in the car example, when we say turn right, the car code really needs to know lots the time". could hold -9101982309812 or a "name" variable could contain Object Oriented Programming languages are defined by the following key words: Class attributes are often used to define constants which are closely associated with a particular class. There is no conflict between the local variable named x and the attribute named x.
You should now see Point.java and Rectangle.java, along with the other classes in the java.awt package. A function performs an action using some set of input parameters.
You may not have realized that these classes are written in Java. or data value). They provide actions (also called functions or methods) which manipulate that The java.awt package also provides a class called Rectangle. when we mean class, or class when we mean object. variables. For example, it makes sense to compare 1 and 2.5 because they are both numbers, even though one is an integer and the other is a float. Packages are functions, because they are usually used by other functions in the object to We can use dir on any object. Often a package name will be something like: jims_code, or pacman_game_code, It is used if the rich comparisons are not defined. The purpose of the constructor is to fill in This example adds 50 to all four sides of the rectangle, so it moves the corner up and to the left by 50, and it increases the height and width by 100: The first line displays 100, which is the width of the Rectangle referred to by box2. Only will be usually be referred to as member variables, but sometimes as not the object. The first of these databases was the Pick operating system. by the programmer to allow the computer to use/construct new objects. The second line changes blank so that instead of referring to the object, it refers to nothing.
time) concept. The arrow points to the new object, which contains two variables, x and y. Variables that belong to an object are usually called attributes, but you might also see them called fields. clean plates that the dishwasher produces).
The following example creates a Rectangle object and makes the variable box refer to it: Figure10.2 shows the effect of this assignment.
of the variable. shapes" we use the term "Abstract". For example, you can drive a car on the highway, but can you drive For example, in computer graphics, line objects can have attributes such as thickness (with real values), color (with descriptive values such as brown or green or values defined in a certain color model, such as RGB), dashing attributes, etc. the car variable can contain a Sedan because a Sedan ISA Car! really should be placed in a Vehicle class, from which both cars refer back to this page at a later date. covered under pointers/references). For example, the purpose of the __str__ method is to output a useful string representation of our object. There is more to them than you may have realized, but that doesnt limit your ability to use these methods in a program. (public) to any protected variable (or function) in the parent object.
For an object design pattern (in ActionScript), refer to the following Design Pattern. computer) whenever you craete a new object. When we create an object of that data type, we call it an instance of a class. This is enough information to "recreate" a square.
An __init__ method doesnt have to take any parameters (except self) and it can be completely absent. Sometimes it is useful to write a class method which creates an instance of the class after processing the input so that it is in the right format to be passed to the class constructor. In practice, its more convenient to look at high-level pictures than to examine the source code. The constructor (for an object) is a method that is called (FOR YOU by the We use the same names for the attributes and the parameters, but this is not compulsory. Where do we store data with types? In this case, the value is a reference, which is represented with an arrow. itself to the graphics screen). would simulate slowing the car down, etc. Generic_Shape class, because we don't want to have to repeat the self. In Section10.6, we saw what happens when more than one variable refers to the same object. how these function work, just so long as the car does know and responds thus, we make this method public (accessible by all). Sometimes we write classes purely to group related constants together with functions which act on them we may never instantiate these classes at all. Remember that defining a function doesnt make the function run. System.out.println(box2.width); Figure 10.5: State diagram showing the effect of invoking, Figure 10.6: State diagram showing a variable that contains a, Figure 10.7: State diagram showing the effect of setting a variable to. When we call the class object, a new instance of the class is created, and the __init__ method on this new object is immediately executed with all the parameters that we passed to the class object. Python 2 also has a __cmp__ method which was introduced to the language before the individual comparison methods (called rich comparisons) described above. The only way an "outsider" can change the speed of a car is to use the Empty meant: "What ever the heck happened to be in memory at class. describing how to use an object (in fact, see Sun's own When creating objects (or just reading the code for objects created by others) you only purpose of which is to tell every shape that ISA The second line creates the new Point with the given arguments as coordinates. Because its so large, the library source code is stored in a file named src.zip. operations.
A car ISA vehicle. about the object. Alternatively in some cases, we will reverse! Local variables (those used only within a function) are not tagged with this. A class is a kind of data type, just like a string, integer or list. An object is a like Java, ActionScript, and C++, developed the Constructor. In Python, you can add new attributes, and even new methods, to an object on the fly. creator of our Square object to give it a "personality".
means that only the object itself can use the function. Remember, a variable can normally only This gives us a way to access the objects properties from inside the objects methods. However its still present in the computers memory, taking up space. car, for example: start_car() would "simulate" turning the engine on, break() The first line creates a new Point object and makes blank refer to it. This is a small mathematical operation that tells the Object most class files. A good example is the process of XML assigning values to properties (elements). The decision to expose any given attribute as a property is also left to the developer as is the decision to use them as part of a larger application framework. because, a car ISA vehicle, and a sedan ISA car. and many different ones.
The parameters allow the Again, this For example, we might have a "square" object. about is that when we turn the key, the engine starts, when we press the gas, the The wise student will ask for Then we could have a "sedan" class as an appropriately to each function call. By calling the parameter cls we remind ourselves that it is not guaranteed to have any instance attributes.
It doesnt have to be called self, but this is a very strongly followed convention. here in our Abstract Base Class. For example, the speed of a Generic_Shape - the parent or base class. be created "empty". we can share all the stuff (variables and methods) that is common, In this chapter, we introduce two new types of objects: Point and Rectangle. One of the key issues in Inheritance is when to "inherit" and when to use a data any vehicle on the highway. really define a specific 'sub-species' of the item, or would an extra member All Class files contain the following parts: The Package (or namespace) that the class belongs to. This is a great example If we always access the fullname attribute through a setter, we can just rewrite the setter none of the code which calls the setter will have to be changed. If we create a new attribute outside the __init__ method, we run the risk that we will try to use it before it has been initialised. generic shape that it must be able to paint itself (draw Sometimes we write objects which map very intuitively onto things in the real world.
For example, in the car variable above, when These names indicate that these properties have a special meaning you shouldnt create your own methods or attributes with the same names unless you mean to overload them. allowed interactions between the outside world (e.g., other programs) and should ever be created without "Data". Notice the use of the key word "this". We would list any parent classes in between round brackets before the colon, but this class doesnt have any, so we can leave them out. Passing objects as parameters makes the source code more readable and less error-prone, because related values are bundled together. As we learned in the previous chapter, an object is a collection of data that provides a set of methods. function defined, then even though the car variable is TYPED only as a CAR, at run the internal code of the object. Yes, we can. Notice how box1 and box2 are aliases for the same object, so any changes that affect one variable also affect the other. "break()" function associated with the car. contain a value of the same type as the variable. For the most part, you can read protected as "private". For example, findCenter takes a Rectangle as an argument and returns a Point with the coordinates of the center of the rectangle: The return type of this method is Point. You might be sick of the factorial method by now, but were going to do one more version. time, this instruction REALLY becomes a call to "Sedan.drive()". Remember that when you assign an object to a variable, you are assigning a reference to an object. To use it, you have to import it: Rectangle objects are similar to points, but they have four attributes: x, y, width, and height. variables that are often changed by the outside world (with out affecting the For example: The first line displays 3,4; the second line calculates the value 25. Figure10.1 shows the result. As we saw in the previous age function example, hasattr detects whether an attribute exists. In an OOP, a "child" object Here is our generic shape's default constructor: The public key word tells the class file (and other programmers) that this object comes from. It may also refer to or set the specific value for a given instance of such. Something which is often considered an advantage of setters and getters is that we can change the way that an attribute is generated inside the object without affecting any code which uses the object. Would it make sense to allow only the object itself to create the object itself? Moving the the current object. under a, // if n is even, the result is t squared information that would be the same for both cars and trucks (and others)
Their specific use as metadata is left to the developer and can cover a wide range of types of information about any given application, classes and members that is not instance-specific. Suppose that we want our person objects to be equal if all their attributes have the same values, and we want to be able to order them alphabetically by surname and then by first name. You can see the results by reading the documentation for the Point class, which you can find by doing a web search for Java Point. is so important, that you will see ISA used as a keyword in the discussion on programmers) that this object can be created by anyone. Without this "object" into one piece of code, we make our programs easier to write and In this case, it shows the name of the type (java.awt.Point) and the names and values of the attributes. In the next section we will see how to define them using a decorator. If we create two Person objects and call the mark_as_deceased method on one of them, we will not affect the other one. with the object. Here is a short Figure 10.3: State diagram showing updated attributes. __init__ is sometimes called the objects constructor, because it is used similarly to the way that constructors are used in other languages, but that is not technically correct its better to call it the initializer. In Python it is explicitly exposed. We have already used the methods of some built-in objects, like strings and lists. Creating an actual (non-abstract) class that uses Generic_Shape. information about what defines an object. Text Allen Downey and Chris Mayfield. The use of this is not required, except Thats more than one person could read and understand fully, so please dont be intimidated! You can write methods that return objects.
So blank contains a reference to the new Point object. For example, should a 2-door sedan be a separate (child) class Thus the child gets all the parents data and functionality "for Here is an example of a simple custom class which stores information about a person: We start the class definition with the class keyword, followed by the class name and a colon. You can find out the type of any object using the type function: The data values which we store inside an object are called attributes, and the functions which are associated with the object are called methods. all the fields (or variables) associated with an object. Rather than write methods like moveRect that modify one or more parameters, we apply methods to objects themselves using dot notation. As a result, the box object is updated directly. maintain. We would have a similar issue with in-place modifications, like adding values to lists. The goal of an object is to contain enough information to (in the program) of a car. Not all functions are applicable to all kinds of data. The age function doesnt take any parameters except self it only uses information stored in the objects attributes, and the current date (which it retrieves using the datetime module). both are numbers. Similarly, setattr allows us to set the value of an attribute. This process is called garbage collection. to this is when we use the Object Oriented technique of What are class methods good for? But what if our code accesses the fullname attribute directly? Attributes are implemented as classes that are derived from System.Attribute. Parameters are symbolic names for the information that must be presented to object can be created by anyone. In Python, everything is an object everything is an instance of some class. JAVA docs). If our attribute name is stored as a string value in a variable, we have to use the getattr function to retrieve the attribute value from an object: Note that getattr is a built-in function, not a method on the object: it takes the object as its first parameter. But dont use BigInteger.pow; that would spoil the fun. The inheritance information describes what "parent" class the
you can "stop" a vehicle, then you can "stop" a car. intialized to a reasonable value to start. You can change the contents of an object by making an assignment to one of its attributes. Thats correct, the X A square can be defined by the X,Y position of it's center, and the length of its It also makes it easier to read and understand we can see at a glance what attributes our object has. (but perhaps other people could use the clean plates that
First, we can't really have a Generic Shape, we can't taste one, we can't display It is also often useful to overload the comparison methods, so that we can use comparison operators on our person objects. The result of the new operator is a reference to the new object. shared by all the objects, we can put the actual code for the method What is "the center" of a two dimensional shape? The disadvantage is that if we do occasionally want to refer to another class method or attribute inside a static method we have to write the class name out in full, which can be much more verbose than using the cls variable which is available to us inside a class method. Any variable (or function) that is tagged "private" can only be used by The element itself may have a number of attributes set (NAME = "IAMAPROPERTY").
We should, however, be careful when a class attribute is of a mutable type because if we modify it in-place, we will affect all objects of that class at the same time. object itself can use the function. On many post-relational or multi-valued databases systems, relative to SQL, tables are files, rows are items, and columns are attributes. Object-oriented programming makes it possible to hide messy details so that you can more easily use and understand code that other people wrote. You should see something like '<__main__.Person object at 0x7fcb233301d0>'. Classes are to Objects as Recipes are to Cakes. we might set the x,y of one square to 100,200, and of another square to 500,600. world entity. Enter search terms or a module, class or function name. very hard to find mistakes in programs where future code accessed undefined special function that can only be used once, never returns a value, Two current platforms include Rocket U2's Universe and InterSystems Cach.
Each of these methods takes two parameters self for the current object, and other for the other object: Note that other is not guaranteed to be another person object, and we havent put in any checks to make sure that it is. Class attributes can also sometimes be used to provide default attribute values: When we set an attribute on an instance which has the same name as a class attribute, we are overriding the class attribute with an instance attribute, which will take precedence over it.
Figure 10.4: State diagram showing two variables that refer to the same object. We could store a cached age value on the object from inside the age function: Starting an attribute or method name with an underscore (_) is a convention which we use to indicate that it is a private internal property and should not be accessed directly. Then every instance will have its own separate copy: Note that method definitions are in the same scope as class attribute definitions, so we can use class attribute names as variables in method definitions (without self, which is only defined inside the methods): Can we have class methods? We can check what properties are defined on an object using the dir function: Now we can see our attributes and our method but whats all that other stuff? For example a "car" could be modeled in a computer by information Note: Often we will be lazy in our English level descript and say object extends means that of things, like are we moving, how fast, is the break on, etc etc etc. As shown in Figure10.8, a class diagram is divided into two sections.
car can be used.
At run time, "The object referenced by the variable behaves as it
Consider: Moving a shape is something that all shapes have in common. But we dont really need a method like printPoint, because if you invoke System.out.println(blank) you get: Point objects provide a method called toString that returns a string representation of a point. In the age example above we have to check if an _age attribute exists on the object before we try to use it, because if we havent run the age method before it will not have been created yet. An attribute of an object usually consists of a name and a value; of an element, a type or class name; of a file, a name and extension. Every function in a class has access to all member
code and the vehicle code have been combined to form the car class. These all synonymous for each other. Write a class for creating completely generic objects: its. the dishwasher produces). However, in actual usage, the term attribute can and is often treated as equivalent to a property depending on the technology being discussed. The last line creates a new Point object and returns a reference to it. When we design our own objects, we have to decide how we are going to group things together, and what our objects are going to represent.
For example, most of use do not care how our car engines work. and trucks extend. Remember, we are creating objects that we can draw or "paint" car, or even of vehicle)! Users of the language see many examples where attributes are used to address cross-cutting concerns and other mechanistic or platform uses. to the screen. When trying to decide when to subclass, you should think: "Would a new 'class' Attributes are an objects data, and methods are an objects code. Theres nothing preventing us from using getattr on attributes even if the name can be hard-coded, but this is not recommended: its an unnecessarily verbose and round-about way of accessing attributes: You should only use these functions if you have a good reason to do so. these pieces of information are stored internally to the car in member "This" is a runtime (as well as compile For example, translate has the same effect as moveRect, but instead of passing the rectangle as an argument, you use dot notation: This line invokes the translate method for the object that box refers to. Many default methods and attributes that are found in built-in Python objects have names which begin and end in double underscores, like __init__ or __str__. Name parameters like the Boolean parameter in the example are a property of the attribute and should be a constant value.[1]. Correct, The interface between the car object and the rest of the program (or world) But in high-performance applications, you may notice a slight delay every now and then when Java reclaims space from discarded objects. The primary example is the Constructor. remember that ISA means, the square class acts in many ways presented to the object when it is created. information. Learning from these program language design errors, the creaters of new languages, the world). what is really inside the variable. We know how to paint (draw) a square, we know how to paint meant.
The code for this chapter is in the ch10 directory of ThinkJavaCode. In reference to computer systems, attributes are defined particularly for read or write attributes for specific read or write. you need a new object. Alternatively in some If we call the class method from an instance, this parameter will contain the instance object, but if we call it from the class it will contain the class object. A professor (contrary to popular belief) ISA In computing, an attribute is a specification that defines a property of an object, element, or file. int size; // the "size" of the shape (means different things to different shapes), Color color; // the color to draw the object. You should overload it in a way which is consistent with the rich comparison methods, otherwise you may encounter some very strange behaviour. We dont have to assign it to an intermediate variable before using it as a parameter to Person; we can just create it when we call Person, just like we create the string literals for the other parameters. Take a few minutes to locate this file on your machine: When you open (or unzip) the file, you will see folders that correspond to Java packages. What happens when no variables refer to an object? If we are using a class to group together related methods which dont need to access each other or any other data on the class, we may want to use this technique. # we can access a class attribute from an instance, # but we can also access it from the class, # extract all the parameters from the text file, # this is the same as calling Person(*params), # instance object accessible through self, # class or instance object accessible through cls, # no parameter for class or instance object, # this is much more complicated in real life, # now we can define all the other methods in terms of the first two, # This isn't strictly necessary, but it clearly introduces these attributes, Object-Oriented Programming in Python 1 documentation.