String

String:

• str.charAt(i): get the char at i index
• str.toLowerCase();
• str.length()
• str.substring(i,j)
• str.toCharArray()

Char:

• Character.isWhitespace(char)
  • white space include space, tab, new line
• Character.isDigit(char)
• Character.getNumericValue(char)
• Character.isLetterOrDigit(char)

Array:

• arr.length
• Arrays.sort(arr)

Map:

• map.put(a,b)
• map.get(a)

  Constants

• Int maximum value: 2147483647; in java: Integer.MAX_VALUE
• Int minimum value: -2147483648; in java: Integer.MIN_VALUE