# Is a vs has relationship in c system

### Solutions to systems of equations: consistent vs. inconsistent (video) | Khan Academy

In object-oriented programming, inheritance is the mechanism of basing an object or class Composition implements a has-a relationship, in contrast to the is-a For example, in a summary of C++ in his book on objective C dayline.info actually (upcasting a reference type violates the type system). subtype polymorphism. One of the advantages of Object-Oriented programming language is code reuse. This reusability is possible due to the relationship b/w the. Interface vs. . Composition exhibits a "has-a" relationship. Constructor super(); dayline.infon("Constructed C"); } @Override public String toString() .

And then there's a couple of ways we could do it. The easiest way is really just find two points on each of these that satisfy each of these equations, and that's enough to define a line.

So for this first one, let's just make a little table of x's and y's. I'll just put it right over here. And then let's just see what happens when y is 0. When y is 0, then 2 times y is 0. You have x equaling So we have the point 13 comma 0.

Is-A And Has-A Relationship In C++ - Inheritance And Composition In C++

We're just trying to approximate-- 13 comma 0. And so this line right up here, this equation can be represented by this line.

Let me try my best to draw it. It would look something like that. Now let's worry about this one. Let's worry about that one. So once again, let's make a little table, x's and y's. I'm really just looking for two points on this graph. So when x is equal to 0, 3 times 0 is just 0. So you get negative y is equal to negative 11, or you get y is equal to So you have the point 0, 11, so that's maybe right over there.

And then when y is 0, you have 3x minus 0 is equal to negative 11, or 3x is equal to negative And so the second equation will look like something like this. For instance, in Cthe base method or property can only be overridden in a subclass if it is marked with the virtual, abstract, or override modifier. Code reuse[ edit ] Implementation inheritance is the mechanism whereby a subclass re-uses code in a base class.

By default the subclass retains all of the operations of the base class, but the subclass may override some or all operations, replacing the base-class implementation with its own. The base class comprises operations to compute the sum of the squares between two integers.

The subclass re-uses all of the functionality of the base class with the exception of the operation that transforms a number into its square, replacing it with an operation that transforms a number into its square and cube respectively. An alternative technique, explicit delegationrequires more programming effort, but avoids the substitutability issue. Whereas public inheritance represents an " is-a " relationship and delegation represents a " has-a " relationship, private and protected inheritance can be thought of as an "is implemented in terms of" relationship.

The parent class can be a combination of implemented operations and operations that are to be implemented in the child classes. Often, there is no interface change between the supertype and subtype- the child implements the behavior described instead of its parent class.

Subtyping Inheritance is similar to but distinct from subtyping. The relationship can be expressed explicitly via inheritance in languages that support inheritance as a subtyping mechanism. Inheritance, even in programming languages that support inheritance as a subtyping mechanism, does not necessarily entail behavioral subtyping.

It is entirely possible to derive a class whose object will behave incorrectly when used in a context where the parent class is expected; see the Liskov substitution principle.

In some OOP languages, the notions of code reuse and subtyping coincide because the only way to declare a subtype is to define a new class that inherits the implementation of another.

Using inheritance extensively in designing a program imposes certain constraints. For example, consider a class Person that contains a person's name, date of birth, address and phone number. This has a serious drawback if the superclasses have conflicting implementation for the same method.

In Java, each subclass can have one and only one direct superclass, i.

On the other hand, a superclass can have many subclasses. Common Root Class - java. Object Java adopts a so-called common-root approach. All Java classes are derived from a common root class called java. This Object class defines and implements the common behaviors that are required of all the Java objects running under the JRE. These common behaviors enable the implementation of features such as multi-threading and garbage collector.

Superclass Person and its Subclasses Suppose that we are required to model students and teachers in our application. We can define a superclass called Person to store common properties such as name and address, and subclasses Student and Teacher for their specific properties. For students, we need to maintain the courses taken and their respective grades; add a course with grade, print all courses taken and the average grade.

Assume that a student takes no more than 30 courses for the entire program.

For teachers, we need to maintain the courses taught currently, and able to add or remove a course taught. Assume that a teacher teaches not more than 5 courses concurrently. We design the classes as follows. Paul Tan 8 sunset way IM added.

IM cannot be added. IM cannot be removed. Inheritance "A line is composed of 2 points" vs. We have seen that a Line class can be implemented using composition of Point class - "A line is composed of two points", in the previous section. A Line can also be implemented, using inheritance from the Point class - "A line is a point extended by another point". Let's call this subclass LineSub to differentiate from the Line class using composition. This is the same test driver used in the earlier example on composition, except change in classname.

Study both versions of the Line class Line and LineSub. I suppose that it is easier to say that "A line is composed of two points" than that "A line is a point extended by another point". Use composition if possible, before considering inheritance.

Use inheritance only if there is a clear hierarchical relationship between classes. It comes from Greek word "poly" means many and "morphos" means form. For examples, in chemistry, carbon exhibits polymorphism because it can be found in more than one form: But, each of the form has it own distinct properties and price. Substitutability A subclass possesses all the attributes and operations of its superclass because a subclass inherited all attributes and operations from its superclass.

This means that a subclass object can do whatever its superclass can do. As a result, we can substitute a subclass instance when a superclass instance is expected, and everything shall work fine.

### Has-a - Wikipedia

This is called substitutability. In our earlier example of Circle and Cylinder: Cylinder is a subclass of Circle. We can say that Cylinder "is-a" Circle actually, it "is-more-than-a" Circle. Subclass-superclass exhibits a so called "is-a" relationship. The reference c1, however, retains its internal identity. In our example, the subclass Cylinder overrides methods getArea and toString. This is because c1 is in fact holding a Cylinder object internally.

Summary A subclass instance can be assigned substituted to a superclass' reference. Once substituted, we can invoke methods defined in the superclass; we cannot invoke methods defined in the subclass. However, if the subclass overrides inherited methods from the superclass, the subclass overridden versions will be invoked.

Consider the following example. Suppose that our program uses many kinds of shapes, such as triangle, rectangle and so on. We should design a superclass called Shape, which defines the public interfaces or behaviors of all the shapes.

For example, we would like all the shapes to have a method called getAreawhich returns the area of that particular shape. The Shape class can be written as follow. We shall print an error message for the time being. In the later section, I shall show you how to resolve this problem. We can then derive subclasses, such as Triangle and Rectangle, from the superclass Shape.

A Test Driver TestShape. You could instantiate different subclass instance, and the code still works. You could extend your program easily by adding in more subclasses, such as Circle, Square, etc, with ease. Nonetheless, the above definition of Shape class poses a problem, if someone instantiate a Shape object and invoke the getArea from the Shape object, the program breaks.

## Java Programming Tutorial

We do not want anyone to instantiate a Shape instance. This problem can be resolved by using the so-called abstract class. For example, in our game app, we have many types of monsters that can attack. We shall design a superclass called Monster and define the method attack in the superclass.

The subclasses shall then provides their actual implementation. In the main program, we declare instances of superclass, substituted with actual subclass; and invoke method defined in the superclass.

This is because, in a UML class diagram, subclass is often drawn below its superclass.

### A System and Its Surroundings - Chemistry LibreTexts

Upcasting is always safe because a subclass instance possesses all the properties of its superclass and can do whatever its superclass can do. The compiler checks for valid upcasting and issues error "incompatible types" otherwise. This is called "downcasting". Downcasting is not always safe, and throws a runtime ClassCastException if the instance to be downcasted does not belong to the correct subclass.

A subclass object can be substituted for its superclass, but the reverse is not true. Point cannot be casted to Circle The "instanceof" Operator Java provides a binary operator called instanceof which returns true if an object is an instance of a particular class. The syntax is as follows: When a superclass instance is expected, it can be substituted by a subclass instance.

In other words, a reference to a class may hold an instance of that class or an instance of one of its subclasses - it is called substitutability.

If a subclass instance is assign to a superclass reference, you can invoke the methods defined in the superclass only. You cannot invoke methods defined in the subclass.