Python and Java - Comparisons and Contrasts

Python and Java are the same here.

In Python, standard library functions usually have lowercase names.

In Java, standard library functions usually have "camelCase" names (each word but the first begins with a capital letter).

String exampleString;

Class files for a project in Python are contained with in a source folder.

With java, classes for a project are typically stored within a package
for purposes of visibilty and accessability.

Python: Can run any file in the project.

Java: Must have a class in the project that defines a main() method.

If a Java source file contains the definition of a public class or interface named XXX, the filename must be XXX.java  .  A source file can contain at most one definition of a public class.  It can include multiple non-public class definitions (nested classes).

Executes the code in order from beginning to end of the file.

main() is optional, and has no special meaning in Python.

That means that Java starts in the class in the project containing main(), but main() can call methods from other classes.

main() needs to be declared as public static void main in Java to work.

   public static void main(String[] args) {
	 // body of main()
   }

(assumes that the folders containing python.exe and java,exe are in your execution path)

Example with Java:

Java interprets /** as beginning a documentation string. Javadoc can contain special tags such as @author and @param to further describe
methods or fields that are commented

Python interprets the first string of a function/class/method (usually set aside with triple quotes) as its documentation.

The following is illegal:

/* cannot have /* comment inside */ comment */

Required, as in:

It's still a good idea to have good, consistent indentation.

In java spacing and alignment is more stylistic per programmer than anything else.

If the class you want to use is in another package,
import packagename.classname;
or
import packagename.*;

In Java, we may not write x = 5.6; unless x has been previously declared:

int i;
int n = 5;
double x=7.3, y=8.2, z;
				

Not allowed in Java

Identical in Python and Java (except for the semicolon)

Identical in Python and Java

x++, ++x, x--, --x  (as in C)

No exponentiation operator in Java: instead, use Math.pow(x,6). For small integer powers, it's better to use x*x or x*x*x because the pow function is a resource expensive operation.

<   <=   >  >=   ==   !=

&&   ||   !

Documentation is online at Sun's website.  

You can also download the documentation so you can access it on your own computer, and get context-sensitive help in Eclipse.  Instructions are at 
http://www.rose-hulman.edu/class/csse/resources/JavaDevKit/installation.htm (see the part on documentation near  the bottom of the page) and
http://www.rose-hulman.edu/class/csse/resources/Eclipse/eclipse-java-configuration.htm (see the part on Java documentation near  the bottom of the page).

Javadoc is available for interfaces and classes online (Google search).

try:
	<code that could throw an exception>
except:
	<code to handle the exception>
				

try {
	<code that could throw an exception>
}
catch (ExceptionType e) {
	<code to handle the exception>
}				

sum = 0

for i in range(5):
	sum += i
print i, sum
	 			

int i, sum=0;
for (i=0; i<5; i++) {
	sum += i;
}

System.out.print(i);
System.out.print(' ');
System.out.println(sum);
				

for <variable> in <list>:
	 <one or more indented statements>
	 			

for (<initialization> ; <test>; <update>) {
	 <zero or more body statements>
}  

for (int i : numList) {
	 <zero or more body statements>
}
				

initialization: usually gives initial value to loop variable(s)

test: If the value of the test is true, the loop continues.

update: After each execution of the loop body, this code is executed, and the test is run again.

The second form of the for loop works for the elements of an array.

In Java, a block can be used anywhere a single statement can be used.

If a Java block only contains one statement, the braces are optional as with an if-test that has one resulting statement.

Classes and methods in java must be closed in braces.

for (<initialization> ; <test>; <update>) 
	 <statement>
	 			

In Java, a single semicolon with nothing in front of it indicates an empty statement. Just like Python's pass, it is used where the syntax requires a statement, but there is nothing to do there.

while <condition>:
	<one or more indented statements> 
				

while ( <condition> )
	<statement>
				

Immediately end the execution of the current loop

Immediately end the execution of this time through the current loop, then continue the loop's next execution.

if <condition>:
	<one or more indented statements> 
				

if ( <condition> )
	<statement>
				

Java Syntax is just like C.

Parentheses are required. <statement> may be a single statement or a block of statements surrounded by { ... }

if <condition>:
	<one or more indented statements>
else:
	<one or more indented statements> 
			

if <condition>:
	<one or more indented statements>
elif <condition>:
	<one or more indented statements>
else:
	<one or more indented statements> 

if ( <condition> )
	<statement>
else if ( <condition> )
	<statement>
else
	<statement>

do {
	statement;
	...
	statement;
	} while (condition);
			

In a while loop, the test is at the beginning.  So it is possible that the loop body never executes at al.  In a do loop, the test is at the end, so the loop body always executes at least once (main reason to use a do-while over a while loop).

switch (<variable>) {
	case <value1> :
		<statement1>
		break;
	case <value2> :
		<statement2>
		break;
	...
	default:
		<statement>
}
				

Java Syntax is just like C.

Python has nothing that is like Java's switch statement ( if-elif-else is used)

If there are multiple statements for a given case, no curly braces are needed.

Don't forget the break; at the end of each case (except the last) or else the value could be switched to a different case.

Conditional expression. Can be used anywhere an expression can be used. For example:,

public static int max (int a, int b) {
   return (a > b) ? a : b;
}

Java Syntax is just like C.

int type is typically 32-bit.

long type is sometimes 64-bit, but always at least 32-bit.

There is no automatic conversion of values.

Lists are almost a primitive type: there is native syntax for creating lists, there are keywords to modify lists, and the built in len function can tell you their lengths.

Lists are in the standard library in the ArrayList class which can only store one type of object (although you can use inheritance to store multiple types). The builtin language features only arrays, which function similarly to C arrays.

The Python list can change size later.  The Java array will always have four elements; the individual elements may change, however.

Note that Java has an ArrayList class with much of the functionality of Python's lists, but no special list syntax like Python has.

list/array element assignment.

Java does not have this feature

In many cases in Java, an explicit call to toString() is not required, because Java puts it  in for you when needed.  Examples:  in print or println, and in string concatenation.

If x is an object, System.out.print(x); is equivalent to System.out.print(x.toString());

Python strings are basically lists of characters: you can perform all list operations on them.

In Python, print is a keyword, which can take a list and prints the elements of the list one at a time.

In Java, the print and println methods take a single argument. To print multiple things with one call, you need to convert them all to strings and concatenate the strings. Can also use printf for print formatting similar to what you learned in C.

CSSE 120 uses a graphics library built on top of the builtin Python graphics library, Tkinter.

Objects are regarded as an active data type - "know stuff and can do stuff"

An instance of a class that contains instance variables (know stuff) and use methods to operate on instance variables (can do stuff).

Objects are created using a constructor.

Objects send each other messages
       p.getX()     accessor method

Interaction of these objects is the basic idea of Object Oriented Development.

Same concepts as in Python.

An object is an atomic unit, meaning that its parts cannot be dissected and viewed by general users.

Have structure and state and define operations that may access or manipulate that state..

Initialized by a constructor.

Required: code resides in classes, main is a method, and all functions are implemented as methods. Main creates an instance of a class (object) that contains fields and is oeprated on by methods.

A class in Java consists of fields the store data and methods that are applied to instances of a class. An object is the instance of a class.

Objects provide for encapsulation, which is the hiding of functional details of a class from outside methods. OOD is also based on polymorphism, which gives a reference variable the ability to reference objects of several different types. One last significant characteristic of object-oriented development in Java is inheritance.

This is a very basic overview of objects in Java. For a more in-depth description consult your textbook.

Object-oriented programming in either language refers to the concept of using objects to define fields pertaining to the object and operations that may access or manipulate its state.

An object is an instance of a class that contains functions and data, known as fields and methods. Object-oriented programming utilizes the object to interact with other classes and other objects in order to solve the problem at hand...

Java forces some level of use of object-oriented programming, whereas Python allows for other methods of program design.

Assuming you have Student and Person classes, and a student IS-A Person, you'd write:

class Student(Person): .

All classes inherit from Object

In java, a class can only inherit from one class, but can inherit from multiple interfaces. If a class extends an interface, it must include some implementation of all of the methods within that interface (even thay are merely passed over).

IS-A and HAS-A relationships appear in inheritance. Referring to shape classes, the shape class HAS-A circle class, and since the circle class extends shape, the circle class IS-A shape class.

In inheritance, the derived class must either override or accept the methods of the base class. Construct a derived class object by first constructing the inherited portion of the base class using super to call the base class constructor.

In the example to the left, you'd write:

public class Student extends Person {

The Object class gives all Java objects certain properties, such as rudimentary y toString() and hashCode().

The object is the foundation of all object-oriented programming.

In Java, the use of this in many contexts (including the one shown here) is optional.  But it is encouraged..

Both self and this are used to access private data fields or methods of an object within a class.

 def move(self, deltax, deltay):
	self.x += deltax
	self.y += deltay

  int move(int deltax, int deltay){
	this.x += deltax;
	this.y += deltay;
  }

Both are called in the same way, p.move(5,7) (in Java a semicolon is also needed).

In Python the declaration of self as the first parameter is what distinguishes it as a method instead of an ordinary function.

In Java, methods are the only kind of functions there are.

Both languages can convert objects to strings, but the name of the method to override is different.

from math import *

def printRootTable(n):
	for i in range(1,n):
		print " %2d %7.3f" % (i, sqrt(i))

def main():
	printRootTable(10)

main()

public class RootTable {
	public static void printRootTable(int n) {
		int i;
		for (i=1; i<=n; i++) {
			System.out.printf(" %2d %7.3f\n", i, Math.sqrt(i));
		}
	}
	
	public static void main(String args[]) {
	      RootTable.printRootTable(10);
	}
}

If a Java function does not return a value, you must declare its return type as void. A python function always returns a value (if no return statement is present, None is returned).

If a java function is to return a value, the method header must have the type of that value declared. This example header returns a String.

	public static String returnAString(){
		...
	}

m,n = input("Enter two numbers: ")
print m, n

import java.util.Scanner;

public class InputTwo {
	public static void main(String args[]) {
		int i, j;
		System.out.print("Enter two numbers: ");

		Scanner s = new Scanner(System.in)

		i = s.nextInt();
		j = s.nextInt();

		System.out.print(i);
		System.out.print(' ');
		System.out.println(j);
	}
}

In the Python example, the numbers in the input must be separated by commas.

In java, an input scanner can be used, which parses primitive types and strings using assignment expressions. A BufferedReader can also be used, which stores the input from an InputStreamReader object.

Public data can be accessed anywhere (like data in Python), whereas private data can only be accessed from within the class, and protected data only from within the class or classes inherited from it.

Declaring a final value in java:

public static final int MAX_WIDTH = 100;

This statement declares a final variable in java that is public, but cannot be changed.