# java4python 3.0 documentation

## Lists and Maps

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# Lists and Maps¶

## Numeric¶

One of the great things about Python is that all of the basic data types are objects. Integers are objects, floating point numbers are objects, lists are objects, everything. In Java that is not the case. In Java some of the most basic data types like integers and floating point numbers are not objects. The benefit of having these primitive data types be non-objects is that operations on the primitives are fast. The problem is that it became difficult for programmers to combine objects and non-objects in the way that we do in Python. So, eventually all the non-object primitives ended up with Objectified versions.

Primitive Object
int Integer
float Float
double Double
char Char
boolean Boolean

In older versions of Java it was the programmers responsibility to convert back and forth from a primitive to an object whenever necessary. This processing of converting a primitive to an object was called “boxing.” The reverse process is called “unboxing.” In Java 5, the compiler became smart enough to know when to convert back and forth and is called “autoboxing.” In general, you should always use the primitive form, since computation always happens on the unboxed form, and boxing and unboxing are not free.

Lets go back in time and look at another of our very early Python programs. Here is a simple Python function to convert a Fahrenheit temperature to Celsius.

def main():
fahr = input("Enter the temperature in F: ")
cel = (fahr - 32) * 5.0/9.0
print "the temperature in C is: ", cel


Next, lets look at the Java Equivalent.

  1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 import java.util.Scanner; public class TempConv { public static void main(String[] args) { double fahr; double cel; Scanner in; in = new Scanner(System.in); System.out.println("Enter the temperature in F: "); fahr = in.nextDouble(); cel = (fahr - 32) * 5.0/9.0; System.out.println("The temperature in C is: " + cel); System.exit(0); } } 

There are several new concepts introduced in this example. We will look at them in the following order:

• Import
• Variable Declaration
• Input/Output and the Scanner Class

### Import¶

In Java you can use any class that is available without having to import the class subject to two very important conditions:

1. The javac and java must know that the class exists.
2. You must use the full name of the class

You first question might be how do the java and javac commands know that certain classes exist. The answer is the following:

1. Java knows about all the classes that are defined in .java and .class files in your current working directory.
2. Java knows about all the classes that are shipped with java.
3. Java knows about all the classes that are included in your CLASSPATH environment variable. Your CLASSPATH environment variable can name two kinds of structures.
1. A jar file that contains java classes
2. Another unix directory that contains java class files

You can think of the import statement in Java as working a little bit like the from module import xxx statement in Python. However, behind the scenes the two statements actually do very different things. The first important difference to understand is that the class naming system in Java is very hierarchical. The full name of the Scanner class is really java.util.Scanner. You can think of this name as having two parts: The first part java.util is called the package and the last part is the class. We’ll talk more about the class naming system a bit later. The second important difference is that it is the Java class loader’s responsibility to load classes into memory, not the import statement’s.

So, what exactly does the import statement do? What it does is tell the compiler that we are going to use a shortened version of the class’s name. In this example we are going to use the class java.util.Scanner but we can refer to it as just Scanner. We could use the java.util.Scanner class without any problem and without any import statement provided that we always referred to it by its full name. As an Experiment you may want to try this yourself. Remove the import statement and change the string Scanner to java.util.Scanner in the rest of the code. The program should still compile and run.

### Declaring Variables¶

Here is where we run into one of the most important differences between Java and Python. Python is a dynamically typed language. In a dynamically typed language a variable can refer to any kind of object at any time. When the variable is used, the interpreter figures out what kind of object it is. Java is a statically typed language. In a statically typed language the association between a variable and the type of object the variable can refer to is determined when the variable is declared. Once the declaration is made it is an error for a variable to refer to an object of any other type.

In the example above, lines 5—7 contain variable declarations. Specifically we are saying that fahr and cel are going to variables holding values of type double. The variable in will reference a Scanner object. This means that if we were to try an assignment like fahr = "xyz" the compiler would generate an error because "xyz" is a string and fahr is supposed to be a double.

For Python programmers the following error is likely to be even more common. Suppose we forgot the declaration for cel and instead left line 6 blank. What would happen when we type javac TempConv.java on the command line?

TempConv.java:13: cannot find symbol
symbol  : variable cel
location: class TempConv
cel = (fahr - 32) * 5.0/9.0;
^
TempConv.java:14: cannot find symbol
symbol  : variable cel
location: class TempConv
System.out.println("The temperature in C is: " + cel);
^
2 errors


When you see the first kind of error, where the symbol is on the left side of the equals sign it usually means that you have not declared the variable. If you have ever tried to use a Python variable that you have not initialized the second error message will be familiar to you. The difference here is that we see the message before we ever try to test our program. More common error messages are discussed in the section Common Mistakes.

The general rule in Java is that you must decide what kind of an object your variable is going to reference and then you must declare that variable before you use it. There is much more to say about the static typing of Java but for now this is enough.

### Input / Output / Scanner¶

In the previous section you saw that we created a Scanner object. In Java Scanner objects make getting input from the user, a file, or even over the network relatively easy. In our case we simply want to ask the user to type in a number at the command line, so in line 9 we construct a Scanner by calling the constructor and passing it the System.in object. Notice that this Scanner object is assigned to the name in, which we declared to be a Scanner on line 7. System.in is similar to System.out except of course it is used for input. If you are wondering why we must create a Scanner to read data from System.in when we can write data directly to System.out using println, you are not alone. We will talk about the reasons why this is so later when we talk in depth about Java streams. You will also see in other examples that we can create a Scanner by passing the Scanner a File object. You can think of a scanner as a kind of “adapter” that makes low level objects easier to use.

On line 11 we use the Scanner object to read in a number. Here again we see the implications of Java being a strongly typed language. Notice that we must call the method nextDouble. Because the variable fahr was declared as a double. So, we must have a function that is guaranteed to return each kind of object we might want to read. In this case we need to read a double so we call the function nextDouble. The compiler matches up these assignment statments and if you try to assign the results of a method call to the wrong kind of variable it will be flagged as an error.

Table 2 shows you some commonly used methods of the scanner class. There are many more methods supported by this class and we will talk about how to find them in the next chapter.

Return type Method name Description
boolean hasNext() returns true if more data is present
boolean hasNextInt() returns true if the next thing to read is an integer
boolean hasNextFloat() returns true if the next thing to read is a float
boolean hasNextDouble() returns true if the next thing to read is a double
int nextInt() returns the next thing to read as an integer
Float nextFloat() returns the next thing to read as a float
double nextDouble() returns the next thing to read as a double
String next() returns the next thing to read as a String

Of course Java is more well known for producing applications that have more of a user interface to them than reading and writing from the command line. Lets look at a version of our temperature control application that uses dialog boxes for input and output.

  1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 import javax.swing.*; public class TempConvGUI { public static void main(String[] args) { String fahrString; double fahr, cel; fahrString = JOptionPane.showInputDialog("Enter the temperature in F"); fahr = Double.parseDouble(fahrString); cel = (fahr - 32) * 5.0/9.0; JOptionPane.showMessageDialog(null,"The temperature in C is, " + cel); } } 

This example illustrates a couple of interesting points:

First, the function call JOptionPane.showInputDialog pops up a dialog box to allow you to enter a temperature. But, since you could enter anything into the text input box it returns a String. On the next line the string is converted into a double by the function Double.parseDouble. This is similar to what happens in Python when you call float() with either a string or an integer as the argument.

The next dialog box is JOptionPane.showMessageDialog. Notice that the first parameter is null In Java null serves the same purpose as None in Python. The first parameter is null because we do not have a ‘main window’ for this little application. When we look at creating full blown java programs with user interfaces, we will learn more about this parameter.

The second parameter is "The temperature in C is, " + cel. Now you may be thinking to yourself that this must surely be a violation of the strong typing I have been describing to you. After all you should not be able to add together a string and a double right? You are correct, however, all java objects have a method called tostring. The tostring method acts much like the Python method __str__() and is called automatically by the compiler whenever it makes sense to convert a Java object to a string.

## String¶

Strings in Java and Python are quite similar. Like Python, Java strings are immutable. However, manipulating strings in Java is not quite as obvious since Strings do not support an indexing or slicing operator. That is not to say that you can’t index into a Java string, you can. You can also pull out a substring just as you can with slicing. The difference is that Java uses method calls where Python uses Operators.

In fact this is the first example of another big difference between Java and Python. Java does not support any operator overloading. Table 3 maps common Python string operations to their Java counterparts. For the examples shown in the table we will use a string variable called “str”

Python Java Description
str[3] str.charAt(3) Return character in 3rd position
str[2:5] str.substring(2,4) Return substring from 2nd to 4th
len(str) str.length() Return the length of the string
str.find('x') str.indexOf('x') Find the first occurrence of x
str.split() str.split('\s') Split the string on whitespace into a list/array of strings
str.split(',') str.split(',') Split the string at ',' into a list/array of strings
str + str str.concat(str) Concatenate two strings together
str.strip() str.trim() Remove any whitespace at the beginning or end

## List¶

Lets look at another early Python program. We are going to read numbers from a file and produce a histogram that shows the frequency of the various numbers. The data file we will use has one number between 0 and 9 on each line of the file. Here is a simple Python program that creates and prints a histogram.

def main():
count = [0]*10
data = open('test.dat')

for line in data:
count[int(line)] = count[int(line)] + 1

idx = 0
for num in count:
print idx, " occured ", num, " times."
idx += 1


Now if we run this program on a data file that looks like this:

9 8 4 5 3 5 2 1 5

We will get output that looks like this:

0 occurred 0 times
1 occurred 1 times
2 occurred 1 times
3 occurred 1 times
4 occurred 1 times
5 occurred 3 times
6 occurred 0 times
7 occurred 0 times
8 occurred 1 times
9 occurred 1 times


Lets review what is happening in this little program. In the first line we create a list and initialize the first 10 positions in the list to be 0. Next we open the data file called ‘test.dat’ Third, we have a loop that reads each line of the file. As we read each line we convert it to an integer and increment the counter at the position in the list indicated by the number on the line we just read. Finally we iterate over each element in the list printing out both the position in the list and the total value stored in that position.

To write the Java version of this program we will have to introduce several new Java concepts. First, you will see the Java equivalent of a list, called an ArrayLlist. Next you will see three different kinds of loops used in Java. Two of the loops we will use are going to be very familiar, the third one is different from what you are used to in Python but is easy when you understand the syntax:

while
Used with boolean expression for loop exit condition.
for
Used to iterate over a sequence. This is very similar to for i in xxx where xxx is a list or string or file.
for
Used to iterate through a sequence of numbers. This is most similar to for i in range(), except the syntax is different.

Here is the Java code needed to write the exact same program:

  1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 import java.util.Scanner; import java.util.ArrayList; import java.io.File; import java.io.IOException; public class Histo { public static void main(String[] args) { Scanner data = null; ArrayList count; int idx; try { data = new Scanner(new File("test.dat")); } catch ( IOException e) { System.out.println("Sorry but I was unable to open your data file"); e.printStackTrace(); System.exit(0); } count = new ArrayList(10); for (int i =0; i<10;i++) { count.add(i,0); } while(data.hasNextInt()) { idx = data.nextInt(); count.set(idx,count.get(idx)+1); } idx = 0; for(int i : count) { System.out.println(idx + " occured " + i + " times."); idx++; } } } 

Before going any further, I suggest you try to compile the above program and run it on some test data that you create.

Now, lets look at what is happening in the Java source. As usual we declare the variables we are going to use at the beginning of the method. In this example we are declaring a Scanner variable called data, an integer called idx and an ArrayList called count. However, there is a new twist to the ArrayList declaration. Unlike Python where lists can contain just about anything, in Java we let the compiler know what kind of objects our array list is going to contain. In this case the ArrayList will contain Integers. The syntax we use to declare what kind of object the list will contain is the <*Type*> syntax.

Technically, you don’t have to declare what is going to be on an array list. The compiler will allow you to leave the <*Type*> off the declaration. If you don’t tell Java what kind of object is going to be on the list Java will give you a warning message like this:

Note: Histo.java uses unchecked or unsafe operations.
Note: Recompile with -Xlint:unchecked for details.


Without the <Integer> part of the declaration Python simply assumes that any object can be on the list. However, without resorting to an ugly notation called casting, you cannot do anything with the objects on a list like this! So, if you forget you will surely see more errors later in your code. (Try it and see what you get)

Lines 13—20 are required to open the file. Why so many lines to open a file in Java? The additional code mainly comes form the fact that Java forces you to reckon with the possibility that the file you want to open is not going to be there. If you attempt to open a file that is not there you will get an error. A try/catch construct allows us to try things that are risky, and gracefully recover from an error if one occurs. The following example shows the general structure of a try catch block.

try {
Put some risky code in here.... like opening a file
}
catch (Exception e) {
If an error happens in the try block an exception is thrown.
We will catch that exception here!
}


Notice that in line 16 we are catching an IOException. In fact we will see later that we can have multiple catch blocks to catch different types of exceptions. If we want to be lazy and catch any old exception we can catch an Exception which is the parent of all exceptions.

On line 22 we create our array list and give it an initial size of 10. Strictly speaking it is not necessary to give the ArrayList any size. It will grow or shrink dynamically as needed just like a list in Python. On line 23 we start the first of three loops. The for loop on lines 23–25 serves the same purpose as the Python statement count = [0]*10, that is it initializes the first 10 positions in the ArrayList to hold the value 0.

The syntax of this for loop probably looks very strange to you, but in fact it is not too different from what happens in Python using range. In fact for(int i = 0; i < 10; i++) is exactly equivalent to the Python for i in range(10) The first statement inside the parenthesis declares and initializes a loop variable i. The second statement is a Boolean expression that is our exit condition. In other words we will keep looping as long as this expression evaluates to true. The third clause is used to increment the value of the loop variable at the end of iteration through the loop. In fact i++ is Java shorthand for i = i + Java also supports the shorthand i-- to decrement the value of i. Like Python you can also write i += 2 as shorthand for i = i + 2 Try to rewrite the following Python for loops as Java for loops:

• for i in range(2,101,2)
• for i in range(1,100)
• for i in range(100,0,-1)
• for x,y in zip(range(10),range(0,20,2)) [hint, you can separate statements in the same clause with a ,]

The next loop (lines 27–30) shows a typical Java pattern for reading data from a file. Java while loops and Python while loops are identical in their logic. In this case we will continue to process the body of the loop as long as data.hasNextInt() returns true.

Line 29 illustrates another important difference between Python and Java. Notice that in Java we can not write count[idx] = count[idx] + 1. This is because in Java there is no overloading of operators. Everything except the most basic math and logical operations is done using methods. So, to set the value of an ArrayList element we use the set method. The first parameter of set indicates the index or position in the ArrayList we are going to change. The next parameter is the value we want to set. Notice that once again we cannot use the indexing square bracket operator to retrieve a value from the list, but we must use the get method.

The last loop in this example is similar to the Python for loop where the object of the loop is a Sequence. In Java we can use this kind of for loop over all kinds of sequences, which are called Collection classes in Java. The for loop on line 33 for(int i : count) is equivalent to the Python loop for i in count: This loop iterates over all of the elements in the ArrayList called count. Each time through the loop the int variable i is bound to the next element of the ArrayList. If you tried the experiment of removing the <Integer> part of the ArrayList declaration you probably noticed that you had an error on this line. Why?

## Arrays¶

As I said at the outset of this Section we are going to use Java ArrayLists because they are easier to use and more closely match the way that Python lists behave. However, if you look at Java code on the internet or even in your Core Java books you are going to see examples of something called arrays. In fact you have already seen one example of an array declared in the ‘Hello World’ program. Lets rewrite this program to use primitive arrays rather than array lists.

  1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 import java.util.Scanner; import java.io.File; import java.io.IOException; public class HistoArray { public static void main(String[] args) { Scanner data = null; int[] count = {0,0,0,0,0,0,0,0,0,0}; int idx; try { data = new Scanner(new File("test.dat")); } catch ( IOException e) { System.out.println("Sorry but I was unable to open your data file"); e.printStackTrace(); System.exit(0); } while(data.hasNextInt()) { idx = data.nextInt(); count[idx] = count[idx] + 1; } idx = 0; for(int i : count) { System.out.println(idx + " occured " + i + " times."); idx++; } } } 

The main difference between this example and the previous example is that we declare count to be an Array of integers. We also can initialize short arrays directly using the syntax shown on line 8. Then notice that on line 24 we can use the square bracket notation to index into an array.

## Dictionary¶

Just as Python provides the dictionary when we want to have easy access to key, value pairs, Java also provides us a similar mechanism. Rather than the dictionary terminology, Java calls these objects Maps. Java provides two different implementations of a map, one is called the TreeMap and the other is called a HashMap. As you might guess the TreeMap uses a balanced binary tree behind the scenes, and the HashMap uses a hash table.

Lets stay with a simple frequency counting example, only this time we will count the frequency of words in a document. A simple Python program for this job could look like this:

  1 2 3 4 5 6 7 8 9 10 11 12 13 14  def main(): data = open('alice30.txt') wordList = data.read().split() count = {} for w in wordList: w = w.lower() count[w] = count.setdefault(w,0) + 1 keyList = count.keys() keyList.sort() for k in keyList: print "%-20s occured %4d times"%(k, count[k]) main() 

Notice that the structure of the program is very similar to the numeric histogram program.

  1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 import java.util.Scanner; import java.util.ArrayList; import java.io.File; import java.io.IOException; import java.util.TreeMap; public class HistoMap { public static void main(String[] args) { Scanner data = null; TreeMap count; int idx; String word; int wordCount; try { data = new Scanner(new File("alice30.txt")); } catch ( IOException e) { System.out.println("Sorry but I was unable to open your data file"); e.printStackTrace(); System.exit(0); } count = new TreeMap(); while(data.hasNext()) { word = data.next().toLowerCase(); wordCount = count.get(word); if (wordCount == null) { wordCount = 0; } count.put(word,++wordCount); } for(String i : count.keySet()) { System.out.printf("%-20s occured %5d times\n", i, count.get(i) ); } } } 

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