-
-
-
- The best way to understand classes and objects is to see them in action. Let's define a
- class Dog:
- def __init__(self, name, breed, fur_color):
- self.name = name
- self.breed = breed
- self.fur_color = fur_color
- self.trained = False
- print("Dog named " + self.name + " created!")
-
- def bark(self):
- print(self.name + " says woof!")
-
- def sit(self):
- if self.trained:
- print(self.name + " sits.")
- else:
- print(self.name + " has not been trained.")
-
- def train(self):
- self.trained = True
-
-
- Let's unpack what is going on in this code. The first line is where we declare the class definition and name it
- The next three blocks of code are the class's methods. These include
-
- Next, we will use this class to create a new
- class Dog:
- def __init__(self, name, breed, fur_color):
- self.name = name
- self.breed = breed
- self.fur_color = fur_color
- self.trained = False
- print("Dog named " + self.name + " created!")
-
- def bark(self):
- print(self.name + " says woof!")
-
- def sit(self):
- if self.trained:
- print(self.name + " sits.")
- else:
- print(self.name + " has not been trained.")
-
- def train(self):
- self.trained = True
-
-
- my_dog = Dog("Rex", "pug", "brown")
-
-
- In the final line of code, we have created an object called
- Now that we have created a
- class Dog:
- def __init__(self, name, breed, fur_color):
- self.name = name
- self.breed = breed
- self.fur_color = fur_color
- self.trained = False
- print("Dog named " + self.name + " created!")
-
- def bark(self):
- print(self.name + " says woof!")
-
- def sit(self):
- if self.trained:
- print(self.name + " sits.")
- else:
- print(self.name + " has not been trained.")
-
- def train(self):
- self.trained = True
-
+
+ First Java Program
- my_dog = Dog("Rex", "pug", "brown")
- my_dog.bark()
- my_dog.sit()
-
-
- When running the code above, the line
- Now, we have a full class definition and have utilized its methods. Class definitions in Java will be covered thoroughly in chapter 6. For now, it is important to know that Python programs can be written without using classes at all. Java, on the other hand, requires all code to reside in a class. This will be discussed in the next section. -
- - -- A time-honored tradition in Computer Science is to write a program called “hello world.” The “hello world” program is simple and easy. - There are no logic errors to make, so getting it to run relies only on understanding the syntax. - To be clear, lets look at a “complicated” version of hello world for Python: -
+
+print ("Hello World!")
+
+ def main(): print("Hello World!")
- - Remember that we can define this program right at the Python command line and then run it: -
++ Next, let's look at a more "complicated" version of hello world for Python: +
->>> main() "Hello World!" >>>-
- Now lets look at the same program written in Java: -
+
+def main():
+ print ("Hello World!")
+main()
+
+
+ This is a bit more complicated, but it is still very simple. We have defined a function called
+ Now lets look at the same program written in Java: +
-
+
+
public class Hello {
public static void main(String[] args) {
System.out.println("Hello World!");
}
}
-
-
+
+ - What we see is that at the core there are a few similarities, such as a main and the string “Hello World”. However, there is a lot more stuff around the edges that make it harder to see the core of the program. Do not worry! An important skill for a computer scientist is to learn what to ignore and what to look at carefully. You will soon find that there are some elements of Java that will fade into the background as you become used to seeing them. -
+
+ What we see is that at the core there are a few similarities, such as a
-
+
+
+
+ interpreter
+ interpreted language
+ compile
+ A natural question that you may have about this little program is "How would I run it on my own computer?" Running a Java program on your computer is not as simple as running a Python program because the first thing you need to do with a Java program is to compile it, which is not needed in Python. Python is an interpreted language . We could run our Python programs in the Python interpreter and we can be quite happy to do only that. Java makes running programs a two-step process. First, we must type the hello world program into a file and save that file using the filename Hello.java because the file name must be the same as the public class that you define in the file. Once we have saved the file, we compile it either in our IDE or from the command line as follows:
+
+
+
+
$ javac Hello.java
$ ls -l Hello.*
-rw-r--r-- 1 bmiller bmiller 391 Jul 19 17:47 Hello.class
-rw-r--r-- 1 bmiller bmiller 117 Jul 19 17:46 Hello.java
-
-
+
+
+
+ Now that we have compiled our Java source code we can run the compiled code using the
+$ java Hello
+Hello World!
+$
+
+ + Now you may be wondering what good is that extra step? What does compiling do for us? There are a couple of important benefits we get from compiling: +
+ ++
+ Early detection of errors +
++ Faster program execution +
+
+
+ When the compiler does the translation it can find many different kinds of errors. + For example, if you make a typo, the compiler will find the typo and point it out to you before you ever run the program. + We will look at some examples of compiler errors shortly. + Chances are you will create some on your own very soon, too. +
+
- The command
- Now that we have compiled our java source code we can run the compiled code using the
+ Now that you've seen your first Java program, let's examine the key differences between Java and Python more systematically. Some differences can be immediately observed in their syntax and structure. +
+
-$ java Hello
-Hello World!
-$
-
- + The above differences are easy to identify and understand, but there are some more subtle differences that will take a bit more time to grasp. The most important of these is how Java organizes all code around classes and objects which we will cover in the next section. +
-- Now you may be wondering what good is that extra step? What does compiling do for us? There are a couple of important benefits we get from compiling: -
++ But, before we move on, let's go back and look at our Hello World program carefully to see what we can learn about the Java language. + This simple example illustrates a few very important rules: +
+ ++
+ Every Java program must define a class, and all code is inside a class +
++ Everything in Java must have a type +
+
+ Every Java program must have a function called
+ Let's take the hello world example a line at a time to see how these rules are applied. + On line 1 we see that we are declaring a class called Hello: +
+ +
+public class Hello {
+
+
+
+ On the next line we start our method definition. + The name of this method is: +
+ +
+public static void main(String[] args)
+
+ -
- Early detection of errors
+ Every part of
+ We're covering a lot of syntax details here - don't worry about memorizing everything! The goal is to understand the overall structure and know that Java is more explicit about types and method signatures than Python. +
+ ++ Just digging in to this one line will take us deep into the world of Java, so we are going to start digging but we are not going to dig too deeply right away. + Much of what could be revealed by this one line is better understood through other examples, so be patient. +
+ +
+
+
+
+
+
+ That is a lot of new material to digest in only a single line of Java! Let's press on and look at the next line: +
+ +
+System.out.println("Hello World!");
+
+
+
+ Now there is one more character on this line that is significant and that is the
+ The last two lines of the hello world program simply close the two blocks using
- Faster program execution -
-
-
- When the compiler does the translation it can find many different kinds of errors. - For example, if you make a typo, the compiler will find the typo and point it out to you before you ever run the program. - We will look at some examples of compiler errors shortly. - Chances are you will create some on your own very soon, too. -
- -- Now that we have run our hello world program, lets go back and look at it carefully to see what we can learn about the Java language. - This simple example illustrates a few very important rules: -
- --
- Every Java program must define a class, and all code is inside a class -
-- Everything in Java must have a type -
-
+
- Every Java program must have a function called
+
- Lets take the hello world example a line at a time to see how these rules are applied. - On line 1 we see that we are declaring a class called Hello: -
++ If you've already worked with classes in Python, this will look familiar - the main difference is that Java requires ALL code to be inside classes, while Python makes classes optional. +
+
+ The best way to understand classes and objects is to see them in action. Let's define a
-public class Hello {
-
-
- As rule 1 says all Java code resides inside a class.
- Unlike Python where a program can simply be a bunch of statements in a file, Java programs must be inside a class.
- So, we define a class
- On the next line we start our method definition. - The name of this method is: -
- - -
-public static void main(String[] args)
-
-
+class Dog:
+ def __init__(self, name, breed, fur_color):
+ self.name = name
+ self.breed = breed
+ self.fur_color = fur_color
+ self.trained = False
+ print("Dog named " + self.name + " created!")
+
+ def bark(self):
+ print(self.name + " says woof!")
+
+ def sit(self):
+ if self.trained:
+ print(self.name + " sits.")
+ else:
+ print(self.name + " has not been trained.")
+
+ def train(self):
+ self.trained = True
+
+ - Everything on this line is significant, and helps in the identification of this method. - For example the following lines look similar but are in fact treated by Java as completely different methods: -
+
+
-
-
+ The next three blocks of code are the class's methods. These include
-
+
-
+ Next, we will use this class to create a new
+class Dog:
+ def __init__(self, name, breed, fur_color):
+ self.name = name
+ self.breed = breed
+ self.fur_color = fur_color
+ self.trained = False
+ print("Dog named " + self.name + " created!")
+
+ def bark(self):
+ print(self.name + " says woof!")
+
+ def sit(self):
+ if self.trained:
+ print(self.name + " sits.")
+ else:
+ print(self.name + " has not been trained.")
+
+ def train(self):
+ self.trained = True
+
+
+my_dog = Dog("Rex", "pug", "brown")
+
+
+ In the final line of code, we have created an object called
+ Now that we have created a
+class Dog:
+ def __init__(self, name, breed, fur_color):
+ self.name = name
+ self.breed = breed
+ self.fur_color = fur_color
+ self.trained = False
+ print("Dog named " + self.name + " created!")
+
+ def bark(self):
+ print(self.name + " says woof!")
+
+ def sit(self):
+ if self.trained:
+ print(self.name + " sits.")
+ else:
+ print(self.name + " has not been trained.")
+
+ def train(self):
+ self.trained = True
+
+
+my_dog = Dog("Rex", "pug", "brown")
+my_dog.bark()
+my_dog.sit()
+
+
+ When running the code above, the line
+ Now, we have a full class definition and have utilized its methods. Class definitions in Java will be covered thoroughly in chapter 6. For now, it is important to know that Python programs can be written without using classes at all. Java, on the other hand, requires all code to reside in a class. This will be discussed in the next section. +
+ +
+ Later in this text, we will see how to make our own Java classes that are similar to the Python
-
- Just digging in to this one line will take us deep into the world of Java, so we are going to start digging but we are not going to dig too deeply right away. - Much of what could be revealed by this one line is better understood through other examples, so be patient. -
- -
-
- Finally, we have the parameter list for the method.
- In this example we have one parameter.
- The name of the parameter is
- That is a lot of new material to digest in only a single line of Java! Lets press on and look at the next line: -
- - -
-System.out.println("Hello World!");
-
-
- This line should look a bit more familiar to you.
- Python and Java both use the dot notation for finding names.
- In this example we start with
- Now there is one more character on this line that is significant and that is the
-System.out.println("Hello World");
-System.out.println("Hello World")
-;
-System.out.println
- (
- "Hello World"
- ) ;
-System.
- out.
- println("Hello World")
- ;
-
-
- The last two lines of the hello world program simply close the two blocks using
- If we wanted to translate the Java back to Python we would have something like the following class definition. -
- - -
-class Hello(object):
- @staticmethod
- def main(args):
- print("Hello World!")
-
-
- Notice that we used the decorator
->>> Hello.main("")
-Hello World!
->>>
-
- Java programs must be compiled before execution, unlike Python which is interpreted directly.
-Every Java program must define a
Each Java program must include a
Java enforces that every variable and method must have a clearly defined type, including
Statements in Java must end with a semicolon (
Java uses dot notation to access class members, such as
The
+
Java programs must be compiled before execution, unlike Python which is interpreted directly.
+Every Java program must define a
Each Java program must include a
Java enforces that every variable and method must have a clearly defined type, including
Statements in Java must end with a semicolon (
Java uses dot notation to access class members, such as
The
Object-oriented programming organizes code around classes (blueprints) and objects (instances of classes).
+It must contain a method called
No. The main method must include a specific parameter:
It must contain a method called
No. The main method must include a specific parameter:
It must include a
Correct! This is the required entry point for all Java applications.
It must include a
Correct! This is the required entry point for all Java applications.
+It must be saved with a
No. Java source files use the
It must be saved with a
No. Java source files use the
It must be run using the Python interpreter.
No. Java uses its own compiler and Java Virtual Machine (JVM).
It must be run using the Python interpreter.
+No. Java uses its own compiler and Java Virtual Machine (JVM).
+What is the purpose of the
To compile Java source code into bytecode
Exactly!
To compile Java source code into bytecode
+Exactly!
To run a Python program
No. Python programs are run with the
To run a Python program
+No. Python programs are run with the
To debug Java programs
No.
To debug Java programs
+No.
To edit Java source files
No. You use a text editor or IDE to edit Java files.
To edit Java source files
+No. You use a text editor or IDE to edit Java files.
+What symbol does Java use to indicate the end of a statement?
No.
Correct! Java uses semicolons to mark the end of a statement.
No.
No.
In object-oriented programming, what is a class?
+A blueprint or template for creating objects
+Exactly! A class defines the structure and behavior that objects will have.
+A specific instance of an object
+No. That describes an object, not a class.
+A method that creates variables
+No. A class is not a method.
+A type of loop structure
+No. Classes are not related to loop structures.
+