Python Class Definition. Java Class Definition. Python Class Use public class Student { Java Class Use. Python Class Definition 10/6/2016

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1 Python 2 Java Python Class Use jim = Student( Jim, 95) print jim.getname() k = jim.getgrade() jim.setgrade(k + 5) Java Class Use Student jim = new Student( Jim, 95); Sytem.out.println(jim.getName()); int k = jim.getgrade(); jim.setgrade(k + 5); class definition 1

2 Class definition variables that hold info stored in the class and any object that is of that class. Always global. Variables Local variables Parameter variables Instance variables Variables that hold info stored in the class and any object that is of that class Called instance variables. Should be usually be private. private means only seen within the class. public would mean it s global. local, parameter, & instance variables. public void computegrade(int gradenew) { int gradetotal = grade + gradenew; grade = gradetotal / 2; Variables that hold info stored in the class and any object that is of that class Called instance variables. Should be usually be private. private means only seen with in the class. public would mean it s global. 2

3 constructor used to create an object of that class assigning values to the instance variables. constructor used to create an object of that class assigning values to the instance variables. methods to manipulate the instance variables. methods to manipulate the instance variables. name = name; grade = grade; grade = grade; What if we want to name our input parameters more appropriate names such as name instead of name0 and grade instead of grade0? Does this work? 3

4 name = name; grade = grade; grade = grade; No, the compiler can t tell which variable is which. Java Constructors public Student(String name) { public Student() { name = NoName ; 4

5 Python Class Use jim = Student( Jim, 95) print jim.getname() k = jim.getgrade() jim.setgrade(k + 5) Would This Work? jim = Student( Jim, Johnson ) print jim.getname() k = jim.getgrade() jim.setgrade(k + 5) Java Class Use Arrays Student jim = new Student( Jim, 95); Sytem.out.println(jim.getName()); int k = jim.getgrade(); jim.setgrade(k + 5); Java arrays are like Python lists except that arrays are fixed in length (number of elements). In addition, Java arrays are made up of elements of the same type, whereas Python list can have elements from several types. To declare a Java array, you need to specify the type and number of elements in the array. int[] grades = new int[10]; 5

6 Array Value Assignment Individual Element Access Value assignment can also be done by using a list of element values. Python grades = [90, 86, 57, 95] Java int[] grades = {90, 86, 57, 95; Access and assignments, as in Python are made using an index. grades[2] = 95; // sets the element at index 2 The index starts at 0, so grades[0] is the first element. Java Arrays Methods Python provides several methods for manipulating lists. Since arrays are more restricted, the Java methods provided are more limited. Some are highlighted on the following slides. length int[] grades = {90, 86, 57, 95; grades.length returns 4 arraycopy Parameters: source_array source_start_position destination_array destination_start_position number_elements_to_copy. int[] grades1 = {90, 86, 57, 95; int[] grades2 = {70, 70, 70, 70; System.arraycopy( grades1, 1, grades2, 0, 2); clone int[] grades = {90, 86, 57, 95; int[] backup; backup = (int[]) grades.clone(); backup is an exact copy of grades. grades2 would now be {86, 57, 70, 70 6

7 Student Class Array of Objects Student[] cs_students = new Student[40]; cs_students[2] = new Student("Jim", 95); cs_students[2].getname() returns Jim cs_students[2].getgrade() returns 95 cs_students[2].setgrade(100); cs_students[2].getgrade() returns 100 Arrays Inheritance Java arrays are fixed in length (number of elements). Java arrays are made up of elements of the same type. Python class CollegeStudent(Student): new methods Java class CollegeStudent extends Student { new instance variables new methods Static Variables Could be called Class Variables Variable is associated with the Class, not objects of the Class. Only one is stored for the Class and all of its objects, no matter how many objects there are. Example: private static int numstudents private static int numstudents;... Static Variables Example of a static variable added to the Student Class. In this example, the static variable can be used to keep track of how many Student objects declared. 7

8 Static Final Variables Actually, not a variable, but a constant. The final makes it a constant. Also associated with the Class, not objects of the Class. Only one is stored for the Class and all of its objects, no matter how many objects there are. Example: private static final int MAX_NUM_STUDENTS = 20; 8