明、赋值、数据类型¶

jshell> int a;
a ==> 0
jshell> a = 1;
a ==> 1
jshell> long b = 1L
b ==> 1
jshell> char e = 'E'
e ==> 'E'
jshell> String f = "java"
f ==> "java"
jshell> boolean g = true
g ==> true

jshell> float c = 1.0
|  错误:
|  不兼容的类型: 从double转换到float可能会有损失
|  float c = 1.0;
|            ^-^
jshell> float c = 1.0f
c ==> 1.0
jshell> c = (float) 2.0
c ==> 2.0
jshell> double d = 1.0
d ==> 1.0

整数类型: byte short int long分别为 1、2、4、8 字节有符号整数。
浮点数: float double分别为 4、8 字节浮点数。
逻辑值: boolean。
字符: char 2 字节 Unicode 字符。

Operators: 运算符¶

Arithmetic 算数¶

jshell> 1 + 1
$2 ==> 2
jshell> 2 * 2
$3 ==> 4
jshell> 3 / 2
$4 ==> 1
jshell> 3.0 / 2
$5 ==> 1.5
jshell> 3 % 2
$6 ==> 1
jshell> -3 % 2
$6 ==> -1

jshell> int a = 1;
a ==> 1
jshell> a += 100;
$17 ==> 101
jshell> a *= 20;
$18 ==> 2020
jshell> a /= 3;
$19 ==> 673
jshell> a %= 20;
$20 ==> 13

++ -- 自增自减¶

jshell> int a = 1;
a ==> 1

jshell> ++a
$29 ==> 2
jshell> --a
$30 ==> 1

jshell> a++
$31 ==> 1
jshell> a--
$32 ==> 2

Bitwise 位运算¶

jshell> 1 & 2
$11 ==> 0

jshell> 1 | 2
$12 ==> 3

jshell> 1 ^ 2
$13 ==> 3

jshell> ~1
$14 ==> -2

jshell> 1 << 20;
$21 ==> 1048576

jshell> -1 << 20;
$22 ==> -1048576

jshell> 1024 >> 3;
$23 ==> 128

jshell> -1024 >> 3;
$24 ==> -128

jshell> -1024 >>> 3;
$25 ==> 536870784

对最后两个例子的解释:

Java 中 -1024 作为字面量，被视为有符号的int类型，以补码表示为11111111 11111111 11111100 00000000。

>>的行为: 如果被操作的数的最高位是1，则右移时在高位补1 ；如果被操作的数的最高位是0，则右移时在高位补0。

>>>的行为: 右移时在高位补0。

因此，-1024 >> 3 得到 11111111 11111111 11111111 10000000 ，即-128。

因此，-1024 >>> 3 得到 00011111 11111111 11111111 10000000 ，即563870784。

Compare 比较¶

jshell> int a = 1, b = 2;
a ==> 1
b ==> 2

jshell> a < b
$3 ==> true

jshell> a <= b
$4 ==> true

jshell> a == b
$5 ==> false

jshell> a != b
$6 ==> true

jshell> a >= b
$7 ==> false

jshell> a > b
$8 ==> false

Logic 逻辑¶

jshell> true && false
$15 ==> false

jshell> true || false
$16 ==> true

jshell> !true
$17 ==> false

jshell> "hello" + "world"
$10 ==> "helloworld"

jshell> "1 + 1 = " + (1 + 1)
$11 ==> "1 + 1 = 2"

jshell> System.out.println("hello\nworld")
hello
world

jshell> System.out.println("Col1\tCol2\n1\t2\n11111\t22222")
Col1    Col2
1   2
11111   22222

Function / Method Syntax 函数、方法¶

jshell> int mul(int a, int b) {
   ...>     return a * b;
   ...> }
|  已创建 方法 mul(int,int)

jshell> mul(2, 3)
$20 ==> 6

ControlFlow: 控制流¶

if-else¶

jshell> boolean canIGetA(double score) {
   ...>     if (score >= 90) {
   ...>         System.out.println("Good Job! You got a 4.0!");
   ...>         return true;
   ...>     } else if (score >= 70) {
   ...>         System.out.println("Keep Going! You can do better!");
   ...>         return false;
   ...>     } else {
   ...>         System.out.println("Emm... What happends?");
   ...>         return false;
   ...>     }
   ...> }
|  已创建 方法 canIGetA(double)

jshell> canIGetA(100)
Good Job! You got a 4.0!
$2 ==> true

jshell> canIGetA(80)
Keep Going! You can do better!
$3 ==> false

jshell> canIGetA(0)
Emm... What happends?
$4 ==> false

?:¶

jshell> int gcd(int a, int b) {
   ...>     return b == 0 ? a : gcd(b, a % b);
   ...> }
|  已创建 方法 gcd(int,int)

jshell> gcd(48, 18)
$26 ==> 6

switch¶

jshell> int calculate(int a, int b, char op) {
   ...>     int res;
   ...>     switch(op) {
   ...>         case '+': res = a + b; break;
   ...>         case '-': res = a - b; break;
   ...>         case '*': res = a * b; break;
   ...>         case '/': res = a / b; break;
   ...>         default: res = -1;
   ...>     }
   ...>     return res;
   ...> }
|  已创建 方法 calculate(int,int,char)

jshell> calculate(2, 3, '+')
$6 ==> 5
jshell> calculate(2, 3, '-')
$7 ==> -1
jshell> calculate(2, 3, '*')
$8 ==> 6
jshell> calculate(2, 3, '/')
$9 ==> 0
jshell> calculate(2, 3, '^')
$10 ==> -1

for¶

jshell> boolean isPrime(int a) {
   ...>     if (a <= 1) return false;
   ...>     for (int i = 2; i * i <= a; i++) {
   ...>         if(a % i == 0) return false;
   ...>     }
   ...>     return true;
   ...> }
|  已创建 方法 isPrime(int)

jshell> for (int a = 1; a <= 100; a++) {
   ...>     if (isPrime(a)) System.out.print(a + " ");
   ...> }
2 3 5 7 11 13 17 19 23 29 31 37 41 43 47 53 59 61 67 71 73 79 83 89 97

while¶

jshell> import java.lang.Math;

jshell> double x;
x ==> 0.0

jshell> while( (x = Math.random()) < 0.8) {
   ...>     System.out.println(x + " < 0.8");
   ...> }
0.17636000596270907 < 0.8
0.5029857602823097 < 0.8
0.3349284868587292 < 0.8
0.3650514335114553 < 0.8

do-while¶

jshell> do {
   ...>     x = Math.random();
   ...> } while(x < 0.8);

jshell> x // is a random double >= 0.8
x ==> 0.8346328176085303

String 字符串¶

jshell> String s = "I Love Java"
s ==> "I Love Java"
jshell> s.length()
$37 ==> 11
jshell> s.split(" ")
$38 ==> String[3] { "I", "Love", "Java" }
jshell> s.concat(" and Rust")
$39 ==> "I Love Java and Rust"
jshell> s.contains("C++")
$40 ==> false
jshell> s.charAt(2)
$41 ==> 'L'
jshell> s.replace("Java", "C++")
$42 ==> "I Love C++"
jshell> s.toUpperCase()
$46 ==> "I LOVE JAVA"
jshell> s.toLowerCase()
$47 ==> "i love java"
jshell> s.startsWith("I")
$48 ==> true
jshell> s.endsWith("Java")
$49 ==> true
jshell> s
s ==> "I Love Java"
jshell> s == "I Love Java"
$43 ==> true
jshell> s == new String("I Love Java")
$44 ==> false
jshell> s.equals(new String("I Love Java"))
$45 ==> true
jshell> s.equalsIgnoreCase("i lOve jAVA")
$50 ==> true

API Ref: java.lang.String

Console IO 命令行输入输出¶

jshell> System.out.println("I Love Java")
I Love Java

jshell> System.out.print("I Love Java")
I Love Java
jshell> System.out.printf("I Love %s\n", "Java")
I Love Java
$82 ==> java.io.PrintStream@4d95d2a2
jshell> import java.util.Scanner;
jshell> var input = new Scanner(System.in);
jshell> input.hasNextInt()
100
$86 ==> true
jshell> input.nextInt()
$87 ==> 100
jshell> input.hasNextBoolean()
100
$89 ==> false

API Ref: java.util.Scanner

Array 数组¶

jshell> var matrix = new int[5][5];
matrix ==> int[5][] { int[5] { 0, 0, 0, 0, 0 }, int[5] { 0,  ... int[5] { 0, 0, 0, 0, 0 } }
jshell> for (int i = 0; i<5; i++) {
   ...>     for(int j = 0; j<5; j++) {
   ...>         matrix[i][j] = i * j;
   ...>     }
   ...> }
jshell> matrix
matrix ==> int[5][] {
    int[5] { 0, 0, 0, 0, 0 },
    int[5] { 0, 1, 2, 3, 4 },
    int[5] { 0, 2, 4, 6, 8 },
    int[5] { 0, 3, 6, 9, 12 },
    int[5] { 0, 4, 8, 12, 16 } }

Array Utility:

jshell> import java.util.Arrays;
jshell> Arrays.sort(intArr)
jshell> intArr
intArr ==> int[10] { 0, 0, 0, 1, 1, 2, 5, 8, 9, 9 }
jshell> Arrays.binarySearch(intArr, 5)
$64 ==> 6

jshell> System.out.println(intArr)
[I@2d6eabae
jshell> System.out.println(Arrays.toString(intArr))
[0, 0, 0, 1, 1, 2, 5, 8, 9, 9]
jshell> System.out.println(Arrays.deepToString(matrix))
[[0, 0, 0, 0, 0], [0, 1, 2, 3, 4], [0, 2, 4, 6, 8], [0, 3, 6, 9, 12], [0, 4, 8, 12, 16]]

API Ref: java.util.Arrays

何为api

API是应用编程接口 (英语 :Application Programming Interface），描述了 Java 类库向外提供的功能、使用方法、注意事项等信息。
JavaDoc 是内置的生成网站形式的 API 文档的方式，Java 标准库参考文档，也就是 API Ref就使用 JavaDoc 生成的。
Java 标准库内容丰富，生态庞大，学会阅读、查找、遵循 API 文档是充分利用 Java 优势的第一步。

数学相关¶

BigInteger¶

jshell> import java.math.BigInteger;
jshell> 1000000000000000 + 1
|  错误:
|  整数太大
|  1000000000000000 + 1
|  ^
jshell> new BigInteger("1000000000000000").add(BigInteger.ONE)
$92 ==> 1000000000000001
jshell> new BigInteger("19260817").isProbablePrime(10)
$93 ==> true

API Ref: java.math.BigInteger

BigDecimal¶

jshell> import java.math.BigDecimal;

jshell> 0.1 + 0.2
$94 ==> 0.30000000000000004

jshell> new BigDecimal("0.1").add(new BigDecimal("0.2"))
$95 ==> 0.3

jshell> new BigDecimal("0.000000001").toEngineeringString()
$100 ==> "1E-9"

API Ref: java.math.BigDecimal

Math库¶

jshell> Math.
E                 IEEEremainder(    PI                abs(              absExact(
acos(             addExact(         asin(             atan(             atan2(
cbrt(             ceil(             class             copySign(         cos(
cosh(             decrementExact(   exp(              expm1(            floor(
floorDiv(         floorMod(         fma(              getExponent(      hypot(
incrementExact(   log(              log10(            log1p(            max(
min(              multiplyExact(    multiplyFull(     multiplyHigh(     negateExact(
nextAfter(        nextDown(         nextUp(           pow(              random()
rint(             round(            scalb(            signum(           sin(
sinh(             sqrt(             subtractExact(    tan(              tanh(
toDegrees(        toIntExact(       toRadians(        ulp(

jshell> Math.sin(Math.PI / 4)
$103 ==> 0.7071067811865475
jshell> Math.floor(1.002)
$104 ==> 1.0
jshell> Math.log(Math.E)
$107 ==> 1.0
jshell> Math.pow(2, 1.5)
$108 ==> 2.82842712474619
jshell> Math.sqrt(2)
$109 ==> 1.4142135623730951

API Ref: java.lang.Math

一些语法糖¶

[JEP286, Java10] `var` type inference¶

jshell> var mapOfMap = new HashMap<String, Map<String, String>>()
mapOfMap ==> {}

jshell> mapOfMap.put("Java", Map.of("Type", "Strong-Typed", "Paradigm", "OOP"))
$111 ==> null

jshell> mapOfMap
mapOfMap ==> {Java={Paradigm=OOP, Type=Strong-Typed}}

jshell> mapOfMap.forEach(
    ..>     (var key, var val) -> System.out.println(key + " : " + val))
Java : {Paradigm=OOP, Type=Strong-Typed}

[JEP378, Java15] Text Blocks¶

"""包含的字符串称为文本块 TextBlock。文本块允许使用"而不需转义，\"""则需要转义。

jshell> """
   ...> Line 1
   ...> Line 2
   ...> Line 3
   ...> Line 4
   ...> """
$4 ==> "Line 1\nLine 2\nLine 3\nLine 4\n"

使用\取消换行

jshell> """
   ...> Line 1 \
   ...> Line 2 \
   ...> Line 3 \
   ...> Line 4 \
   ...> """
$5 ==> "Line 1 Line 2 Line 3 Line 4\n"

使用\s保证末尾空格不会删除，否则末尾空格会被替换为换行符。下面用·表示空格，而非实际字符:

jshell> """
   ...> red···
   ...> green·
   ...> blue··
   ...> """
$1 ==> "red\ngreen\nblue\n"

jshell> """
   ...> red···\s
   ...> green·\s
   ...> blue··\s
   ...> """
$1 ==> "red···\ngreen·\nblue··\n"

本章 CodeLab¶

CodeLab 1-1 a +-*/^&| b¶

本CodeLab链接

codelab/chapter1/src/Lab1.java
标准输入输出
输入格式: 若干行a op b，其中op是+-*/^&|中的其中一种，以标准输入结束为终止
输出格式: 若干行计算结果
提示: 计算结果可能非常大

CodeLab 1-2 QuickSort¶

本CodeLab链接

codelab/chapter1/src/Lab2.java
实现public static void quickSort(int[] arr)
运行main方法会自动检测实现的正确性

Basic Language Structure¶

前置知识¶

Comments 注释¶

Variable Declare & Assignment / Data types 变量声明、赋值、数据类型¶

Operators: 运算符¶

Arithmetic 算数¶

++ -- 自增自减¶

Bitwise 位运算¶

Compare 比较¶

Logic 逻辑¶

Function / Method Syntax 函数、方法¶

ControlFlow: 控制流¶

if-else¶

?:¶

switch¶

for¶

while¶

do-while¶

String 字符串¶

Console IO 命令行输入输出¶

Array 数组¶

数学相关¶

BigInteger¶

BigDecimal¶

Math库¶

一些语法糖¶

[JEP286, Java10] `var` type inference¶

[JEP378, Java15] Text Blocks¶

本章 CodeLab¶

CodeLab 1-1 a +-*/^&| b¶

CodeLab 1-2 QuickSort¶

资源链接¶

评论

Basic Language Structure¶

前置知识¶

Comments 注释¶

Variable Declare & Assignment / Data types 变量声明、赋值、数据类型¶

Operators: 运算符¶

Arithmetic 算数¶

++ -- 自增自减¶

Bitwise 位运算¶

Compare 比较¶

Logic 逻辑¶

Function / Method Syntax 函数、方法¶

ControlFlow: 控制流¶

if-else¶

?:¶

switch¶

for¶

while¶

do-while¶

String 字符串¶

Console IO 命令行输入输出¶

Array 数组¶

数学相关¶

BigInteger¶

BigDecimal¶

Math库¶

一些语法糖¶

[JEP286, Java10] var type inference¶

[JEP378, Java15] Text Blocks¶

本章 CodeLab¶

CodeLab 1-1 a +-*/^&| b¶

CodeLab 1-2 QuickSort¶

资源链接¶

评论

[JEP286, Java10] `var` type inference¶