SE2205B - DATA STRUCTURES AND ALGORITHMS JAVA BASICS. Kevin Brightwell. Tuesday January 10th, Acknowledgements:Dr.

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1 SE2205B - DATA STRUCTURES AND ALGORITHMS JAVA BASICS Kevin Brightwell Tuesday January 10th, 2017 Acknowledgements:Dr. Quazi Rahman 1 / 29

2 LECTURE OUTLINE Course Preamble More Java 2 / 29

3 OFFICE HOURS - DOODLE Scan the code, fill in all of the times that you are available I'll pick two hours of time per week that the most are available Limited additional appointment times will be available 3 / 29

4 CONTACT GUARANTEES For , I try to reply as fast as possible, but please allow at least 1 business day for me to reply I'll be asking my TAs to give the same guarantee 4 / 29

5 LECTURE OUTLINE Course Preamble More Java 5 / 29

6 IDENTIFIERS AKA names Adheres to the same rules as C++: Must start with letter, underscore or dollar sign ($) Must not be a reserved word, e.g. true, null) Can contain any valid "ascii" letter or number, i.e. a-z but not Can be of any length 6 / 29

7 CONSTANTS // don't use this, use Math.PI if needed final double PI = ; final String NAME = "John Smith"; 1. Must use final keyword first, denotes constant value 2. Datatype, e.g. int or String 3. Name, e.g. PI or NAME 4. Value, all constants must be initialized at declaration 7 / 29

8 NUMERICAL DATA TYPES Numerical data types are nearly identical to C++ Primitive sizes are guaranteed across OS No native unsigned datatypes Type Storage byte short int long char float double 8-bit 16-bit 32-bit 64-bit 32-bit (it's an int) 32-bit 64-bit 8 / 29

9 EXAMPLE: DISPLAYING TIME Problem: Display the current time in the format: HH:MM:SS. 1. Get the current time System.currentTimeMillis() 2. Now figure out the math: long millis = System.currentTimeMillis(); long seconds = millis / 1000; // total number of seconds.. long minutes = seconds / 60; // total number of minutes.. long hours = minutes / 60; // total number of hours.. // still need to figure out how to break it down further Docs: System#currentTimeMillis() 9 / 29

10 EXAMPLE: DISPLAYING TIME Need to extract the actual time breakdown from the total amount long millis = System.currentTimeMillis(); long seconds = millis / 1000; // total number of seconds.. long minutes = seconds / 60; // total number of minutes.. long hours = minutes / 60; // total number of hours.. // Use remainder operator! long secondsleft = seconds % 60; long minutesleft = minutes % 60; System.out.println(hours + ":" + minutesleft + ":" + secondsleft); 10 / 29

11 EXAMPLE: DISPLAYING TIME Use Java's built in time library, do not handle time by hand Manually handling time sucks import java.time.localdatetime; import java.time.format.datetimeformatter; System.out.println(LocalDateTime.now().format(DateTimeFormatter.ISO_LOCAL_TIME)); Docs: LocalDateTime DateTimeFormatter 11 / 29

12 EXPRESSIONS Expressions in Java are a statement with a result These are identical to C++, including how data types auto cast. Consider: int a = 1 / 2; // = 0 int b = (int) (1.0 / 2.0); // = 0 double c = 1.0 / 2.0; // = 0.5 double d = 1 / 2; // = 0 12 / 29

13 STRINGS Seen previously, java.lang.string is Java's string type Behaves very similarly to C++'s std::string It is a reference type String is a thin wrapper around a char[], held by java.lang.charsequence The underlying data is immutable 13 / 29

14 STRINGS String Manipulation final String out = "a" "b"; // = "a2b" String out = ""; for (int i = 0; i < 100; ++i) { out += Integer.toString(i); } return out; // out = " " String concatenation 2. Looping concatenation Docs: String Integer#toString(int) 14 / 29

15 STRINGS StringBuilder - Better Concatenation import java.lang.stringbuilder; StringBuilder out = new StringBuilder(); for (int i = 0; i < 100; ++i) { out.append(i); // out += Integer.toString(i); } return out.tostring(); // out = " " Looped concatenation is bad, this doesn't allocate a new string each time 2. We create a new instance of a StringBuilder 3. Append the integer Docs: StringBuilder 15 / 29

16 GETTING INPUT FROM THE CONSOLE Java uses Streams similar to the way that C++ uses <iostream> Instead of using std::cin to read data, Java uses java.util.scanner Scanner is a general class that will read from an InputStream, extracting data Docs: java.util.scanner java.io.inputstream 15 / 29

17 import java.util.scanner; GETTING INPUT FROM THE CONSOLE Example final Scanner scanner = new Scanner(System.in); System.out.print("Enter a double then an int value: "); final double d = scanner.nextdouble(); final int i = scanner.nextint(); System.out.println(String.format("d = %f, i = %f", d, i)); 1. Create a new Scanner instance 2. Pass a reference to stdin (standard input from the console) 3. Read the double and int from the user 17 / 29

18 STRING-PRIMITIVE CONVERSIONS Many times, data will only be available as a String, e.g. String s = "23"; To convert between Strings and other primitives, the Java standard library has "Wrapper types" These are named the same as primitive type but with an uppercase: e.g. double Double Primitives are automatically "boxed" between wrapper and primitive types final String s_i1 = "23"; final int i1 = Integer.parseInt(s_i1); final String s_d1 = "23.04"; final double d1 = Double.parseDouble(s_d1); Docs: Integer#parseInt(String) Double#parseDouble(String) 18 / 29

19 PROGRAMMING STYLE AND DOCUMENTATION Comments Naming Conventions Indentation and Spacing Block Styles 19 / 29

20 PROGRAMMING STYLE AND DOCUMENTATION Comments All methods should have Javadoc explaining: Inputs / Outputs Brief description of what the method does including side effects Classes/Interfaces should have Javadoc explaining the purpose of the type Within methods, inline comments should describe unobvious code For example, an index variable in a for loop needs no comment 20 / 29

21 PROGRAMMING STYLE AND DOCUMENTATION Naming Conventions Variables are lowercamelcased Static constants are UPPERCASE_UNDERSCORED Non-static constants follow the same rule as variables, lowercamelcased Types are CapitalCamelCased 21 / 29

22 PROGRAMMING STYLE AND DOCUMENTATION Package Naming As said in Lecture 01, packages are nested e.g. java.lang.math implies Math is in the lang package within the package Unique package names are important when distributing library code Convention says to use the reverse of your domain name, e.g. eng.uwo.ca ca.uwo.eng Package names must be lowercased Docs: Lecture 01: Package 22 / 29

23 PROGRAMMING STYLE AND DOCUMENTATION Package Naming There is a one-to-one mapping of the package name to the filesystem location that source and compiled files must be placed Java looks up structures based on this path, known as the classpath 23 / 29

24 PROGRAMMING STYLE AND DOCUMENTATION Indentation / Spacing Camps are in turmoil over this debate For this course, you must use a tab width of 4 spaces 24 / 29

25 PROGRAMMING STYLE AND DOCUMENTATION Block Styles In Java, all braces go on the same line as the statement Closing braces line up with the statement they opened for class Test { } public void foo() { int i = 20; while (i!= 0) { i--; } } 25 / 29

26 LOOPS AND CONTROL STATEMENTS Because these controls are nearly identical to C++, only examples will be provided 26 / 29

27 LOOPS AND CONTROL STATEMENTS int i = 0; while (i!= 20) { System.out.println(i); } While loop 27 / 29

28 final int[] arr = { 0, 1, 2 }; LOOPS AND CONTROL STATEMENTS For-Loops for (int i = 0; i < 3; ++i) { System.out.print(arr[i] + ", "); // 5, 6, 7, } for (int i : arr) { System.out.print(i + ", "); // 5, 6, 7, } Arrays.stream(arr).forEach(System.out::println); 1. Normal for loop 2. "Enhanced" for loop (more commonly known as for-each) 3. foreach reduction (we'll cover it later) Docs: The for statement Arrays utility 28 / 29

29 LOOPS AND CONTROL STATEMENTS Branching Changing loop behaviour works by the same three control statements from C++ break Breaks out of the current loop continue Returns to "beginning" of loop and reruns it (conditions are checked as required) return Exits the currently executing method, calling no other code 29 / 29