Introducing Java

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I expect that you’re familiar with Python.

This semester we’re going to be working in Java.

It makes sense to think of computer programming as one skillset. Many of the concepts and techniques you learned in Python still apply in Java. But there are significant differences and new stuff in the new language as well.

The differences include the basic model of how a program is structured.

In Python, a program is:

• A collection of modules, each in its own file.
• A module contains statements, which includes defining functions and classes.
• One module in the program is the main module.
• The program runs by executing statements from the main module in order.
• Some statements cause conditional, repeat, or delayed execution, which makes things complicated.

#!/usr/bin/env python

def greet(name):
    print("Greetings, " + name)

greet("Alice")
greet("Bob")

Now let’s look at a similar program in Java:

public class Greet {
    public static void main(String[] _args) {
        greet("Alice");
        greet("Bob");
    }

    static void greet(String name) {
       System.out.println("Greetings, " + name); 
    }
}

Run that with:

$ javac Greet.java
$ java Greet

In Java, we start by defining a class. Python has classes, but we didn’t need one for this program. In Java, you always need a class.

In Java, a program is:

• A collection of packages (like Python modules), each of which lives in its own directory.
• Each package contains a collection of classes, some of which get their own file.
• A class contains methods.
  • There are no functions (for now), just methods.
• Methods contain statements.
• There is a main package, with a main class, which has a main method.
• The program runs by executing the statements in that main method in order.
• Definitions are not statements. The order of class and method declarations doesn’t matter.

Try this:

#!/usr/bin/env python

greet("Alice")
greet("Bob")

def greet(name):
    print("Greetings, " + name)

In Python:

• Defining a function is a statement.
• It happens in order.

Similarities:

• Functions and static methods work similarly - you can name a sequence of statements and call it with arguments.
• Both languages have strings, which look the same and can be concatenated with a “+” operator.
• Both languages have operators.
• Both languages have a way to print to the console.

Differences:

• Java code always exists in a class, even if you’re not trying to describe a new class of data.
• Java has explicit, static types. The name parameter of the greet method is a String - we need to say that, and the compiler will check that we don’t pass in anything else (try greet(5)).
• Java writes blocks with curly braces rather than a colon and indentation. This has concrete advantages over the Python system - like being able to correctly indent code automatically.
• A Python program frequently includes a shebang line, which is typical for a Unix scripting language.
• Running a Java program typically requires an explicit compilation step.

Data in Python

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• Primitive Types
  • Number (int vs float), String
• Tuples
• Classes / Instances
• Lists
• Dictionaries
• Sets

Data in Java

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Dynamic vs static type system

• Primitive Types
  • integers: byte, short, int, long
  • floating point: float, double
  • boolean
  • char
• Native complex types
  • String
  • Class wrappers: Int, Float
• Classes / Instances
• Stuff like Lists / Dictionaries are in the standard library
  • (Although there’s a built-in special thing called an array)

public static void main(String args[]) {
    float xx = 5.0;
    float yy = 3.5;
    System.out.println(xx + yy);
}

Classes: Data or code structure?

Records: Halfway between tuples and classes?