OJ systems Java programming language skills
Frequently Asked Questions:
The most common kinds of questions still rely on arrays and strings, require skilled operation, and there is an array of strings for java provides a number of methods. Can simplify programming, but also for a programming language to master. The following list some of some of the techniques that he often encountered good use.
1, calculates the last word of the string array length: S [s.length - 1] .length ();
2, means changing case of characters:
- Java string using the conversion method, uppercase: s.toUpperCase (); lowercase: s.toLowerCase ();
- Using character ASCII code: uppercase to lowercase ( 'char' + 32); lowercase uppercase ( 'CHAR' - 32);
3, the data type conversion between:
- String to char: s.charAt (index);
- char is converted into String: String.valueOf (char);
- String to int: Integer.parseInt (s); // return a value. Integer.valueOf (s); // returns the object. Integer.valueOf (s) .intValue (); // return a value.
- int convert String: String.valueOf (i); // object. s = i + ""; // (splice) two objects.
- is converted to char int: strong rotation (int) ( 'char'-48); // switch to an integer sub-string
- int converted to char: strong rotation (char) (i + 48);
NOTE: Integer.parseInt (s, radix); A method according to binary conversion
4, the concept of substantially ASCII code: using 8-bit binary number indicates the 128 characters, remember common: (48 to 57) for the Arabic numerals 0 to 9; (65 ~ 90) 26 capital English letters; (122 ~ 97) No. 26 lowercase letters; uppercase and lowercase letters in mind that the phase difference of 32 ASCII code.
5, the correlation value calculation
- Math.sqrt (n): square root; // find the cube root Math.cbrt (); squaring and Math.hypot ();
- Math.pow (n, x): x-th power of n -;
- Math.log (): natural logarithm, base-E;
- Math.exp (x): index taking, e power of x;
- Math.random (): Take a random number (0 to 1, the so often need to take a range domain);
- Math.ceil (double a): rounding up;
- Math.floor (double a): rounded down;
- Math.round (double a): rounded to the nearest integer;
6, the median retention results
- Simple Output: System.out.println (String.format ( "% 2f.", S));
- Structure format object: DecimalFormat df = new DecimalFormat ( "# 00."); System.out.println (df.format (number));
7, some of the operations of the string
- String split: str.substring (index); // str.split ( "");
- String substitution: str.replace ();
- String Matching: str.matches (regex);
- String Reverse: new StringBuffer (str) .reverse ();
8, Map operation
- Adding elements: map.put (key, value);
- Get elements: map.get (key); value // key value corresponding to
- Removing elements: map.remove (key); // remove the key corresponding to the key-value pairs
- Comprising determining whether key: map.containsKey (key); // boolean function
- Determining whether the air: map.isEmpty (); // true or false;
- Map traversal: traversal key: for (Integer key: map.keySet ()) {}; traversal entity: for (Map.Entry <Integer, Integer> entry: map.entrySet ()) {};
(Continued) continuously updated