Java

Packages

java.lang -> String, StringBuilder, StringBuffer (synchronized, slow), Math java.io -> File, BufferedInputStream, BufferedReader java.math -> BigDeciman, BigInteger, RoundingMode java.util -> Collections, ArrayList, HashMap, LinkedList, UUID, TimeZone, Objects

Char

ASCII values:

• a-z = 97 - 122
• A-Z = 65 - 90
• 0-9 = 48 - 57

     var visited = new int[128];
     for(int i=0; i<128;i++){
       visited[i] = -1;
     }

     System.out.println("Has char r been visited?" + visited['r']);
     System.out.println("ASCII value of char a is: " + (int) Character.valueOf('a'));

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TypeCasting

• Implicit (Widening) Casting: Automatic When converting a smaller primitive type to a larger primitive type (e.g., int to long). No data loss.
• Explicit (Narrowing) Casting: Manual When converting a larger primitive type to a smaller type (e.g., double to int). Requires explicit casting syntax (targetType) and can result in data loss.

Order of Size (Narrow to Wide): byte > short > char > int > long > float > double

String Parses static methods:

• Integer.parseInt(String str) -> int;
• Long.parseLong(String str) -> long;
• Double.parseDouble(String str) -> double;
• Float.parseFloat(String str) -> float;

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Strings

• Reverse string: Use StringBuilder.reverse(); StringBuffer is thread-safe and for that reason its slow. or you can also use pointer from length() to 0.

Useful String methods:

• str.toCharArray() -> char[]
• str.indexOf("m") -> int
• str.indexOf('m', 1) -> int
• str.indexOf('m', 1, 24) -> int
• str.lastIndexOf('m'); -> int
• str.startsWith("abc") -> boolean
• str.endsWith("xyz") -> boolean
• str.charAt(5)
• str.codePointAt(0) - returns ascii value of char at that index a-z = 97 - 122; A-Z = 65 - 90; 0-9 = 48 - 57
• str.toUpperCase(); -> String
• str.toLowerCase(); -> String
• str.length(); -> int
• str.hashcode();
• str.substring(int beginingIndex) -> String
• str.substring(int beginIndex, int endIndex) -> String
• str.concat("xyz") -> String
• str.replace("abc", "xyz") -> String
• str.replaceFirst(String regex, String replacement) -> String
• str.replaceAll(String regex, String replacement) -> String
• str.contains("abc") -> bool
• str.matches(String regex) -> bool
• str.isEmpty() -> bool " " -> false
• str.isBlank() -> " " -> true
• str.split(String regex, int limit) -> String[]
• str.split(String regex) -> String[]
• str.trim() -> removes spaces
• str.strip() -> removes spaces, even the unicode spaces like \u2000 (space)

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Math

• Math.random()
• Math.max(a,b)
• Math.min(a,b)
• Math.abs(-1) = 1
• Math.pow(a,b) = a^b
• Math.log()
• Math.log10()
• Math.sqrt(4) = 2
• Math.cbrt(9) = 3
• Math.round(10.45) = 10
• Math.floot(20.6) = 20
• Math.ceil(20.1) = 21

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Collections

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