Java 语法清单-快速回顾

程序员文章站 2022-04-08 16:43:00

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Java 语法清单-快速回顾

Java 语法清单翻译自 egek92 的 JavaCheatSheet，从属于笔者的 Java 入门与实践系列。时间仓促，笔者只是简单翻译了些标题与内容整理，支持原作者请前往原文点赞。需要注意的是，此文在 Reddit 上也引起了广泛的讨论，此文讲解的语法要点还是以 Java 7 为主，未涉及 Java 8 中内容，略显陈旧，读者可以带着批判的视角去看。

Java CheatSheet

基础

hello, world! :

if-else:

loops:

do-while:

do {        
    System.out.println("Count is: " + count);
    count++;
} while (count < 11);

switch-case:

数组:

二维数组:

对象:

类:

方法:

Java IDE 比较:

Java 语法清单-快速回顾
图片来自 Wikipedia

个人推荐 IntelliJ IDEA 并且对于学生免费.

字符串操作

字符串比较:

boolean result = str1.equals(str2);
boolean result = str1.equalsIgnoreCase(str2);

搜索与检索:

int result = str1.indexOf(str2);
int result = str1.indexOf(str2,5);
String index = str1.substring(14);

字符串反转:

public class Main {
    public static void main(String[] args) {
        String str1 = "whatever string something";
        StringBuffer str1buff = new StringBuffer(str1);
        String str1rev = str1buff.reverse().toString();
         System.out.println(str1rev);
    }
}

按单词的字符串反转:

public class Main {public static void main(String[] args) {

    String str1 = "reverse this string";

    Stack stack = new Stack<>();

    StringTokenizer strTok = new StringTokenizer(str1); 
    while(strTok.hasMoreTokens()){
       stack.push(strTok.nextElement());
    }

    StringBuffer str1rev = new StringBuffer();
    while(!stack.empty()){
        str1rev.append(stack.pop());
        str1rev.append(" ");
    }
    System.out.println(str1rev);
}

大小写转化:

String strUpper = str1.toUpperCase();
String strLower = str1.toLowerCase();

首尾空格移除:

String str1 = "     asdfsdf   ";
str1.trim(); //asdfsdf

空格移除:

str1.replace(" ","");

字符串转化为数组:

String str = "tim,kerry,timmy,camden";
String[] results = str.split(",");

数据结构

重置数组大小:

int[] myArray = new int[10];
int[] tmp = new int[myArray.length + 10];
System.arraycopy(myArray, 0, tmp, 0, myArray.length);
myArray = tmp;

集合遍历:

 for (Iterator it = map.entrySet().iterator();it.hasNext();){
     Map.Entry entry = (Map.Entry)it.next();
     Object key = entry.getKey();
     Object value = entry.getValue();}

创建映射集合:

HashMap map = new HashMap();
map.put(key1,obj1);
map.put(key2,obj2);
map.put(key2,obj2);

数组排序:

int[] nums = {1,4,7,324,0,-4}; 
Arrays.sort(nums);
System.out.println(Arrays.toString(nums));

列表排序:

ListunsortList = new ArrayList();

unsortList.add("CCC");
unsortList.add("111");
unsortList.add("AAA");
Collections.sort(unsortList);

列表搜索:

int index = arrayList.indexOf(obj);

finding an object by value in a hashmap:

hashmap.containsValue(obj);

finding an object by key in a hashmap:

hashmap.containsKey(obj);

二分搜索:

int[] nums = new int[]{7,5,1,3,6,8,9,2};
Arrays.sort(nums);int index = Arrays.binarySearch(nums,6);
System.out.println("6 is at index: "+ index);

arrayList 转化为 array:

Object[] objects = arrayList.toArray();

将 hashmap 转化为 array:

Object[] objects = hashmap.entrySet().toArray();

时间与日期类型

打印时间与日期:

Date todaysDate = new Date(); //todays date
SimpleDateFormat formatter = new SimpleDateFormat("EEE, dd MMM yyyy HH:mm:ss"); //date format
String formattedDate = formatter.format(todaysDate);
System.out.println(formattedDate);

将日期转化为日历:

Date mDate = new Date();
Calendar mCal = Calendar.getInstance();
mCal.setTime(mDate);

将 calendar 转化为 date:

Calendar mCal = Calendar.getInstance();
Date mDate = mDate.getTime();

字符串解析为日期格式:

public void StringtoDate(String x) throws ParseException{
    String date = "March 20, 1992 or 3:30:32pm";
    DateFormat df = DateFormat.getDateInstance();
    Date newDate = df.parse(date);    
}

date arithmetic using date objects:

Date date = new Date();
long time = date.getTime();
time += 5*24*60*60*1000; //may give a numeric overflow error on IntelliJ IDEA

Date futureDate = new Date(time);
System.out.println(futureDate);

date arithmetic using calendar objects:

Calendar today = Calendar.getInstance();
today.add(Calendar.DATE,5);

difference between two dates:

 long diff = time1 - time2; diff = diff/(1000*60*60*24);

comparing dates:

boolean result = date1.equals(date2);

getting details from calendar:

Calendar cal = Calendar.getInstance();
cal.get(Calendar.MONTH);
cal.get(Calendar.YEAR);
cal.get(Calendar.DAY_OF_YEAR);
cal.get(Calendar.WEEK_OF_YEAR);
cal.get(Calendar.DAY_OF_MONTH);
cal.get(Calendar.DAY_OF_WEEK_IN_MONTH);
cal.get(Calendar.DAY_OF_MONTH);
cal.get(Calendar.HOUR_OF_DAY);

calculating the elapsed time:

long startTime = System.currentTimeMillis();//times flies by..

long finishTime = System.currentTimeMillis();
long timeElapsed = startTime-finishTime;
System.out.println(timeElapsed);

正则表达式

使用 REGEX 寻找匹配字符串:

String pattern = "[TJ]im";      
Pattern regPat = Pattern.compile(pattern,Pattern.CASE_INSENSITIVE); 
String text = "This is Jim and that's Tim";
Matcher matcher = regPat.matcher(text);       
if (matcher.find()){           
    String matchedText = matcher.group();
    System.out.println(matchedText);
}

替换匹配字符串:

String pattern = "[TJ]im";
Pattern regPat = Pattern.compile(pattern,Pattern.CASE_INSENSITIVE); 
String text = "This is jim and that's Tim";
Matcher matcher = regPat.matcher(text);
String text2 = matcher.replaceAll("Tom");
System.out.println(text2);

使用 StringBuffer 替换匹配字符串:

 Pattern p = Pattern.compile("My");
Matcher m = p.matcher("My dad and My mom");
StringBuffer sb = new StringBuffer();
boolean found = m.find();       
while(found){
      m.appendReplacement(sb,"Our");
      found = m.find();
}
m.appendTail(sb);
System.out.println(sb);

打印所有匹配次数:

String pattern = "\\sa(\\w)*t(\\w)*"; //contains "at"Pattern regPat = Pattern.compile(pattern);
String text = "words something at atte afdgdatdsf hey";
Matcher matcher = regPat.matcher(text);
while(matcher.find()){     
     String matched = matcher.group();
      System.out.println(matched);
}

打印包含固定模式的行:

String pattern = "^a";      
Pattern regPat = Pattern.compile(pattern);
Matcher matcher = regPat.matcher("");
BufferedReader reader = new BufferedReader(new FileReader("file.txt"));
String line;        
while ((line = reader.readLine())!= null){
    matcher.reset(line);
         if (matcher.find()){
             System.out.println(line);
        }
}

匹配新行:

String pattern = "\\d$"; //any single digitString text = "line one\n line two\n line three\n";     
Pattern regPat = Pattern.compile(pattern, Pattern.MULTILINE);
Matcher matcher = regPat.matcher(text);
while (matcher.find()){
    System.out.println(matcher.group());
}

regex:

beginning of a string: ^
end of a string: $
0 or 1 times: ?
0 or more times: (*) //without brackets
1 or more times: +
alternative characters: [...]
alternative patterns: |
any character: .
a digit: d
a non-digit: D
whitespace: s
non-whitespace: S
word character: w
non word character: W

数字与数学操作处理

内建数据类型:

byte: 8bits, Byte
short: 16bits, Short
long: 64bits, Long
float: 32bits, Float

判断字符串是否为有效数字:

String str = "dsfdfsd54353%%%"; 
try{    
     int result = Integer.parseInt(str);
}catch (NumberFormatException e){
    System.out.println("not valid");
}

比较 Double:

Double a = 4.5; 
Double b= 4.5;
boolean result = a.equals(b);
if (result) System.out.println("equal");

rounding:

double doubleVal = 43.234234200000000234040324;
float floatVal = 2.98f;
long longResult = Math.round(doubleVal);
int intResult = Math.round(floatVal);
System.out.println(longResult + " and " + intResult); // 43 and 3

格式化数字:

double value = 2343.8798;
NumberFormat numberFormatter;        
String formattedValue;
numberFormatter = NumberFormat.getNumberInstance();
formattedValue = numberFormatter.format(value);
System.out.format("%s%n",formattedValue); //2.343,88

格式化货币:

double currency = 234546457.99;
NumberFormat currencyFormatter;       
String formattedCurrency;
currencyFormatter = NumberFormat.getCurrencyInstance();
formattedCurrency = currencyFormatter.format(currency);
System.out.format("%s%n",formattedCurrency); // $ 234.546.457,99

二进制、八进制、十六进制转换:

int val = 25;
String binaryStr = Integer.toBinaryString(val);
String octalStr = Integer.toOctalString(val);
String hexStr = Integer.toHexString(val);

随机数生成:

double rn = Math.random();        
int rint = (int) (Math.random()*10); // random int between 0-10
System.out.println(rn);
System.out.println(rint);

计算三角函数:

double cos = Math.cos(45);
double sin = Math.sin(45);        
double tan = Math.tan(45);

计算对数

double logVal = Math.log(125.5);

Math library:

输入输出操作:

从输入流读取:

//throw IOexception first
BufferedReader inStream = new BufferedReader(new InputStreamReader(System.in));
String inline ="";
while (!(inline.equalsIgnoreCase("quit"))){
      System.out.println("prompt> ");   
      inline=inStream.readLine();
}

格式化输出:

StringBuffer buffer = new StringBuffer();
Formatter formatter = new Formatter(buffer, Locale.US);
formatter.format("PI: "+Math.PI);
System.out.println(buffer.toString());

formatter format calls:

打开文件:

BufferedReader br = new BufferedReader(new FileReader(textFile.txt)); //for reading
BufferedWriter bw = new BufferedWriter(new FileWriter(textFile.txt)); //for writing

读取二进制数据:

InputStream is = new FileInputStream(fileName);

int offset = 0;    
int bytesRead = is.read(bytes, ofset, bytes.length-offset);

文件随机访问:

File file = new File(something.bin);
RandomAccessFile raf = new RandomAccessFile(file,"rw");
raf.seek(file.length());

读取 Jar/zip/rar 文件:

ZipFile file =new ZipFile(filename);
Enumeration entries = file.entries();    
while(entries.hasMoreElements()){ 
    ZipEntry entry = (ZipEntry) entries.nextElement();
   if (entry.isDirectory()){
        //do something
   } else{            
        //do something
   }
}
file.close();

文件与目录

创建文件:

File f = new File("textFile.txt");
boolean result = f.createNewFile();

文件重命名:

File f = new File("textFile.txt");

File newf = new File("newTextFile.txt");
boolean result = f.renameto(newf);

删除文件:

File f = new File("somefile.txt");
f.delete();

改变文件属性:

File f = new File("somefile.txt");
f.setReadOnly(); // making the file read only
f.setLastModified(desired time);

获取文件大小:

File f = new File("somefile.txt");

long length = file.length();

判断文件是否存在:

File f = new File("somefile.txt");
boolean status = f.exists();

移动文件:

File f = new File("somefile.txt");
File dir = new File("directoryName");
boolean success = f.renameTo(new File(dir, file.getName()));

获取绝对路径:

File f = new File("somefile.txt");
File absPath = f.getAbsoluteFile();

判断是文件还是目录:

File f = new File("somefile.txt");
boolean isDirectory = f.isDirectory();
System.out.println(isDirectory); //false

列举目录下文件:

File directory = new File("users/ege");
String[] result = directory.list();

创建目录:

boolean result = new File("users/ege").mkdir();

网络客户端

服务器连接:

String serverName = "www.egek.us";
Socket socket = new Socket(serverName, 80);
System.out.println(socket);

网络异常处理:

try {
            Socket sock = new Socket(server_name, tcp_port);
            System.out.println("Connected to " + server_name);
        sock.close(  );

    } catch (UnknownHostException e) {
        System.err.println(server_name + " Unknown host");  
            return;
    } catch (NoRouteToHostException e) {
        System.err.println(server_name + " Unreachable" );
            return;
    } catch (ConnectException e) {
        System.err.println(server_name + " connect refused");
            return;
    } catch (java.io.IOException e) {
        System.err.println(server_name + ' ' + e.getMessage(  )); 
            return;
    }

包与文档

创建包:

package com.ege.example;

使用 JavaDoc 注释某个类:

javadoc -d \home\html
    -sourcepath \home\src
    -subpackages java.net

Jar 打包:

jar cf project.jar *.class

运行 Jar:

java -jar something.jar

排序算法

Bubble Sort
Linear Search
Binary Search
Selection Sort
Insertion Sort