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\\b 退格  \\t 制表  \\n 换行  \\r 回车  \\" 双引号  \\' 单引号  \\\\ 反斜杠

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+=  *=  %=  ...
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k++  ++k
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==  !=  >  <  >=  <=
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&&  ||
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x?a:b  //(true:false)
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&  |  ^  ~  >>  <<

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"...".substring(l,r)  //[l,r)
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"a"+"b"
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"a".repeat(3)      //ans = "aaa"

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int index = a.offsetByCodePoints(0,i);  //position of ith code point
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int cp = a.codePointAt(index);      //get the ith code point
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//UTF-16 surrogate pairs require two code units; cannot use charAt(pos)

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StringBuilder builder = new StringBuilder();  //String builder
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builder.append('a');
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builder.append(b);
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String res = builder.toString();        //generate string

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Scanner in = new Scanner(System.in);
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in.nextLine();    //read a line
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in.next();      //read with space delimiter
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in.nextInt();    //read int
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in.nextDouble();  //read Double
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in.hasNext();    //check if there is more input

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System.out.println();
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System.out.printf(“%8.2f",x);    //similar to printf in C
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// d decimal  x hexadecimal  o octal  f fixed-point  e exponential
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// s string  c character   b boolean  h hash code

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Scanner in = new Scanner(Path.of(""),StandardCharsets.UTF_8);
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PrintWriter out = new PrintWriter("",StandardCharsets.UTF_8);

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// 块作用域
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{}
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// 条件语句
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if(){
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}else if(){
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}else {}
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// 循环
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while(...){}      // Evaluate condition first, then execute
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do ... while(...);    // Execute first, then evaluate
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for(int i = ..;i <= .. ; i++) {}
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switch (...){
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  case ...://Label can be char byte short int enum constants string literals
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        ...
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        break;
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    ...
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  default:
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        ...
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        break;//Will stop only when break is encountered
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}
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//break   continue    goto
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//break tag;  Similar to goto jumping to tag:

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//数组定义
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int[] a = new int[len];
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int[] b = {1,2,3,4};
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new Type[0] || new Type[] {}    //Length-0 arrays, different from null
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//for each loop
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for(int i:a) ...    //process array or other element collections
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//- Array copying
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a = b;
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a = Array.copyOf(b,copyLen);
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//Array sorting
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Arrays.sort(a);    //Quicksort    Math.random() -> [0,1)
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//Multidimensional arrays
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int[][] a = new int[lenA][lenB];
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a = {
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  {...},
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  {...}
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};
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for(int[] i:a)
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  for(int j:i)
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    ...
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//Jagged arrays
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int[][] a = new int[N][]; //Array of N references
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a = {...};