INFO1113. Calum Baird. October 5, Stuff Always check null Inline declare array Switch statement...

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1 INFO1113 Calum Baird October 5, 2018 INFO1113 Notes Contents 1 Stuff Always check null Inline declare array Switch statement Unified Modelling Language 3 3 Files Opening file Reading from file Writing to file Classes Encapsulation File IO Text IO Binary IO Reading our own bytes Reading with ObjectInputStream Run time data areas Frames Garbage collection Collections 9 8 Overloading 9 1

2 9 Inheritance / covariant Overidding Constructors Calling parent methods Polymorphism Abstract class Interfaces is-a has-a relationship Exceptions Constructors Overloading Generics Implementing iterator for a for:each Generic Arrays enum Memoization - Caching General Objects HashMap Collections and Maps ArrayList HashSet HashMap LinkedList Math Check instance

3 1 Stuff 1.1 Always check null if (str == null) { return -1; 1.2 Inline declare array new double[] {1.0, 2.6, Switch statement switch (num) { case 0: System.out.println("num is zero"); break; case -1; System.out.println("asdsad"); break; default: System.out.println("If nothing else") 2 Unified Modelling Language private + public # protected package protected (default) 3

4 3 Files 3.1 Opening file import java.io.file; import java.util.scanner; File f = new File("file.txt"); try { Scanner scan = new Scanner(f); catch (FileNotFoundException e) { System.err.printlnt("File not found :("); e.printstacktrace(); 3.2 Reading from file scan.next(); // Next word // Iterate file while (scan.hasnextline()) { System.out.println(scan.nextLine()); // nextint(), nextfloat(), Writing to file import java.io.printwriter; PrintWriter writer = new PrintWriter(f); writer.println("writing to file"); writer.flush(); // Empties buffer writer.close(); // Automatically flush() 4 Classes 4.1 Encapsulation + public, can be accessed by anythin - private, only accessed by itself # protected, restricted to subclasses, children package protected, can only be used within the package 5 File IO 5.1 Text IO 4

5 import java.util.*; import java.io.*; public class FileIO { public static void main(string[] args) { PrintWriter outputstream = null; // Writing to text file try { outputstream = new PrintWriter("out.txt"); catch(filenotfoundexception e) { outputstream.println("use normal printwriter methods"); outputstream.close(); // Includes outputstream.flush(); // Appending to text file try { FileOutputStream f = new FileOutputStream("out.txt", true); outputstream = new PrintWriter(f); catch(filenotfoundexception e) { outputstream.println("appending another line"); outputstream.close(); // Reading from text file Scanner inputstream = null; try { inputstream = new Scanner(new File("out.txt")); catch(filenotfoundexception e) { // Print the file contents while (inputstream.hasnextline()) { System.out.println(inputStream.nextLine()); inputstream.close(); // good practice 5.2 Binary IO We can store objects or primitive types in a binary file. Every primitive type in java has a sign bit so we need to be careful of signs 5

6 when reading bytes from a binary file. Be careful of big and little endian. Big endian, most significant bit first (lowest address) Reading our own bytes import java.io.*; import java.util.*; public class ManualBinaryIO { static int bytestoint(byte[] b) { // int value = 0; // int ls = 0x0; // for (int i = 0; i < b.length; i++) { // value = ((b[i] & 0x8) << ls); // ls += 0x8; // // return value; return b[3] (b[2] << 8) (b[1] << 16) (b[0] << 24); static byte[] convert (int v) { byte[] b = new byte[4]; b[0] = (byte) (v >> 24); b[1] = (byte) (v >> 16); b[2] = (byte) (v >> 8); b[3] = (byte) (v >> 0); return b; public static void main(string[] args) { // Write binary data to a file FileOutputStream fout = null; try { fout = new FileOutputStream("test_binary.bin"); // fout.write((byte)0); // fout.write((byte)1); // fout.write((byte)0); byte[] bytes = convert(128); // for (byte b : bytes) { // fout.write(b); // fout.write(bytes); 6

7 fout.close(); catch (Exception e) { // Read from the file FileInputStream fin = null; byte[] buffer = new byte[16]; // Buffer for reading in try { fin = new FileInputStream("test_binary.bin"); fin.read(buffer); // Will read 16 bytes //.read() also returns the amount of bytes it has read // for (byte b : buffer) { // System.out.println((int)b); // System.out.println(bytesToInt(buffer)); fout.close(); catch (IOException e) { e.printstacktrace(); Reading with ObjectInputStream import java.util.*; import java.io.*; public class BinaryIO { public static void main(string[] args) { // Write to binary file try { ObjectOutputStream outputstream = new ObjectOutputStream( // Will make new file if it doesnt exist new FileOutputStream("numbers.dat") ); // Write int to file int x = new Scanner(System.in).nextInt(); // writedouble, writechar etc for primitives // writeutf(string astring) UTF encoding // Unicode Text Format outputstream.writeint(x); 7

8 outputstream.writeutf(new Scanner(System.in).nextLine()); // Writing an object //public void writeobject(object o) throws IOException, // NotSerializableException, InvalidClassException Double y = new Double(3.141); outputstream.writeobject(y); outputstream.close(); catch(filenotfoundexception e) { catch(ioexception e) { // For writeobject // catch(notserializableexception e) { // // catch(invalidclassexception e) { // // Read from binary file try { ObjectInputStream inputstream = new ObjectInputStream( new FileInputStream("numbers.dat") ); System.out.println(inputStream.readInt()); System.out.println(inputStream.readUTF()); while (true) { // Read until EOF character. System.out.println(inputStream.readObject()); inputstream.close(); catch(filenotfoundexception e) { catch(eofexception e) { catch(ioexception e) { // For readobject catch(classnotfoundexception e) { 8

9 6 Run time data areas pc (Program Counter) register. Each JVM thread has its own pc register. JVM Stacks, each thread has a stack of frames for each method that is invoked. Allocated in heap? heap, Stores objects. shared among all JVM threads. Created at virtual machine start up Method Area, shared between JVM threads, hold every method. 6.1 Frames Stores data and partial results. Has its own: Array of local variables Operand Stack reference to runtime constants of its class The stack frame is stored in the heap and deallocated when the method finished. 6.2 Garbage collection index.html A Stop the World Event is when no other threads are running until the event finishes. All minor garbage collections are Stop the World Events. 7 Collections ADT - Abstract Data Type 8 Overloading Overloading only possible if the return type is the same. We can explicity declare which type int[] crossproduct((int[]) null, ( int[]) null) 9

10 9 Inheritance / covariant We make a general (aggregate?) class and then classes that extends that class. If we have a class Student that extends a class Person. If Person has a method like setname() Student inherits this method. A class should only derive from another class if a is a relationship exists. Parent, Ancestor Child, Descendant public class Student extends Person Use protected variables, so children can access it Overidding If a decendant class has a method with same name, input types and return type it overrides the method in the ancestor. If we did not want a child to overide the method we could: public final void specialmethod() We should override the tostring() method in classes we make. It comes from the Object eve class. println calls println(object.tostring) Constructors A child class uses the constructor of its parent. We can achieve this explicitly with the super keyword. public Student(String initialname, int initialstudentnumber) { // Calls the constructor of the Person class with the initialname argument super(initialname); studentnumber = initialstudentnumber; To call a constructor of the same class we can use: this("input to constructor") Similarly can use parent constructor super( "input to parent constructor") Calling parent methods super.printoutput() We cannot train supers together Polymorphism A class that takes on multiple forms. We can use abstract to make classes that cannot be created but have attributes. 10

11 9.0.5 Abstract class We cant instantiate an abstract class. It is a generalisation of a series of potential subclasses. Define with abstract keyword Interfaces we can make a interface which is similar to an abstract class but doesnt make any sense as an object by itself. public class Person implements Swim, Talk, Run; 10 is-a has-a relationship 11 Exceptions Havent read textbook for this specifing throws specifies what a method throws. throw new Exception() - forces user of method to catch exception throw new RuntimeException() - wont force user of method to catch exception 12 Constructors 13 Overloading 14 Generics public class Container<T extends Object1> We can go on to use this generic type T to define methods and variables. We cant just use Object due to casting etc..? Compiler can vverify that what we are doing is correct in terms of objects. 15 Implementing iterator for a for:each A for:each loop utilises transverses an Iterator. We must implement our class with Iterator and Iterable for it to function properly, some functions exist by default in these interfaces but others we have to define ourselves explicitly

12 class LinkedList<T> implements Iterator<T>, Iterable<T> { public Node<T> head; // Iterator public Node<T> iternode; public boolean hasnext(){ if (iternode.next == null) { return false; else { return true; public T next() { if (!hasnext()) { return null; else { iternode = iternode.next; return iternode.element; // Iterable public Iterator<T> iterator() { iternode = new Node<T>(null); iternode.next = head; return this; 15.1 Generic Arrays We can declare a generic array T[] array; however we cannot instantiate a generic array. This is to enforce type safety. String[] strings = {"one","two"; Object[] objects = strings; // Valid as arrays are COVARIANT objects[0] = 1; // Valid in compile time // This throws a runtime error ArrayStoreException as the array is a string array in memory space. covariance - can store differing child types We couldnt perform the above code with a List as it is not covariant. We can use generics with array = new (T[]) Object[10] 12

13 16 enum import java.util.arrays; import java.util.arraylist; public enum Weekdays { Sunday(true), Monday(false), Tuesday(false), Wednesday(false), Thursday(false), Friday(false), Saturday(true); private boolean isweekend; // No access modifier for enum constructor Weekdays(boolean isweekend) { this.isweekend = isweekend; public static void main(string[] args) { int n = Integer.parseInt(args[0]); // Turn enum labels to array Weekdays[] days = Weekdays.values(); System.out.printf( "Day %d of the week is: %s. ", n, days[n]); System.out.printf( "It is %sa weekend\n", days[n].isweekend? "" : "not "); 17 Memoization - Caching Storing the result so that we dont recalculate every time. 18 General Objects 18.1 HashMap import java.util.hashmap; HashMap<String,String> dict = new HashMap<String,String>(); dict.put("key", "Value"); // Add an item to the dictionary 13

14 // Iterate through dictionary for (String x : dict.keyset()) { // Access value. System.out.println(dict.get(x)); 18.2 Collections and Maps An interface that some objects utilise to store multiple objects ArrayList import java.util.arraylist; public class UseArrayList { public static void main(string[] args) { // <String> is type the list holds // 20 is the initial size of allocated memory. Default 10. // Define big as reallocation takes time. ArrayList<String> list = new ArrayList<String>(20); // Could do: // List<String> list = new ArrayList<String>(20); // Where List is aggregate type list.add("on the end"); // Adds strig to the end of the list // List is empty so puts at start list.set(0, "Overwrite zeroth element"); list.get(0); // Returns element at zeroth element. list.add(0, "Insert to the the start"); list.size(); // Size of list for (String s : list) { System.out.println(s); HashSet Stores objects without order, there can be no duplicates. Utilises a hash table to store the Objects. Ignores when try to re add an object. 14

15 HashMap LinkedList Our node: 15

16 // We should make this private?? // And with setters and getters obviously public class ListNode { public String data; public ListNode link; public ListNode(String data) { this.data = data; Our linked list: public class StringLinkedList { public ListNode head; public int length() { int count = 0; ListNode node = this.head; while (node!= null) { count++; node = node.link; return count; public static void main(string[] args) { StringLinkedList list = new StringLinkedList(); list.head = new ListNode("top of linked list"); list.head.link = new ListNode("adding a node"); list.head.link.link = new ListNode("recursion??"); System.out.println(list.length()); 18.3 Math import java.lang.math; Math.abs(); Math.ceil(); 18.4 Check instance Object instanceof Object 16