《Head First Java》

Dogs bark, fish swim and don’t make sounds, and parakeets speak better than dogs. That’s why it’s easier to create a class Fish that will inherit some common behavior and attributes from the class Pet, rather than creating Dog, Parrot or Fish from scratch every time. The value of the variable petReaction is passed as an argument to the method prinln for output. The variable petReaction will store the value returned by the method say.

The modified version of the Swimmable interface includes a static method isSummer that checks the current month and returns true if it’s June, July, or August. The default method dive calls the static method isSummer and https://deveducation.com/ either agrees or disagrees to dive depending on the time of the year. It also implements Swimmable, but has its own version of the method dive, which will override the default implementation of the dive from Swimmable.

By using lambda expressions I was able to eliminate the need for creating a separate class for each animal. Of course, this https://deveducation.com/blog/10-luchshih-knig-po-programmirovaniiu-na-java/ is possible only if the only difference between classes Dog and Parrot was implementation of the talking behavior.

If the players (or game spectators) will occupy several rows, then we’ll have two dimensions – the row number and a set number within the row. This is the case where we’d need to declare a two-dimensional array. Java allows creation of multi-dimensional arrays, and I’ll show you how to do this in Chapter 10. If a constructor with arguments has been defined in a class, you can no longer use the default no-argument constructor.

You can add implements Callable to the declaration of your class and implement the method call, which plays a similar role to the method run. The program HashTableBrokenDemo allowed us to store anything in the collection but crashed during the runtime, but HashTableGenericsDemo prevented this error from happening. Having a compiler’s error it’s a lot better than getting surprises during the runtime, isn’t it? Besides, with generics we’ve eliminated the need to cast objects.

6. Project: Adding Events Handling For Calculator

Let’s declare a simple class ContactDetail, which can store contact details of one person. The code to add (and extract) a particular Fish to the ArrayList collection may look similar to the program FishTank that comes next. IntelliJ IDEA can automatically generate setters and getters for the class. Just right-click on the class name and select the options Refactor | Encapsulate Fields. Let’s see how you can work with some other objects in ArrayList, for example instances of the class Fish shown next.

From now on, I’ll be adding comments to code samples to give you a better idea how and where to use them. If you do not want this to happen, the class Fish has to override the Pet’s typical behavior java книги – the method talk. If you declare a method with exactly the same signature in a subclass as in its superclass, the subclass’ method will be used instead of the method of the superclass.

Note that the first two lines print the output from the second loop! This means that after calling the method start on the thread myThread the program didn’t wait for the completion of the first loop, which ran in parallel thread. The entire console output is a mix of two parallel number-crunching threads.

The next version of the class Dog implements only the method swim from Swimmable. As with switch statements, the keyword break is used to jump out of the loop when some particular condition is true. Let’s say we do not want to print more than 3 congratulations, regardless of how many players we’ve got. In the next example, after printing the array elements 0, 1 and 2, the break will make the code go out of the loop and the program will continue from the line after the closing curly brace. The array of players in our example is called one-dimensional array.

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You can still invoke the method dive on the instance of the Dog class – the default implementation will be invoked. Now the class Dog doesn’t have to implement the method dive – the compiler will see a default implementation and won’t complain.

Change the declarations of the classes Dog and Fish so each of them extend Pet while implementing Talkable and Swimable interfaces. In the package pets recreate the final versions of classes Dog, Fish and interfaces Swimmable and Talkable from the section Interfaces.

Let’s add the following method talk to the class Fish. Accordingly, the class Pet is a superclass of the class Fish. In other words, you use the class Pet as a template for creating a class Fish.

The keyword extends defines relationship between classes. Since our Fish extends a Pet, we can say that a fish is a pet. The class Pet can include behaviors java книги and attributes that are shared by many pets – all of them eat and sleep, some of them make sounds, their skin has different colors, and so on.

Then the above code will print „Good Job!” and will break out of the switch statement to continue executing the rest of the program code if any. If an expression inside the parentheses is true, JVM will execute the code between the first pair of curly braces, otherwise it goes to the code after java книги the else statement. In other words, if the value of the variable totalSpent is more than a hundred, give a 20% discount, otherwise take only 10% off. Based on the result of evaluating this expression, your program execution forks, and only the relevant portion of the code will be invoked.

While the ArrayList collection only allows referencing its elements by index (e.g. fishTank.get(i)), sometimes it would be easier to reference collection elements by names as key/value pairs. I’ll illustrate by storing my friends’ contact information in a HashTable collection that allows accessing objects by key names.

If you’d like to have a constructor without arguments – write one. Constructors are special methods that are called only once during construction of the object in memory.

The easiest way of implementing multi-threading is by using lambda expressions that were introduced in Chapter 5. As a refresher, you can use lambdas to implement functional interfaces – those that have only one abstract method. The interface Runnable declares a single abstract method run, which makes it a perfect candidate to be implemented with lambdas.

To start the execution of the thread, we need to call the method start, which is defined in the class Thread and will internally invoke the method run, which is implemented as lambda. The method start doesn’t wait for the completion of the method run so the next line in our main method is executed. The following class MultiThreadedDemo executes two loops in parallel. It creates a separate thread for the fist loop, and starts it in a separate thread so the main thread, which has the second loop doesn’t wait for the first one to complete. First, I’ll show you a pretty simple program that executes all code sequentially in one main thread, and then I’ll re-write it to be executed in parallel.

4. Special Methods: Constructors

If an interface has only one abstract method declared (default and static methods don’t count) it’s called functional interface. Both Talkative and Swimmable are examples of a functional https://itstep.org/ interface – each has only one abstract method. Java 8 introduced a special way of implementing functional interfaces using lambda expressions, which we’ll discuss later in this lesson.