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:

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); 

单字节处理:

for (int i=0;i<str1.length();i++){char aChar = str1.charAt(i);} 

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<Object> 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:

 List<String> unsortList = new ArrayList<String>(); 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 dateSimpleDateFormat formatter = new SimpleDateFormat("EEE, dd MMM yyyy HH:mm:ss"); //date formatString 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 IDEADate 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  file Reader("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 digit String 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 firstBufferedReader 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 onlyf.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

原文出处：某熊的全栈之路