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

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);

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 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();

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 digit

String text = “line onen line twon line threen”;

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: