So far, all our programs perform actions in a sequence – one after another. So if a program java книги calls two methods, the second method is invoked only after the first one completes.

Have you noticed that the member variable and the constructor’s argument have the same name? In this code sample this.currentDepth refers to the object’s member variable currentDepth, while the currentDepth refers to the argument’s value. In other words, the code points at the current instance of the Fish object. If you do not create a constructor for the class, Java automatically creates one during the compilation time – it’s so-called default no-argument constructor. That’s why Java compiler has never complained about the statements new Fish() or new ReportCard(), even though neither class Fish nor class ReportCard has any explicitly declared constructor.

Hence one contact can be referred by the key get(“Jackie”), and the other one as get(“Art”). The method get returns the collection element as the Object type, so I had to cast it to ContactDetail to be able to see the field phone. The ArrayList collection uses Java arrays internally and initially creates an array for 10 elements. But if you keep adding more elements to ArrayList it internally will create another array of the larger size and copy all existing elements there. Because of this additional memory allocations and data copying ArrayList collections works a little slower than arrays, which allocate enough memory in advance.

If you read game scores from the disk file, you do not know in advance how many scores were saved there. Most likely you’ll be reading the scores using the while or for-each loop, which don’t require you to state the number of iterations upfront. The variable pl has the type String – the same as as the array’s elements. Use this variable as a cursor, point it to each element in the array players one by one and repeatedly execute the code inside the curly brackets for the current element.

I do this to show you how to write a lambda expression that implements a method that has an argument and returns some value. The packages are stored in directories and subdirectories on the disk.

When we’ll work on the GUI for the calculator, I’ll also show you how to design a scene using a combination of layouts. Then the Java program will load this .fxml file and display your creation. By writing the code in Java as you did in previous chapters. After creating lots of code samples producing outputs to the system console, it’s time for creating a nice looking Graphic User Interface (GUI).

6. Project: Adding Events Handling For Calculator

java книги

Finally, let’s set the size of the stage so it can accommodate all components from our scene. In IDEA, open the class Main, and in the code set the size of the scene to be 200×150 pixels. I’m not going to cover each JavaFX layout in detail, but will show you how to use a pretty powerful layout – GridPane.

No, it won’t because the absence of the access qualifier gives a package access level to this member. Only classes that are located in the package vehicles can access it, and CarMaster is not one of them. By looking at this code I can say that the class Car exposes only one public method brake, which internally may invoke several other functions that a driver does not need to know about. For example, if the driver pushes the brakes pedal too hard, the car’s computer may apply special anti-lock brakes implemented in private methods.

The class members declared with the protected keyword can be accessed from other members of the same class, from its descendants or classes located in the same package. Even if a descendant class is located in a different package, it’ll be able to access protected methods of its ancestors. What do you think will happen if you simply remove the private keyword from one of the methods of the class Car?

  • This is how to create an instance of the object Pet using the Java operator new.
  • Say, they can define a class Player in the gaming application and create two object instances of this type.
  • JavaFX is also a library that’s included with JDK, and you can write your GUI programs using familiar Java language.
  • On the other hand, JavaFX allows you to create and arrange GUI components in a special FXML format, while writing application logic in Java.

We’re accustomed to programs that open windows with buttons, text fields, menus and other GUI java книги components. Note that I’m overriding the method toString that exists in the class Object.

We start the program with creating a new instance of the Thread class using its constructor that takes an object that implements Runnable interface. You can add implements Runnable to the declaration of a class and implement the method run, which, again, should contain the code to be executed in parallel. A good example of a program that creates multiple threads is a Web browser. For instance, you can browse the Internet while downloading some files so one program (the browser) runs two threads of execution (browsing and downloading) in parallel.

One more thing, even though the method dive returns the value of currentDepth, our FishMaster does not use it. For example, think of a class FishTrafficDispatcher that has to know positions of other fish under the sea before giving permissions to dive to avoid traffic accidents. The method dive has an argument (a.k.a. parameter) howDeep that tells the fish how deep it should go. We’ve also declared a class variable currentDepth that will store and update the current depth every time you call the method dive. This method returns the current value of the variable currenDepth to the calling class.

This means that this variable is available only for the code within this method. When the method is complete, this variable automatically gets removed from memory.

In the project chapter5 create a new package called lambdapets. Create the class PetMaster from the section “Interfaces”, but give pets names while instantiating classes Dog and Fish. In the package pets create a new class Pet with a constructor that java книги will take the name of the pet (of type String) as an argument. Let’s start with defining the functional interface Talkative. This time I’ll slightly change the signature of the method talk comparing to the version shown earlier in this chapter.

If you see a full class name java.time.LocalDate it means that this class was originally created in the subdirectory time of the directory java. Java comes with many useful classes that are organized in packages. Some packages include classes responsible for drawing, while other packages have classes to work with the Internet, and so on. For example the class LocalDate is located in the package called java.time, and the full name of the class LocalDate is java.time.LocalDate. In Chapter 9 you’ll learn how to save the data on the disk and how to read the saved data back into computer’s memory.

In other words, such a program has only one thread of execution, where the code works sequentially. Another important thing to note is that there is no casting needed when the program gets the information about Jackie’s phone. Now the friends collection knows from the very beginning that it stores not just some Object types, but the ContactDetail instances. In this example I used the first name as the key in the method put that adds elements to a Hashtable.

Programmers use the word scope to say how long a variable will live, for example you can say that variables declared inside a method have a local scope. If a variable is declared within the code block surrounded with curly braces (e.g. in java книги the if statement), it has a block scope and won’t be visible outside of this block. Class ReportCard declares a variable grade inside the method convertGrades(). If you declare a variable inside any method, such variable is called local.

4. Special Methods: Constructors

Can we use a member variable out from the class System if we haven’t even created an instance of this class? Yes we can, if the class System declares the variable out with a keyword static.