java vs python(2)

If you know Java and want to quickly get a sense of how to use Python from the very beginning, the following summary can provide you a quick review of data types. You may also find the previouscomparison of Java and Python useful.

By comparing data types between Java and Python, you can get the difference and start using Python quickly. Comparision also can also help developers understand the common concepts shared by different programming languages.

Apparently, Java has more data types/structures than Python, so I will list the most similar concept from Java for the corresponding Python data types.

1. Strings

Java	Python
//string String city = "New York"; String state = "California";//has to be " not '   String lines = "multi-line " + "string";	# Strings city = "New York" state = 'California'   # multi-line string lines = """multi-line string""" moreLines = '''multi-line string'''

In python, string can reside in a pair of single quotes as well as a pair of double quotes. It supports multiplication: “x”*3 is “xxx”.

2. Numbers

Java	Python
//integer numbers int num = 100;   //floating point numbers float f = 1.01f; //float f = 1.01;//wrong!   double d = 1.01;	# integer numbers num = 100 num = int("100")   # floating point numbers f = 1.01 f = float("1.01")   # null spcial = None

In Java, when you type something like 1.01, its interpreted as a double.
Double is a 64-bit precision IEEE 754 floating point, while float is a 32-bit precision IEEE 754 floating point.
As a float is less precise than a double, the conversion cannot be performed implicitly.

3. Null

Java	Python
//null Object special = null;	# null spcial = None

4. Lists

Java	Python
//arraylist is closest with list in python ArrayList<Integer> aList = new ArrayList<Integer>();   //add aList.add(1); aList.add(3); aList.add(2); aList.add(4);   //index int n = aList.get(0);   //get sub list List<Integer> subList = aList.subList(0, 2); //1, 3	aList = [] aList = [1, 'mike', 'john']   #append aList.append(2)   # extend aList.extend(["new","list"])   print aList #[1, 'mike', 'john', 2, 'new', 'list']   aList = [0,1,2,3,4,5,6] # size print len(aList) #7   print aList[2] #2   print aList[0:3] #[0, 1, 2]   print aList[2:] #[2, 3, 4, 5, 6]   print aList[-2] #5   #lists are mutable aList[0] = 10 print aList #[10, 1, 2, 3, 4, 5, 6]

5. Tuples

Java	Python
No tuples in Java.	aTuple = () aTuple = (5) # cause error aTuple = (5,)   print aTuple print aTuple[0] #5

In Python, tuples are similar with lists, and the difference between them is that tuple is immutable. That means methods that change lists’ value can not be used on tuples.

To assign a single element tuple, it has to be: aTuple = (5,). If comma is removed, it will be wrong.

6. Sets

Java

Python

//hashset HashSet<String> aSet = new HashSet<String>(); aSet.add("aaaa"); aSet.add("bbbb"); aSet.add("cccc"); aSet.add("dddd");   //iterate over set Iterator<String> iterator = aSet.iterator(); while (iterator.hasNext()) { System.out.print(iterator.next() + " "); }   HashSet<String> bSet = new HashSet<String>(); bSet.add("eeee"); bSet.add("ffff"); bSet.add("gggg"); bSet.add("dddd");   //check if bSet is a subset of aSet boolean b = aSet.containsAll(bSet);   //union - transform aSet //into the union of aSet and bSet aSet.addAll(bSet);   //intersection - transforms aSet //into the intersection of aSet and bSet aSet.retainAll(bSet);   //difference - transforms aSet //into the (asymmetric) set difference // of aSet and bSet. aSet.removeAll(bSet);

aSet = set() aSet = set("one") # a set containing three letters #set(['e', 'o', 'n'])   aSet = set(['one', 'two', 'three']) #set(['three', 'two', 'one']) #a set containing three words   #iterate over set for v in aSet: print v   bSet = set(['three','four', 'five'])   #union cSet = aSet | bSet #set(['four', 'one', 'five', 'three', 'two'])   #intersection dSet = aSet & bSet   #find elements in aSet not bSet eSet = aSet.difference(bSet)   #add element bSet.add("six") #set(['four', 'six', 'five', 'three'])

7. Dictionaries

Dictionaries in Python is like Maps in Java.

Java

Python

HashMap<String, String> phoneBook = new HashMap<String, String>(); phoneBook.put("Mike", "555-1111"); phoneBook.put("Lucy", "555-2222"); phoneBook.put("Jack", "555-3333");   //iterate over HashMap Map<String, String> map = new HashMap<String, String>(); for (Map.Entry<String, String> entry : map.entrySet()) { System.out.println("Key = " + entry.getKey() + ", Value = " + entry.getValue()); }   //get key value phoneBook.get("Mike");   //get all key Set keys = phoneBook.keySet();   //get number of elements phoneBook.size();   //delete all elements phoneBook.clear();   //delete an element phoneBook.remove("Lucy");

#create an empty dictionary phoneBook = {} phoneBook = {"Mike":"555-1111", "Lucy":"555-2222", "Jack":"555-3333"}   #iterate over dictionary for key in phoneBook: print(key, phoneBook[key])   #add an element phoneBook["Mary"] = "555-6666"   #delete an element del phoneBook["Mike"]   #get number of elements count = len(phoneBook)   #can have different types phoneBook["Susan"] = (1,2,3,4)   #return all keys print phoneBook.keys()   #delete all the elements phoneBook.clear()