Oct 19, Add ability to add minimal compatible version to ProductData. Nov 11, Sep 28, Add javadocs for several PdfFormField methods. Nov 12, Added local high precision property. Added tests. Oct 7, Add JavaDocs to AccessibilityProperties class. Nov 15, Nov 23, Remove anonymous classes usages from main non test code.
Cover several sign methods. Nov 19, In Java Polymorphism, the concept of method overloading and method overriding is achieved, which is the dynamic approach. In a class hierarchy, when a method in a child class has the same name and type signature as a method in its parent class, then the method in the child class is said to override the method in the parent class.
Java programming can have two or more methods in the same class sharing the same name, as long as their arguments declarations are different. Such methods are referred to as overloaded, and the process is called method overloading. Superclass only defines a generalized form shared by all of its subclasses, leaving it to each subclass to implement its methods. Thus, they are similar to class except that they lack instance variables, and their methods are declared without anybody.
Learn more about Java Constructor. An array is a group of like-type variables referred by a common name, having continuous memory. Primitive value objects are stored in an array. It provides code optimization since we can sort data efficiently and also access it randomly.
The only flaw is that we can have a fixed-size element in an array. The String class, which implements the CharSequence interface, defines several methods for string manipulation tasks. The list of most commonly used string methods are mentioned below:. Multitasking: Process of executing multiple tasks simultaneously to utilize the CPU. A thread is always in one of the following five states; it can move from one state to another in a variety of ways, as shown.
The run method is declared in the Runnable interface is required for implementing threads in our programs. The exception is an abnormality or error condition caused by a run-time error in the program; if this exception object thrown by the error condition is not caught and handled properly, the interpreter will display an error message.
If we want to avoid this and want the program to continue, we should try to catch the exceptions. This task is known as exception handling. A code can have more than one catch statement in the catch block; when an exception in the try block is generated, multiple catch statements are treated like cases in a switch statement.
Finally, statement: used to handle exceptions that are not caught by any previous catch statements. A final block is guaranteed to execute, regardless of whether or not an exception is thrown. Collection of related records stored in a particular area on the disk termed as the file. The files store and manage data by the concept of file handling. Java uses the concept of streams to represent an ordered sequence of data, a path along which data flows.
Thus, it has a source and a destination. They are used to read 8-bit bytes include a superclass known as InputStream. InputStream is an abstract class and defines the methods for input functions such as :. These classes are derived from the base class OutputStream. OutputStream is an abstract class and defines the methods for output functions such as :. The collections framework is contained in java.
Wait a minute! Yes we can, if the class System declares the variable out with a keyword static. When you start any Java program it loads the definition of the required classes in memory. The definition of a class can be used for creation of one or more instances of this class. For example:. In this example we have two instances of the class ReportCard , and each of them has its own value in the variable studentName , which is an instance variable.
In this case both instances of the ReportCard would share the same variable studentName , and the above code would first assign "Jerry Lee" to this variable, and then it would be replaced with "Sarah Smith". Moreover, if the declaration of a member variable or a method starts with static , you do not have to create an instance of this class to use such a variable or a method.
Static members of a class are used to store the values that are the same for all instances of the class. For example, the method convertGrades should be declared as static in the class ReportCard , because its code does not use member variables to store values specific to a particular instance of the class.
There is no need to create instances to call static methods or access static variables. Just write the name of the class followed by the dot and the name of the static member:. All these methods are static and you do not need to create an instance of the class Math to invoke them, for example:. Parentheses after the word Fish tell us that this class has some method called Fish. Yes, there are special methods that are called constructors , and these methods have the following features:.
Constructors are special methods that are called only once during construction of the object in memory. Any class can have more than one constructor. Now the class FishMaster can create an instance of the Fish and assign the initial position of the fish. If a constructor with arguments has been defined in a class, you can no longer use the default no-argument constructor. The keyword this is useful when your code needs to refer to the instance of the object, where this code is running.
Look at the next code example, which is a slight modification of the previous one:. The keyword this helps to avoid name conflicts. In this code sample this. In other words, the code points at the current instance of the Fish object. An array is an object that holds several values of the same type - primitives or objects. Instead of declaring four different String variables, you can declare one String array that has four elements.
Arrays are marked by placing square brackets either after the variable name, or after the data type:. These declarations just tell the Java compiler that you are planning to store several text strings in the array players.
Each element has its own index position number starting from zero. The next sample actually creates an instance of an array that can store four String elements and assigns the values to the elements of this array:.
You must declare the size of the array before assigning values to its elements. If you do not know in advance how many elements you are going to have, you cannot use arrays, but you should look into other classes - Java collections.
For example the ArrayList object does not require you to announce the exact number of elements in advance. Any array has an attribute called length that stores the number of elements in this array, and you can always find out how many elements are there:. If you know all the values that will be stored in the array at the time when you declare it, Java allows you to declare and initialize such array in one shot:.
Do you know why the second element has the index [1]? Of course you do, because the index of the first element is always [0]. The array of players in our example is called one-dimensional array. Imagine the players sitting like ducks in a row. The single dimension is the seat number here. A loop is a language construct that allows to repeat the same action multiple times. For example, if we need to print congratulation to several winners, the printing code should be invokes several times in a loop.
When you know in advance how many times this action has to be repeated, you can use a loop with a keyword for :. Print the value of the element from the players array whose number is the same as the current value of the counter. JVM executes every line between the curly braces and then returns back to the first line of the loop to increment the counter and check the conditional expression.
There is another flavor of the for loop known as for each loop. It allows the program to repeat the same action to every element of the collection without even knowing how many are there.
You are basically saying, do this for each element. The for-each loop allows to congratulate players in a more concise manner:. 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.
There is another keyword for writing loops - while. In these loops you do not have to declare exactly how many times to repeat the action, but you still need to know when to end the loop. If you read game scores from the disk file, you do not know in advance how many scores were saved there. You can also use two important keywords with loops: break and continue.
As with switch statements, the keyword break is used to jump out of the loop when some particular condition is true. 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. This means that you are comparing the value of the variable counter with number 3.
A single equal sign in the here would mean assigning the value of 3 to the variable counter. The keyword continue allows the code to skip some code lines and return back to the beginning of the loop. Imagine that you want to congratulate everyone but David — the keyword continue will return the program back to the beginning of the loop:. There is yet another type of the while loop that starts with the word do , for example:. Such loops check an expression after executing the code between curly braces, which means that code in the loop will be executed at least once.
Loops that start with the keyword while might not be executed at all if the loop expression is false to begin with.
Name it Chapter4. In the src directory of the project create a new class the menu File New named TemperatureConverter. Write the if statement inside the method convertTemp to check the value of the argument convertTo. Return the result. The method convertTemp should look like this:. It should look like this:.
If you did everything right, you should see the following output on the IDEA console:. Do a little research to change this program so it always prints the temperature with two digits after the decimal point. Classes have attributes e. There is a special group of programming languages that are called functional e. JavaScript, Scala et al. In object-oriented languages behavior is implemented in methods defined in classes. The phrase to implement a method simply means to write code in a method.
Starting from Java 8 you can also program behavior in so called lambda expressions covered later in this chapter. So far I was declaring and implementing behavior in methods located inside Java classes. But there is a way and a reason to separate the method declaration from its implementation. Such separation can be done using interfaces. While you can just create a Java class and implement all its methods there, a more formal way of declaring the behavior of the class in a separate interface.
Then a class declaration would include the keyword implements followed by the name of the interface. This is a way to declare an application program interface API - what the class can do - so the programmer can quickly look at the short declaration of the interface rather than going through a longer code of the class.
For years, Java interfaces were used mainly for holding method declarations and final static variables. For example, the interface Talkable contains the declaration of just one method talk :. For example, a dog, a cat, and a parrot can "talk". But they "implement it" differently.
A parrot can repeat words, while dogs bark. Cats talk by meowing. Accordingly you can declare three classes that will implement Talkative interface differently, as shown below:.
These three classes use the Java keyword implements , which means they promise to implement all abstract methods in the interface s listed after the keyword implements otherwise the Java compiler will complain: "You promised to implement all abstract methods from Talkable , where are they?
Keep your promise! What if we need to create the class Fish that uses interfaces? But they swim. Dogs can talk bark and swim, right? The Fish will implement Swimmable and the Dog will implement two interfaces:. When a class implements several interfaces, make sure to implement each and every abstract method declared in these interfaces. A class can extend another class and implement interfaces at the same time, for example:. Yes, it can. Java 8 introduced a new keyword default.
The next version of the class Dog implements only the method swim from Swimmable. There is no need to change the class Fish. It also implements Swimmable , but has its own version of the method dive , which will override the default implementation of the dive from Swimmable. You can still invoke the method dive on the instance of the Dog class - the default implementation will be invoked.
The next class PetMaster will demonstrate this. Starting from Java 8, interfaces are also allowed to include static methods, which are not specific to any instance and can be used only internally by other methods of the interface. The following example illustrates the use of a static method in the interface. The default method dive calls the static method isSummer and either agrees or disagrees to dive depending on the time of the year.
The only values that are allowed here are Month. 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. To let the compiler know where the class LocalDate is located you could specify the full class name, for example:.
But this syntax is difficult to read so we use the import statements above the class declaration to let the compiler know the location of the class, interface, or enumeration.
The packages are stored in directories and subdirectories on the disk. If you see a full class name java. LocalDate it means that this class was originally created in the subdirectory time of the directory java. If you want to place your class into a package, just create a subdirectory e.
One last thing: Lazy kids use the wild cards in import statements. Instead of writing one import statement per class, they would use an asterisk:. This means that definitions needed for my program are located in the package java. Of course, writing one import statement instead of several ones looks appealing, but the readability of the program suffers. Beside method declarations, default and static methods you can add static final variables to the interface declaration.
Such variables can be used by the code inside the interface or in the classes that implements it. For example, the interface Swimmable can define the maximum depth allowed for diving. Both Talkative and Swimmable are examples of a functional interface - each has only one abstract method. If a method is not implemented we call it abstract, but classes can be declared abstract too, and Java has the keyword abstract for this.
An abstract class is called abstract if it was declared with the keyword abstract , for example:. Typically, abstract classes have some non-implemented methods that are also declared with the abstract keyword:.
The class Animal has two implemented methods: setName and sleep and one abstract method talk. Since the abstract class can not be instantiated, the programmer has to create a descendant class and implement the method talk there if he or she wants to create an instance of such a class, for example:.
Maybe the programmer wants to create a class Puppy that extends Dog and implements the method talk there? Abstract classes allow you to implement some common static or instance-specific behavior, e. So you can have a class Dog that extends any class, while implementing an interface Talkative that declares the method talk.
If you need to use an object of certain type only once, you can kill two birds with one stone: declare an anonymous class and create an instance of it. I want to keep these examples in the package called calc. You can create a subdirectory named calc in your project and save your classes there. The other option is to right-click on the folder src in your IDEA project and select the menu New Package and enter the package name there:.
The code of the class Calculator is pretty simple. Note the first line that declares the package where the class Calculator belongs. This class has two methods that manipulate numbers one per operation , and its main method invokes each method passing the same values as arguments. After performing the arithmetic operation each method prints the result. Running this program produces the following output:. Instead of writing a separate method for each operation, I want to write a generic method that can receive the code of the operation and two numbers to be operated upon.
This method will have three arguments: the arithmetic operation, the first, and the second number. I will also write a method calculate that will take three arguments: the object that implements ArithmeticOperation and two numbers. What did we achieve by re-writing calculator this way? We separated the declaration of the behavior and its implementation.
The anonymous classes allowed me to create a wrapper object around the method performOperation , so this object could be passed as an argument to the method calculate. This means that such a variable or a method can be accessed by any other code from the project. First, let me introduce a new term - a function. This is a main idea of any object-oriented programming language - the classes and objects are the first-class citizens.
They allow you to implement behavior by writing functions, which are similar to methods in that they can have names, take arguments, and return results. A lambda expression is a function without a name or anonymous function that you can assign to a variable, pass as an argument to a method or return from a method.
In earlier versions of Java you could pass a value to a method only if this value was an object or a primitive. But now a function a piece of code becomes a value that can be passed around. I will reuse the same functional interface with the three-argument method calcuate , but will pass the lambda expression that implements ArithmeticOperation as the first argument. The difference between CalculatorWithAnonymousClasses from the previous section and CalculatorWithLambdas is that the former implements the functional interface as anonymous classes and the latter as lambdas.
Lambda expressions offer a concise way of implementation of functional interfaces. To write a lambda expression you need the play by the rules:. Make sure that the arguments of your lambda expression match the arguments of the abstract method. Make sure that the return value of your lambda expression matches the return value of the abstract method. Review the code of the CalculatorWithLambdas. Both lambdas addition and subtraction abide by these rules.
You may wonder, "Why are there no data types specified for the lambda parameters first and second? An educated guess such as this is called type inference. But we can declare just one class with a method that can take the implementation of the Talkative in a form of lambda expression. 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 only other class we need to create is PetMasterLambda that will create instances of Pet representing both dogs and parrots, but passing different implementation of the Talkative interface.
Here it comes:. Not that I have not specified the data type of the variable name. The second argument of the method speakup will be passed to the method talk. By using lambda expressions I was able to eliminate the need for creating a separate class for each animal. Of course, this is possible only if the only difference between classes Dog and Parrot was implementation of the talking behavior.
Part 1. In the package pets recreate the final versions of classes Dog , Fish and interfaces Swimmable and Talkable from the section Interfaces. In the package pets create a new class Pet with a constructor that will take the name of the pet of type String as an argument.
Change the declarations of the classes Dog and Fish so each of them extend Pet while implementing Talkable and Swimable interfaces. Create the class PetMaster from the section "Interfaces", but give pets names while instantiating classes Dog and Fish. For example, "My name is Sammy.
Part 2. Its method speakup will not have arguments. This is how the code of the new Pet should look like:. Write a new version of the class PetMasterLambda that will define talking rules for dogs and parrots. It should create two instances of Pet one for a dog, and one for a parrot and invoke the method speakup on each instance. Remember arrays and their limitations? Java collections offer alternative ways of storing similar objects. This means that such a variable or method can be accessed by any other code from the project.
You can declare a class, a method, or a member variable to be public , private , or protected. One of the main features of object-oriented languages is encapsulation - the ability to hide and protect data or code.
But who do you need to hide or protect the code from? This is not a protection from bad guys who are not allowed to see your code. You hide and protect the code from misuse. For example, another programmer extended your class and is trying to change the value of a variable in your class, which may result in bug. Think of a car — most people have no clue how many parts are under the hood, and what actually happens when a driver pushes the brake pedal.
When you design a class, hide methods and class member variables that should not be visible from outside. Some of the methods of member variables are meant to be used internally and not to be exposed to other classes. If a variable or a method are declared private, they can be used only within the class where they were declared. The next code sample represents the class Car and declares some of its members as private.
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. Only the method brake is visible from the class Car other methods are from the class Object.
The private access level is the most restrictive one. What do you think will happen if you simply remove the private keyword from one of the methods of the class Car? Will the CarMaster see it? Only classes that are located in the package vehicles can access it, and CarMaster is not one of them. 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.
Some software developers are creating libraries or frameworks of classes to be used by other developers. These classes can be extended, and their creators may use the keyword protected trying to allow access to certain member only descendant classes. But in the real world no one can predict what the developer may want to do with their libraries, and the keyword protected may become an obstacle in achieving of their goals.
I do not use the keyword protected in my projects. As you gain more experience with Java, see for yourself if the keyword protected brings some value to your programs. In Java all classes are directly or indirectly inherited from the class Object , which is the root of the class hierarchy. So any of your application classes have an ancestor - Object. When you declare a variable to represent an instance of your class, you can give it a type either of your class or any of its ancestors. For example, both of the following declarations are correct:.
Java compiler can cast convert one data type to another as long they have inheritance relation. In particular, Java compiler can automatically cast the type to the class ancestors. This is called upcasting. The programmer can downcast the general type to a more specific one, but this has to be done explicitly by placing the specific type of the object in parentheses before the variable of more general type, for example:.
Why do we need all these complications? For example, JDK comes with lots of other classes that were written to work with the Object data types.
Data collection classes were written to be able to store instances of any objects. Creators of data collections had no idea that you might need to store instances of Car or Fish there. But when you use the data collection object in your program, the data type is known. Say you have a hundred songs in your MP3 player. Java packages java. Some of the popular collection classes from the package java. The package java. For example, if you want to write a program that would print all your followers in Twitter, their number may change many times a day.
The class ArrayList can give you more flexibility - it can grow or shrink in size as needed. Think of a list of something - a list of songs, a To-Do list, a list of friends names. Pretty much any objects that can be used with the word list can be stored in an ArrayList. Why use arrays, then? Unfortunately, nothing comes for free, and you have to pay the price for having a convenience of dynamically sized arrays.
The ArrayList works a little slower than a regular array. Besides, you can only store objects there, while arrays allows you to store primitives too. To create and populate an ArrayList you should instantiate it first and then create instances of the objects you are planning to store there. Add each object to the ArrayList by calling its method add. The next little program will populate an ArrayList with String objects and then print each element of this collection.
The method get extracts the element located at the specified position in the ArrayList. Since you can store any objects in this collection, the method get returns each element of the Object type.
The program has to cast this object to a proper data type. We did not do it in the previous example only because we stored String objects in the collection friends , and Java knows how to convert an Object to a String automatically. Note that the class fields color and weight are private variables.
But this class also defines public getters and setters - the methods that read or modify the fields. For example, you could encapsulate the logic that checks the credentials of the users of this class so not everyone can modify the weight property. By Java naming conventions the setter name starts with the prefix set followed by the capitalized letter of the corresponding private variable.
Accordingly, the getter starts with get. The code to add and extract a particular Fish to the ArrayList collection may look similar to the program FishTank that comes next.
First, this program creates a couple of instances of the class Fish passing the values for the fields via constructor. It is easy to add an image in pdf using PDFMake. You can specify an image in the form of base64 data URI image or in the virtual file system you can specify image path.
I have created a separate method getProfilePicObject for getting profile picture image configuration and called it in document definition so that we add configuration only when the user has uploaded an image.
Personal details we have displayed in two columns, the same way we will display the skills list in three columns list. Tables are similar to columns. We will generate experience table dynamically with getExperienceObject method and education table with getEducationObject method.
We will show name and mobile no. PDF documents can have various metadata associated with them, such as the title , or author of the document. You can add that information by adding it to the document definition. In a web application, We can generate pdf using various approaches: Using browser print function: It is an easy option when you want to print a complete web page as a pdf. You can also customize pdf up to some limits. Generating PDF using Backend application or third-party reporting tools and download it on client-side: You have more control over pdf formatting and design and you can process large amounts of data.
Though this type of pdf generation approach required a separate API call for generating the pdf. Environment Setup. Create a Angular Project.
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