Java Classes & Methods

Math Class:
API Documentation: https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/lang/Math.html
w3schools HowTo: https://www.w3schools.com/java/java_math.asp

- Math.max(a, b);   //Compares two values, returns bigger value
- Math.round();    //Rounds a double to the nearest int value. Use Math.floor(); or Math.ceil(); to round to the lowest or highest int
- Math.abs();     //Enter a variable name or expression to get a non-negative value
- Math.sqrt();
- Math.PI;    //Gets the value of pi. Can be used in calculations  (2 * Math.PI * r;)
- Math.pow();    //Takes two parameters as the base and exponent  ( Math.pow(2, 8); )   2^8



String Class:
API Documentation: https://docs.oracle.com/javase/7/docs/api/java/lang/String.html
w3schools HowTo: https://www.w3schools.com/java/java_strings.asp

- str.length();      //Returns int value based on the size of the string
- str.substring(0, 3);    //0 is the starting index, 3 is the ending index to search, but not inclusive. "Matt" -> "Mat"
- str.charAt();        //Enter integer of the string's index number and returns a char
- str.toUpperCase();    //Enter string variable name to make upper case
- str.toLowerCase();
- str.isUpperCase();
- str.indexOf('a');     //returns an index number for where the argument appears in the string. Can be char or string
- str.startsWith("");   //Checks if the string starts with a certain word or character
- str.replace("", "")   //replaces any part of a string with another string literal. Can be used to remove parts of a string. Left: Replace what, Right: Replace with
- str1.equals(str2);    //Checks if one string has the same characters as another string



ArrayList Class:
API Documentation: https://docs.oracle.com/javase/8/docs/api/java/util/ArrayList.html
w3schools HowTo: https://www.w3schools.com/java/java_arraylist.asp

ArrayList<Type> Name = new ArrayList<Type>();

- var.add()   //adds a value to the array list
- var.remove()   //removes a value from the array list
- var.get(i)   //returns the value in the specified index number
- var.set(i, x)   //modified a value in the specified index number with another value
- var.size()   //returns the size of the array list
- var.clear()   //clears the entire array list



Collections Class:
API Documentation: https://docs.oracle.com/javase/7/docs/api/java/util/Collections.html

- Collections.sort()   //sorts the elements in order
- Collections.frequency(arrVar, "")   //returns the amount of times an element appears in an array or arraylist


StringBuilder Class:
API Documentation: https://docs.oracle.com/javase/7/docs/api/java/lang/StringBuilder.html
Note: Requires object creation: StringBuilder myObj = new StringBuilder(String);

- myObj.reverse()   //Reverses the order of the characters in the string



Input/Output Methods:
- System.out.println();   //Add a + in the parameter to add another string of text or variable   ("Hello" + X)
- System.out.printf("%.1f", varName);    //Prints out a decimal to 1 decimal place. Use %d for int, %f for float
- Scanner scanner = new Scanner(System.in);      //Creates a scanner object. To use it, assign a variable of any type to scanner.nextLine(); / scanner.nextInt(); / scanner.nextDouble();   NOTE: Use scanner.nextLine(); after each integer input to clear the empty new line (\n) !
- scanner.hasNextInt()   //Checks if the input has an integer
- PrintWriter pw = new PrintWriter("output.txt");     //An object to create a text file. Use pw.println(""); to write to the text file, then pw.close(); to close it. Requires java.io.PrintWriter;



Coding Tricks:
- Validating user input:    //The while loop checks if the input is an integer, repeats if not.
while( !(hasNextInt()) ) {
	System.out.println("Enter a number");
	scanner.next();
}
scanner.nextInt();