Java CheatSheet

01.基础

hello，world！

public static void main(String[] args) {

    System.out.println("Hello,World");

}

if-else

if (income < 0) rate = 0.00;

else if (income < 8925) rate = 0.10;

else if (income < 36250) rate = 0.15;

else if (income < 87850) rate = 0.23;

else if (income < 183250) rate = 0.28;

else if (income < 398350) rate = 0.33;

else if (income < 400000) rate = 0.35;

else rate = 0.396;

loops:

/**

 * compute the largest

 * power of 2

 * less than or equal to n

 */

public static void test2(int n){

    int power = 1;

    while (power < n/2)

        power = 2 * power;

    System.out.println(power);

}

/**

 * compute a finite sum

 * (1+2+...+n)

 */

public static void test3(int n){

    int sum = 0;

    for(int i=0; i <= n; i++)

        sum += i;

    System.out.println(sum);

}

/**

 * compute a finite product

 * (n! = 1*2*...*n)

 */

public static void test4(int n){

    int product = 1;

    for(int i=1; i <= n; i++)

        product *= i;

    System.out.println(product);

}

/**

 * compute the ruler function

 */

public static void test6(int n){

    StringBuilder ruler = new StringBuilder("1");

    for(int i=2; i <= n; i++)

        ruler.append(" ").append(i).append(" ").append(ruler);

    System.out.println(ruler);

}

do-while:

//do-while

public static void test7(){

    int count = 0;

    do {

        System.out.println("count is:"+ count);

        count++;

    }while (count<11);

}

switch-case:

//switch-case

public static void test8(int day){

    switch(day){

        case 0: System.out.println("Sun");break;

        case 1: System.out.println("Mon");break;

        case 2: System.out.println("Tue");break;

        case 3: System.out.println("Wed");break;

        case 4: System.out.println("Thu");break;

        case 5: System.out.println("Fri");break;

        case 6: System.out.println("Sat");break;

    }

}

02.字符串操作

字符串比较:

boolean result = str1.equals(str2);

boolean result = str1.equalsIgnoreCase(str2);//忽略大小写

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:

//字符串反转

public static void test11(){

    String str1 = "whatever string something";

    StringBuffer buffer = new StringBuffer(str1);

    String reverse = buffer.reverse().toString();

    System.out.println(reverse);

}

按单词的字符串反转:

//按单词的字符串反转

public static void test12(){

    String str1 = "reverse this string";

    Stack<Object> stack = new Stack<>();

    StringTokenizer tokenizer = new StringTokenizer(str1);

    while (tokenizer.hasMoreElements())

        stack.push(tokenizer.nextElement());

    StringBuffer buffer = new StringBuffer();

    while (!stack.isEmpty()){

        buffer.append(stack.pop());

        buffer.append(" ");

    }

    System.out.println(buffer);

}

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(",");

03.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:

Map<String,Object> map = new HashMap<>();

map.put("1","zhangsan");

map.put("2","lisi");

map.put("3","wangwu");

for (Map.Entry<String, Object> next : map.entrySet()) {

    System.out.println(next.getKey() + ":" + next.getValue());

}

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:

List<String> unsortList = 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();

04.时间与日期类型(开发推荐使用org.apache.commons.lang3.time.DateUtils 或者 Java8新的日期工具类，切勿重复造轮子！)

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);

05.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

06.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);

07.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());

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();

08.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();

09.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;

    }

10.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

巴特西

Java语法清单-快速回顾(开发)