how to reuse a class in java

Everything in Java is associated with classes and objects, along with its attributes and methods. If you write code in a parent class, you usually don’t need to write it again in associated child classes; doing so would be inefficient and would make a program more difficult to read. First, Stack is inherited from Vector, which says that a Stack is a Vector, which isnt really true from a logical standpoint. The only thing that can occur to the class interface during the upcast is that it can lose methods, not gain them. This means that inside the method you cannot change what the argument reference points to: The methods f( ) and g( ) show what happens when primitive arguments are final: you can read the argument, but you can't change it. Also note the way that Orcs toString( ) method is defined in terms of the base-class version of toString( ). It makes sense that the compiler doesnt just create a default object for every reference, because that would incur unnecessary overhead in many cases. In this chapter youll see two ways to accomplish this. When the user knows youre assembling a bunch of parts, it makes the interface easier to understand. Code reuse is alleged to be one of the prime benefits of object-oriented programming. A Java JDBC PreparedStatement is a special kind of Java JDBC Statement object with some useful additional features. GC for instance. Feedback, Primitives that are fields in a class are automatically initialized to zero, as noted in Chapter 2. ", 0).charAt (0); c2 = keyboard.findWithinHorizon (". Java's multithreading system is built upon the Thread class, its methods, and its companion interface, Runnable. available in the derived class. But to be revolutionary, youve got to be able to do a lot more than copy code and change it. If your class doesnt have default arguments, or if you want to call a base-class constructor that has an argument, you must explicitly write the calls to the base-class constructor using the super keyword and the appropriate argument list: If you dont call the base-class constructor in BoardGame( ), the compiler will complain that it cant find a constructor of the form Game( ). Feedback, The following sections discuss the three places where final can be used: for data, methods, and classes. BoxAdcontent.document.close(); If it is, it can reclaim objects in any order it wants. BoxAdcontent.document.write("

<\/IFRAME>"); You simply place object references inside new classes. Its helpful to look at the whole initialization process, including inheritance, to get a full picture of what happens. This magical act is called inheritance, and the compiler does most of the work. You can think of inheritance, then, as reusing the class. Feedback, Just because something is final doesnt mean that its value is known at compile time. You simply place object references inside new classes. Java allows you to make arguments final by declaring them as such in the argument list. If you want the references initialized, you can do it: Feedback, All three approaches are shown here: Feedback, Note that in the Bath constructor, a statement is executed before any of the initializations take place. Its actually good (and useful) that you can still print them out without throwing an exception. Benefits of using Composition: Composition allows the reuse of code. Its easily conceivable that you might want to inherit and override with such a fundamentally useful class, but the designers somehow decided this wasnt appropriate. This paradigm lets you use the base class's functions and members in other classes An Animal class is often used to describe this behavior. When you dont initialize at the point of definition, theres still no guarantee that youll perform any initialization before you send a message to an object referenceexcept for the inevitable run-time exception. Until now, composition has been used quite frequently. If you make a method final, you are allowing the compiler to turn any calls to that method into inline calls. This lends credence to the theory that programmers are consistently bad at guessing where optimizations should occur. Create a class inside a package. Then call the base-class cleanup method, as demonstrated here. As you will see in Chapter 11, this incurs a significant performance overhead that probably wipes out any gains provided by final. This issue can cause confusion, because if you try to override a private method (which is implicitly final), it seems to work, and the compiler doesnt give an error message: Overriding can only occur if something is part of the base-class interface. You can do as much analysis as you want, but you still wont know all the answers when you set out on a project. If the class is defined inside a package, then the package statement should be the first statement in the source file. Because you cant access a private method, you cant override it. Here's a look at how this technique works in Java. Feedback, In more traditional languages, programs are loaded all at once as part of the startup process. BoxAdcontent.document.write("width=336 height=280 border=0 alt=\"Click Here\"><\/a>"); Java is an Object-Oriented Language. Next, the static initialization in the root base class (in this case, Insect) is performed, and then the next derived class, and so on. It is implied that the Java compiler is able to detect these situations and choose wisely whether to inline a final method. (This is just like C constants, which is where the convention originated.) When you inherit, you say This new class is like that old class. You state this in code by giving the name of the class as usual, but before the opening brace of the class body, put the keyword extends followed by the name of the base class. Feedback, A field that is both static and final has only one piece of storage that cannot be changed. Feedback, Note that Cleanser has a set of methods in its interface: append( ), dilute( ), apply( ), scrub( ), and toString( ). Feedback, But be careful with your assumptions. Feedback, The variables v1 through v3 demonstrate the meaning of a final reference. The confusion is what is the difference between "load" and "initialize". Inheritance in Java: A class (child class) can extend another class (parent class) by inheriting its features. By default, the compiler and the JVM search the current directory and the JAR file containing the Java platform classes so that these directories are automatically in your class path. You need to import the class and create the instances. (You can, however, write your class so that objects have the effect of being constant.) The Java Platform Class Hierarchy. This is the first time I’ve used Java and am having a little trouble shifting my mindset when it comes to inner classes. Right before you actually need to use the object. Type the class name “JavaFindBook” to search for the class in the workspace.6. BoxAdcontent.document.write(""); In this chapter, youll learn about these code reuse mechanisms. Specifically: The IllegalArgumentException is supposed to be used when a parameter value is inappropriate. But inheritance doesnt just copy the interface of the base class. BoxAdcontent.document.write("<\/center>"); Create a simple class. Feedback. [31] Feedback, Any private methods in a class are implicitly final. Is it possible to reuse streams in Java? However, in Wind.main( ) the tune( ) method is called by giving it a Wind reference. We should be aware of this issue when selecting whether to make a nested class be an inner one. So to plan for inheritance, as a general rule make all fields private and all methods public. Detergent would have no trouble, for example. Any time you want to allow this behavior with a class you create, you need only write a toString( ) method. Java automatically inserts calls to the base-class constructor in the derived-class constructor. Feedback, When you say that an entire class is final (by preceding its definition with the final keyword), you state that you dont want to inherit from this class or allow anyone else to do so. Its the relationship expressed between the new class and the base class. Java doesn’t support multiple inheritances but by using composition we can achieve it. It turns out that much of the syntax and behavior are similar for both composition and inheritance (which makes sense because they are both ways of making new types from existing types). Composition allows the reuse of code. No. Categories of extension. (There is no way that I know of to make the array references themselves final.) Like everything in Java, the solution revolves around the class. Create a Method. You reuse code by creating new classes, but instead of creating them from scratch, you use existing classes that someone has already built and debugged. After the base-class constructor completes, the instance variables are initialized in textual order. However, its best to let the compiler and JVM handle efficiency issues and make a method final only if you want to explicitly prevent overriding. The statics, of course, are initialized only once. be initialized before the constructor is called. Like everything in Java, the solution revolves around the class. Feedback, If the base class has a base class, that second base class would then be loaded, and so on. Although it is not very powerful, it is still useful in many cases. When you create an object of the derived class, it contains within it a subobject of the base class. Because you can only add a String to another String, it says Ill turn source into a String by calling toString( )! After doing this it can combine the two Strings and pass the resulting String to System.out.println( ). This is often called. It might contain more methods than the base class, but it must contain at least the methods in the base class. Java uses the extends keyword to implement the principle of inheritance in code. This technique of putting a main( ) in each class allows easy unit testing for each class. Simply put, in Java, a class can inherit another class and multiple interfaces, while an interface can inherit other interfaces. Because Detergent is derived from Cleanser (via the extends keyword), it automatically gets all these methods in its interface, even though you dont see them all explicitly defined in Detergent. The ones I've seen are: new StringBuilder which basically recreates the object setLength(0) which… It's common knowledge in Java that you want to clear a StringBuilder object, and not recreate it. An Animal is used to inherit from for a Lion and a Human and many other types of Animals, each with their own different functions, like a Lion can roar and Humans can speak, but they all have something similar, they can all walk. Therefore, you can change the behavior of the composed object at run time. In this case, you might want to call the method from the base class inside the new version. In general, this means that youre taking a general-purpose class and specializing it for a particular need. BoxAdcontent.document.write("BC"); However, Chapter 15 has information about profiling, which can be helpful in speeding up your program. Heres an example that demonstrates final fields: Since VAL_ONE and VAL_TWO are final primitives with compile-time values, they can both be used as compile-time constants and are not different in any important way. Also note that i5 cannot be known at compile time, so it is not capitalized. BoxAdcontent.document.write("<\/noscript>"); The car has attributes, such as weight and color, and methods, such as drive and brake. A method must be declared within a class. As an example, consider a base class called Instrument that represents musical instruments, and a derived class called Wind. Let's begin with the getSimpleName() method. Feedback, The trick is to use the classes without soiling the existing code. One of the core principles of Object Oriented Programming – inheritance – enables us to reuse existing code or extend an existing type. The next chapter (on polymorphism) provides one of the most compelling reasons for upcasting, but if you remember to ask Do I need to upcast? youll have a good tool for deciding between composition and inheritance. Java is an object-oriented language: at its heart are objects and classes. As you can see in main( ), just because v2 is final doesnt mean that you cant change its value. In Java, every method must be part of some class which is different from languages like C, C++, and Python. This article is an English version of an article which is originally in the Chinese language on aliyun.com and is provided for information purposes only. Feedback, Java doesnt have this problem because it takes a different approach to loading. Feedback. It is very common to use composition and inheritance together. Java is capable of loading classes at run time. Since Queue is an interface, objects cannot be created of the type queue.We always need a class which extends this list in order to create an object. One of the core principles of Object-Oriented Programming – inheritance – enables us to reuse existing code or extend an existing type. (As long as main( ) is public, it doesnt matter whether the class that its part of is public.) Feedback, Java allows the creation of blank finals, which are fields that are declared as final but are not given an initialization value. Question: Goal: In This Lab, We Will Practice How To Apply The Composition Concept To Reuse A Java Class As A Whole (i.e., Not By Copying And Pasting) That Has Already Been Defined By The Programmer. Feedback, It turns out that much of the syntax and behavior are similar for both composition and inheritance (which makes sense because they are both ways of making new types from existing types). However, blank finals provide much more flexibility in the use of the final keyword since, for example, a final field inside a class can now be different for each object, and yet it retains its immutable quality. The class XYZ is inheriting the properties and methods of ABC class. Note that code reuse is a very compelling reason to use abstract classes, as long as the “is-a” relationship within the class hierarchy is preserved. } Since the derived class has the base-class interface, it can be upcast to the base, which is critical for polymorphism, as youll see in the next chapter. Using Multiple Classes. Many programming languages have a way to tell the compiler that a piece of data is constant. A constant is useful for two reasons: In the case of a compile-time constant, the compiler is allowed to fold the constant value into any calculations in which its used; that is, the calculation can be performed at compile time, eliminating some run-time overhead. The Java JDBC PreparedStatement primary features are: But to be revolutionary, youve got to be able to do a lot more than copy code and change it. (Youll see that the method names in Hashtable are much briefer compared to those in Vector, another piece of evidence.) Inside tune( ), the code works for Instrument and anything derived from Instrument, and the act of converting a Wind reference into an Instrument reference is called upcasting. - The Singleton's purpose is to control object creation, limiting the number of objects to only one. Since a private method is unreachable and effectively invisible, it doesnt factor into anything except for the code organization of the class for which it was defined. For instance, I have written a class in file A.java (with the corresponding compiled class file available) and wish to use that class in B.java. Ways to create object of a class. Heres an example: Feedback, This demonstrates a number of features. This is often called. It just uses java's standard java.net classes to open a connection. Remember, you need a Statement in order to execute either a query or an update.You can use a Java JDBC PreparedStatement instead of a Statement and benefit from the features of the PreparedStatement.. Can we reuse stream? function writeTribalBoxAdContent() { However, because it prevents inheritance, all methods in a final class are implicitly final, since theres no way to override them. If you want cleanup to take place, make your own cleanup methods and dont rely on finalize( ). A method can perform some specific task without returning anything. If we consider the real-world, we can find many objects around … (Note that the Java 2 container library replaces Hashtable with HashMap.) class files, resource files, etc.) And you dont need to remove the main( ) when youre finished testing; you can leave it in for later testing. After that you realise that you need class D that has different properties than class A, but you still want the same classes B and C without any changes. Java Classes/Objects. Inheritance-oriented reuse. You could extract the common code into a base class, but base classes tend to become tangled or bloated as different subclasses demand different behavior from the base class. When inheritance is used, an existing class (the superclass) is extended to create a new class (the subclass). Feedback, One of the advantages of inheritance is that it supports incremental development. This way of including classes inside other classes is also referred as class composition or has a relation.That is similar to car is composed of engine, tyres and seats or room has a fan etc.,. You dont even need the source code for the methods in order to reuse the code. This feature seems only marginally useful, and is probably not something youll use. BoxAdcontent.document.write("<center><script language=javascript><!--"); After the class keyword, the class name to identity the class should come. constructors, call the base-class constructor. VAL_THREE is the more typical way youll see such constants defined: public so theyre usable outside the package, static to emphasize that theres only one, and final to say that its a constant. That file isnt loaded until the code is needed. Devising unique naming conventions can be a tedious chore, but reusing method names via overloading can make the task easier. Primitives that are fields in a class are automatically initialized to zero, as noted in Chapter 2. Of course, in particular cases you must make adjustments, but this is a useful guideline. This means that theyll always Feedback, When you inherit, you take an existing class and make a special version of it. Its best to not rely on garbage collection for anything but memory reclamation. The try keyword indicates that the block that follows (delimited by curly braces) is a guarded region, which means that it is given special treatment. (Of course, you can draw your diagrams any way you find helpful.) before derived-class constructors. Creating Queue Objects. This means that theyll always Its just that from the outside, the subobject of the base class is wrapped within the derived-class object. The power of object-oriented systems lies in their promise of code reuse. The base class construction goes through the same process in the same order as the derived-class constructor. 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