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2. Inheritance Inheritance EG. 1: The Circle and Cylinder Classes A constructor to initialize the name, email and gender with the given values. You could reuse the Author 's toString() method, which returns " author-name (gender) at email ". Polymorphism is very powerful in OOP to separate the interface and. Collection Implementations; Duplicates; Ordering All relationships in Java and JPA are unidirectional, in that if a source object references a . join fetch which would result in a very inefficient join that would return n^2 data. az-links.info 2 // Account class that contains a name instance variable 3 // and methods to set and get its value. Different accounts typically have different names. As you'll soon see, Account method getName returns a value. .. There's a special relationship between classes that are compiled in the.
You need to properly construct the superclasses before you can construct the subclass. Default no-arg Constructor If no constructor is defined in a class, Java compiler automatically create a no-argument no-arg constructor, that simply issues a super call, as follows: The default no-arg constructor will not be automatically generated, if one or more constructor was defined.
In other words, you need to define no-arg constructor explicitly if other constructors were defined. If the immediate superclass does not have the default constructor it defines some constructors but does not define a no-arg constructoryou will get a compilation error in doing a super call. Note that Java compiler inserts a super as the first statement in a constructor if there is no super args.
Multiple inheritance permits a subclass to have more than one direct superclasses. 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.
Day 5 -- Arrays, Conditionals, and Loops
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.
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.
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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.
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. 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. However, the substituted instance retains its own identity in terms of overridden methods and hiding variables. If the subclass overrides methods in the superclass, the subclass's version will be executed, instead of the superclass's version.
This can be resolved via abstract method and abstract class.
An abstract method is a method with only signature i. You use the keyword abstract to declare an abstract method. For example, in the Shape class, we can declare abstract methods getAreadrawetc, as follows: How to compute the area if the shape is not known? Implementation of these abstract methods will be provided later once the actual shape is known.
These abstract methods cannot be invoked because they have no implementation. A class containing one or more abstract methods is called an abstract class. An abstract class must be declared with a class-modifier abstract. Shape and its Subclasses Let us rewrite our Shape class as an abstract class, containing an abstract method getArea as follows: The abstract Superclass Shape. Therefore, an abstract class cannot be instantiated. In other words, you cannot create instances from an abstract class otherwise, you will have an incomplete instance with missing method's body.
To use an abstract class, you have to derive a subclass from the abstract class. In the derived subclass, you have to override the abstract methods and provide implementation to all the abstract methods. The subclass derived is now complete, and can be instantiated.
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If a subclass does not provide implementation to all the abstract methods of the superclass, the subclass remains abstract. This property of the abstract class solves our earlier problem. In other words, you can create instances of the subclasses such as Triangle and Rectangle, and upcast them to Shape so as to program and operate at the interface levelbut you cannot create instance of Shape, which avoid the pitfall that we have faced. The purpose of an abstract class is to provide a common interface or protocol, or contract, or understanding, or naming convention to all its subclasses.
For example, in the abstract class Shape, you can define abstract methods such as getArea and draw. No implementation is possible because the actual shape is not known. However, by specifying the signature of the abstract methods, all the subclasses are forced to use these methods' signature. The subclasses could provide the proper implementations.
Coupled with polymorphism, you can upcast subclass instances to Shape, and program at the Shape level, i,e. The separation of interface and implementation enables better software design, and ease in expansion. For example, Shape defines a method called getAreawhich all the subclasses must provide the correct implementation. You can ask for a getArea from any subclasses of Shape, the correct area will be computed. Dynamic Realms to the rescue!
A DynamicRealm is a variant of the conventional Realm that makes it possible to work with Realm data without using RealmObject subclasses. Instead, all access is done using Strings instead of Classes. Opening a Dynamic Realm uses the same configuration as a conventional Realm but the Dynamic Realm ignores any configured schema, migration, and schema version.
Only use Dynamic Realms when you need that flexibility. Realm instances are reference counted—if you call getInstance twice in a thread, you need to call close twice as well. This allows you to implement Runnable classes without having to worry about which thread will execute them: For the UI thread, the easiest way is to execute realm. If you need to create a Looper thread other than UI, you can use this pattern: Setup the handlers using the Realm instance Use the Realm instance This means the Realm instance will be periodically updated to the latest version.
This lets you keep your UI constantly updated with the latest content with almost no effort! Holding on to an old version of your data is expensive in terms of memory and disk space, and the cost increases with the number of versions between the one being retained and the latest. This is why it is important to close the Realm instance as soon as you are done with it in the thread. If you want to check whether your Realm instance has auto-refresh activated or not, use the isAutoRefresh method.
Models Create Realm models by extending the RealmObject base class: The integer types byte, short, int, and long are all mapped to long within Realm. These types may have the value null. Required fields The Required annotation can be used to tell Realm to disallow null values in a field, making it required rather than optional.
If you add it to other field types, compilation will fail. Fields with primitive types and the RealmList type are required implicitly. Fields with RealmObject type are always nullable. The field type must be either a string String or an integer byte, short, int, long, Byte, Short, Integer, and Long.
Using a string field as a primary key automatically indexes the field: These looks for an object with a given primary key, and either updates it if an object with that key already exists or creates it if the key does not exist. When you use primary keys, reads queries will be slightly faster, but writes creating and updating objects will be a little slower.
If the object is a class with a primary key, this could create a conflict—there might be an object with that primary key set already. To avoid this, you can create an unmanaged object, set its field values, then add it to the Realm with copyToRealm or insert: Indexing properties To index a field, use the annotation Index.Passing, Returning Arrays To / from Methods in Java Programming Video Tutorial
Like primary keys, this makes writes slightly slowerbut makes reads faster. It also makes your Realm file slightly larger, to store the index. You can index String, byte, short, int, long, boolean and Date fields. Fields marked static and transient are always ignored, and do not need the Ignore annotation.
Counters Realm offers MutableRealmInteger as a special integer type. MutableRealmInteger exposes an additional API that can more clearly express intent and generate better conflict resolution steps when using Synchronized Realms. Traditionally, a counter would be implemented by reading a value, incrementing it and setting it myObj.
This will not work well in an asynchronous situation — for example when two clients are offline — because both parties will read a value, say 10, increment it, and store the value as MutableRealmIntegers are backed by traditional integer types, so no migration is required when changing a field from byte, short, int or long to MutableRealmInteger.
MutableRealmInteger is not an immutable type standard like primitive number types in Java. This means the value contained inside the MutableRealmInteger can change when a Realm is written to. For this reason MutableRealmInteger fields must be marked final. Calling set can potentially override increment and decrement operations coming from other devices pr.