10/22/2018 Programming Data Structures

Transcription

1 10/22/2018 Programming Data Structures Quiz 2 on Wednesday, Oct. 24! Assignment14 Quiz Content Recursion 1

2 Assignment14 public static int readlines(string filename) throws FileNotFoundException { FileInputStream stream = null; try { stream = new FileInputStream(filename); catch(filenotfoundexception e) { e.getmessage();... return linenumber; 2

3 Assignment14 public static int readlines(string filename) throws FileNotFoundException { FileInputStream stream = null; try { stream = new FileInputStream(filename); catch(filenotfoundexception e) { e.getmessage();... return linenumber; Q1: Which part of this method is the throw clause? 3

4 Assignment14 public static int readlines(string filename) throws FileNotFoundException { FileInputStream stream = null; try { stream = new FileInputStream(filename); catch(filenotfoundexception e) { e.getmessage();... return linenumber; Q2: This method has two incorrect places. What are they? 4

5 Assignment14 System.out.println(lognumbers("readme1.txt")); public static MyNumber lognumbers(string filename) {... public class MyNumber { int[] arr; public MyNumber(int size){ arr = new int[size]; public String tostring() { return "arr size: " + arr.length; 5

6 Assignment14 System.out.println(lognumbers("readme1.txt")); public static MyNumber lognumbers(string filename) {... public class MyNumber { int[] arr; public MyNumber(int size){ arr = new int[size]; public String tostring() { return "arr size: " + arr.length; Q3: System.out.println prints: arr size: 100 Why the above line is printed? 6

7 Quiz Content array classes, instance variables abstract classes exception handling file IO static method overloading 7

8 Static public static void main(string[] argv) { try{ System.out.println(readlines("readme1.txt")); catch (FileNotFoundException e) { System.out.println(e.toString()); public static int readlines(string filename) throws FileNotFoundException {... Why does readlines need to be static? 8

9 Static A static method is one that can be used without a calling object. With a static method, you normally use the class name in place of a calling object. A static method cannot use an instance variable of the class. A static method cannot invoke a non-static method of the class (unless it creates a new object of the class). 9

10 Static - Practice Is the following code correct? public class Class1 { int a = 100; public static void method() { System.out.println(a); 10

11 Static - Practice Is the following code correct? public class Class1 { int a = 100; public static void method() { method2(); public void method2() { System.out.println(a); 11

12 Static - Practice Is the following code correct? public class Class1 { int a = 100; public static void method() { Class1 object = new Class1(); object.method2(); public void method2() { System.out.println(a); 12

13 Overloading You can have two (or more) definitions of a single method name. This is called overloading. The different definitions can have the same method name but need to have different signatures. 13

14 public class OverloadingDemo { private int month; private int day; private int year; public void setdate(int monthint, int day, int year) { this.month = monthint; this.day = day; this.year = year; public void setdate(string monthstring, int day, int year) { this.month = getmonthnumber(monthstring); this.day = day; this.year = year; private int getmonthnumber(string monthname) { return Month.valueOf(monthName.toUpperCase()).getValue(); 14

15 Declaration vs definition Declaration: You are declaring that something exists, such as a class, function or variable. You don't say anything about what that class or function looks like, you just say that it exists. Definition: You define how something is implemented, such as a class, function or variable, i.e. you say what it actually is. 15

16 Declaration vs definition in Java Classes/Variables: Java doesn't really separate declarations and definitions. You define them at the point where you declare them. Functions: When you're writing an abstract class, you could say that you're declaring a function, without defining it. Ordinary functions however, are always defined right where they are declared. The body of the function can be seen as its definition. The signature of the function can be seen as its declaration. 16

17 Practice public abstract class Class2 { public abstract void method1(); public void method2() { return; 17

18 Recursion a method definition that includes a call to itself is said to be recursive. Recursive methods can be written in different programming languages. 18

19 Recursive method: writevertical Given an input: 1234 Write a method to print:

20 Recursive method: writevertical public static void writevertical(int number) { int remainder = number%10; int quotient = number/10; if(quotient > 0) { writevertical(quotient); System.out.println(remainder); 20

21 Recursive method: writevertical public static void writevertical2(int number) { if(number < 10) { System.out.println(number); else { int remainder = number%10; int quotient = number/10; writevertical(quotient); System.out.println(remainder); 21