Create new type java

Содержание

Generic Types
A Simple Box Class
A Generic Version of the Box Class
Type Parameter Naming Conventions
Invoking and Instantiating a Generic Type
The Diamond
Multiple Type Parameters
Parameterized Types
Java User-defined Data Types
User-defined Data Types in Java
1. Class
2. Interface
Conclusion
Recommended Articles
Create instance of generic type in Java?

Generic Types

A generic type is a generic class or interface that is parameterized over types. The following Box class will be modified to demonstrate the concept.

A Simple Box Class

Begin by examining a non-generic Box class that operates on objects of any type. It needs only to provide two methods: set, which adds an object to the box, and get, which retrieves it:

public class Box < private Object object; public void set(Object object) < this.object = object; >public Object get() < return object; >>

Since its methods accept or return an Object, you are free to pass in whatever you want, provided that it is not one of the primitive types. There is no way to verify, at compile time, how the class is used. One part of the code may place an Integer in the box and expect to get Integers out of it, while another part of the code may mistakenly pass in a String, resulting in a runtime error.

A Generic Version of the Box Class

A generic class is defined with the following format:

The type parameter section, delimited by angle brackets (<>), follows the class name. It specifies the type parameters (also called type variables) T1, T2, . and Tn.

To update the Box class to use generics, you create a generic type declaration by changing the code «public class Box» to «public class Box ". This introduces the type variable, T, that can be used anywhere inside the class.

With this change, the Box class becomes: /** * Generic version of the Box class. * @param the type of the value being boxed */ public class Box < // T stands for "Type" private T t; public void set(T t) < this.t = t; >public T get() < return t; >> As you can see, all occurrences of Object are replaced by T. A type variable can be any non-primitive type you specify: any class type, any interface type, any array type, or even another type variable. This same technique can be applied to create generic interfaces. Type Parameter Naming Conventions By convention, type parameter names are single, uppercase letters. This stands in sharp contrast to the variable naming conventions that you already know about, and with good reason: Without this convention, it would be difficult to tell the difference between a type variable and an ordinary class or interface name. The most commonly used type parameter names are: E - Element (used extensively by the Java Collections Framework) K - Key N - Number T - Type V - Value S,U,V etc. - 2nd, 3rd, 4th types You'll see these names used throughout the Java SE API and the rest of this lesson. Invoking and Instantiating a Generic Type To reference the generic Box class from within your code, you must perform a generic type invocation, which replaces T with some concrete value, such as Integer: You can think of a generic type invocation as being similar to an ordinary method invocation, but instead of passing an argument to a method, you are passing a type argument — Integer in this case — to the Box class itself. Читайте также: Java операции над строками Type Parameter and Type Argument Terminology: Many developers use the terms "type parameter" and "type argument" interchangeably, but these terms are not the same. When coding, one provides type arguments in order to create a parameterized type. Therefore, the T in Foo is a type parameter and the String in Foo f is a type argument. This lesson observes this definition when using these terms. Like any other variable declaration, this code does not actually create a new Box object. It simply declares that integerBox will hold a reference to a "Box of Integer", which is how Box is read. An invocation of a generic type is generally known as a parameterized type. To instantiate this class, use the new keyword, as usual, but place between the class name and the parenthesis: The Diamond In Java SE 7 and later, you can replace the type arguments required to invoke the constructor of a generic class with an empty set of type arguments (<>) as long as the compiler can determine, or infer, the type arguments from the context. This pair of angle brackets, <>, is informally called the diamond. For example, you can create an instance of Box with the following statement: For more information on diamond notation and type inference, see Type Inference. Multiple Type Parameters As mentioned previously, a generic class can have multiple type parameters. For example, the generic OrderedPair class, which implements the generic Pair interface: public interface Pair < public K getKey(); public V getValue(); >public class OrderedPair implements Pair < private K key; private V value; public OrderedPair(K key, V value) < this.key = key; this.value = value; >public K getKey() < return key; >public V getValue() < return value; >> The following statements create two instantiations of the OrderedPair class: Pair p1 = new OrderedPair("Even", 8); Pair p2 = new OrderedPair("hello", "world"); The code, new OrderedPair , instantiates K as a String and V as an Integer. Therefore, the parameter types of OrderedPair's constructor are String and Integer, respectively. Due to autoboxing, it is valid to pass a String and an int to the class. As mentioned in The Diamond, because a Java compiler can infer the K and V types from the declaration OrderedPair , these statements can be shortened using diamond notation: OrderedPair p1 = new OrderedPair<>("Even", 8); OrderedPair p2 = new OrderedPair<>("hello", "world"); To create a generic interface, follow the same conventions as for creating a generic class. Parameterized Types You can also substitute a type parameter (that is, K or V) with a parameterized type (that is, List ). For example, using the OrderedPair example: OrderedPairBox > p = new OrderedPair<>("primes", new Box(. )); Источник Java User-defined Data Types Java User-defined data types are the customized data types created by the developers by exploiting the special features offered by the Java language. This customized data type combines data types of a group of data elements with homogenous or assorted data types. It utilizes the object-oriented functionalities of Java and allows creation of any kind of data types as per the need. Web development, programming languages, Software testing & others These data types have versatile characteristics and behavior and it provides greater flexibility to developers in improving their productivity and maintainability of the programs. Let’s go through features of these special data types in this article. Data types of all the variables should be declared well ahead of its compilation and it should be one from the standard type that Java offers. The variables cannot hold any other data type than what was declared. But the user-defined data type provides the flexibility to define the data type at a group level. Читайте также: File merging in java User-defined Data Types in Java Two major User defined data types are: 1. Class Java a true object-oriented language is full of Classes that encapsulate everything from data elements that acts as instance variables and functions to process the data. It also provides templates to create Objects that are instances of Class that contain a method for performing the defined functions and interact with other parts of the program. A class typically contains Invoking variables with data types How Objects are created? Objects are created or instantiated from the class and used as individual instance of the class. Instance variables are created for each of the objects to interact with other components of the program and there could be as many objects created and each object will have Clear identity with a unique name Different set of instance variables to reflect its state. All the functionalities that define its behavior Each class is a user-defined data type with the combined effect of all the data types of its data elements and each object is a variable data. Example #1: Class Definition // Sample program to define Class, Object and Method class ValueStore //Class definition with default access < int Rate; // Variables with standard data types int Qty; void AssignData(int A, int B) // method definition < Rate = A; // Assigning the values Qty = B; >int ComputeValue() // Another method to compute value < int value; // variable definition value = Rate * Qty; return(value); // Returning the value >> Class with name ValueStore with default access is defined. It has two instance variables Rate, QTY used in the method interactions. Two methods AssignData, ComputeValue with respective function codes are created inside the class. public class OrderValueCompute // Public class < public static void main(String[] args) // Execution starts here < int value1; // variable definition int value2; ValueStore VS1; // Object VS1 definition VS1 = new ValueStore(); // Object VS1 instantiated ValueStore VS2; // Object VS2 definition VS2 = new ValueStore(); // Object VS2 instantiated VS1.AssignData (200,10); // Assigndata method in Object VS1 invoked //with values value1 = VS1.ComputeValue(); // Computedata method in object VS1 // invoked VS2.AssignData (500,20); // Assigndata method in Object VS2 // invoked with values value2 = VS2.ComputeValue(); // Computedata method in object VS2 //invoked System.out.println("Order value 1 " +value1); // Output 1 displayed System.out.println("Order Value 2 " +value2); // Output 2 displayed >> In the execution class, the objects in the other class are defined, instantiated. Methods are invoked. Results obtained and displayed. Class is a user-defined data type and each object is a variable in it. 2. Interface Interface is similar to Class in architecture. It has data variables and methods. It broadly suggests what the classes calling it should do but it does not suggest how it should be carried out. The method in an interface differs from a normal method in a class by It is not full-fledged It is only an abstract and it has the only a definition and the body is empty. It will not perform any functionalities as such Interfaces cannot instantiate any objects like class. But it facilitates abstraction, Multiple inheritances, and loose coupling of classes. Class extends to another super class by a level. Interface extends to another super interface by a level. Class implements interface and multiple inheritance is achieved indirectly which is otherwise not possible in a class. Читайте также: Java caching with guava Each interface created by a user has a unique data type with a combined effect of its data elements. Objects created in these classes uses interfaces also and each of these objects are the variables for interface data types. Below example shows how classes implement interfaces, how objects of these classes are created linking the interfaces, and how they are executed. // Sample program to demonstrate Interfaces interface Intface //Interface is defined < // It has its own data type public void dayname(); //Abstract method within the interface >class day1 implements Intface < // Class interacts through public void dayname() // interface implementation < // (each class is a variable for interface) System.out.println("Monday"); >> class day2 implements Intface < // Another class public void dayname() < System.out.println("Tuesday"); >> class day3 implements Intface < // Another class public void dayname() < System.out.println("Wednesday"); >> public class Subject < // Execution starts here public static void main(String[] args) < Intface t = new day1(); // Object of day1 class thru interface t.dayname(); // method invoked Intface tx = new day2(); // Object of day2 class thru interface tx.dayname(); Intface tx2 = new day3(); // Object of day3 class thru interface tx2.dayname(); >> // objects t, tx1, tx2 are the variables of // the interface data type Conclusion Class and interfaces are the major user-defined data types. Some forums have the opinion that String and Arrays are also part of user-defined types. Strictly speaking, they are part of standard data types of Java and hence they are not discussed. Recommended Articles This is a guide to Java User-Defined Exception. Here we discuss the Introduction, syntax, How Objects are created? examples with code implementation. You may also have a look at the following articles to learn more – 89+ Hours of HD Videos 13 Courses 3 Mock Tests & Quizzes Verifiable Certificate of Completion Lifetime Access 4.5 97+ Hours of HD Videos 15 Courses 12 Mock Tests & Quizzes Verifiable Certificate of Completion Lifetime Access 4.5 JAVA Course Bundle - 78 Courses in 1 | 15 Mock Tests 416+ Hours of HD Videos 78 Courses 15 Mock Tests & Quizzes Verifiable Certificate of Completion Lifetime Access 4.8 Источник Create instance of generic type in Java? To create an instance of a generic type in Java, you can use the newInstance() method of the Class class, along with the Type and TypeVariable classes. Here is an example of how you can create an instance of a generic type: import java.lang.reflect.ParameterizedType; import java.lang.reflect.Type; import java.lang.reflect.TypeVariable; public class MyClass < private final Class type; public MyClass() < Type t = getClass().getGenericSuperclass(); ParameterizedType pt = (ParameterizedType) t; type = (Class) pt.getActualTypeArguments()[0]; > public T createInstance() throws InstantiationException, IllegalAccessException < return type.newInstance(); > > In this example, the MyClass class has a generic type parameter T , and the createInstance() method creates an instance of T using the newInstance() method of the Class class. To use the MyClass class, you can create a subclass that specifies a concrete type for T , like this: public class MySubClass extends MyClass < >MySubClass subClass = new MySubClass(); String s = subClass.createInstance(); // s is a new instance of String Keep in mind that the newInstance() method requires a default constructor, so the class that you are trying to instantiate must have a public or default (package-private) no-arg constructor. If the class does not have a suitable constructor, you will get a java.lang.InstantiationException at runtime. Источник