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Generics in Java

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Generics means parameterized types. The idea is to allow type (Integer, String, … etc., and user-defined types) to be a parameter to methods, classes, and interfaces. Using Generics, it is possible to create classes that work with different data types. An entity such as class, interface, or method that operates on a parameterized type is a generic entity.

Why Generics?

The Object is the superclass of all other classes, and Object reference can refer to any object. These features lack type safety. Generics add that type of safety feature. We will discuss that type of safety feature in later examples.

Generics in Java are similar to templates in C++. For example, classes like HashSet, ArrayList, HashMap, etc., use generics very well. There are some fundamental differences between the two approaches to generic types.

Types of Java Generics

Generic Method: Generic Java method takes a parameter and returns some value after performing a task. It is exactly like a normal function, however, a generic method has type parameters that are cited by actual type. This allows the generic method to be used in a more general way. The compiler takes care of the type of safety which enables programmers to code easily since they do not have to perform long, individual type castings.

Generic Classes: A generic class is implemented exactly like a non-generic class. The only difference is that it contains a type parameter section. There can be more than one type of parameter, separated by a comma. The classes, which accept one or more parameters, are known as parameterized classes or parameterized types.

Generic Class

Like C++, we use <> to specify parameter types in generic class creation. To create objects of a generic class, we use the following syntax.

// To create an instance of generic class 
BaseType <Type> obj = new BaseType <Type>()

Note: In Parameter type we can not use primitives like ‘int’,’char’ or ‘double’.

Java

// Java program to show working of user defined // Generic classes // We use < > to specify Parameter type class Test<T> { // An object of type T is declared T obj; Test(T obj) { this.obj = obj; } // constructor public T getObject() { return this.obj; } } // Driver class to test above class Main { public static void main(String[] args) { // instance of Integer type Test<Integer> iObj = new Test<Integer>(15); System.out.println(iObj.getObject()); // instance of String type Test<String> sObj = new Test<String>("GeeksForGeeks"); System.out.println(sObj.getObject()); } }

15
GeeksForGeeks

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Output

15
GeeksForGeeks

We can also pass multiple Type parameters in Generic classes.

Java

// Java program to show multiple // type parameters in Java Generics // We use < > to specify Parameter type class Test<T, U> { T obj1; // An object of type T U obj2; // An object of type U // constructor Test(T obj1, U obj2) { this.obj1 = obj1; this.obj2 = obj2; } // To print objects of T and U public void print() { System.out.println(obj1); System.out.println(obj2); } } // Driver class to test above class Main { public static void main (String[] args) { Test <String, Integer> obj = new Test<String, Integer>("GfG", 15); obj.print(); } }

Output

GfG
15

Generic Functions:

We can also write generic functions that can be called with different types of arguments based on the type of arguments passed to the generic method. The compiler handles each method.

Java

// Java program to show working of user defined // Generic functions class Test { // A Generic method example static <T> void genericDisplay(T element) { System.out.println(element.getClass().getName() + " = " + element); } // Driver method public static void main(String[] args) { // Calling generic method with Integer argument genericDisplay(11); // Calling generic method with String argument genericDisplay("GeeksForGeeks"); // Calling generic method with double argument genericDisplay(1.0); } }

Output

java.lang.Integer = 11
java.lang.String = GeeksForGeeks
java.lang.Double = 1.0

Generics Work Only with Reference Types:

When we declare an instance of a generic type, the type argument passed to the type parameter must be a reference type. We cannot use primitive data types like int, char.

Test<int> obj = new Test<int>(20);

The above line results in a compile-time error that can be resolved using type wrappers to encapsulate a primitive type.

But primitive type arrays can be passed to the type parameter because arrays are reference types.

ArrayList<int[]> a = new ArrayList<>();

Generic Types Differ Based on Their Type Arguments:

Consider the following Java code.

Java

// Java program to show working // of user-defined Generic classes // We use < > to specify Parameter type class Test<T> { // An object of type T is declared T obj; Test(T obj) { this.obj = obj; } // constructor public T getObject() { return this.obj; } } // Driver class to test above class Main { public static void main(String[] args) { // instance of Integer type Test<Integer> iObj = new Test<Integer>(15); System.out.println(iObj.getObject()); // instance of String type Test<String> sObj = new Test<String>("GeeksForGeeks"); System.out.println(sObj.getObject()); iObj = sObj; // This results an error } }

Output:

error:
 incompatible types:
 Test cannot be converted to Test

Even though iObj and sObj are of type Test, they are the references to different types because their type parameters differ. Generics add type safety through this and prevent errors.

Type Parameters in Java Generics

The type parameters naming conventions are important to learn generics thoroughly. The common type parameters are as follows:

T – Type
E – Element
K – Key
N – Number
V – Value

Advantages of Generics:

Programs that use Generics has got many benefits over non-generic code.

1. Code Reuse: We can write a method/class/interface once and use it for any type we want.

2. Type Safety: Generics make errors to appear compile time than at run time (It’s always better to know problems in your code at compile time rather than making your code fail at run time). Suppose you want to create an ArrayList that store name of students, and if by mistake the programmer adds an integer object instead of a string, the compiler allows it. But, when we retrieve this data from ArrayList, it causes problems at runtime.

Java

// Java program to demonstrate that NOT using // generics can cause run time exceptions import java.util.*; class Test { public static void main(String[] args) { // Creatinga an ArrayList without any type specified ArrayList al = new ArrayList(); al.add("Sachin"); al.add("Rahul"); al.add(10); // Compiler allows this String s1 = (String)al.get(0); String s2 = (String)al.get(1); // Causes Runtime Exception String s3 = (String)al.get(2); } }

Output :

Exception in thread "main" java.lang.ClassCastException: 
   java.lang.Integer cannot be cast to java.lang.String
    at Test.main(Test.java:19)

How do Generics Solve this Problem?

When defining ArrayList, we can specify that this list can take only String objects.

Java

// Using Java Generics converts run time exceptions into // compile time exception. import java.util.*; class Test { public static void main(String[] args) { // Creating a an ArrayList with String specified ArrayList <String> al = new ArrayList<String> (); al.add("Sachin"); al.add("Rahul"); // Now Compiler doesn't allow this al.add(10); String s1 = (String)al.get(0); String s2 = (String)al.get(1); String s3 = (String)al.get(2); } }

Output:

15: error: no suitable method found for add(int)
        al.add(10); 
          ^

3. Individual Type Casting is not needed: If we do not use generics, then, in the above example, every time we retrieve data from ArrayList, we have to typecast it. Typecasting at every retrieval operation is a big headache. If we already know that our list only holds string data, we need not typecast it every time.

Java

// We don't need to typecast individual members of ArrayList import java.util.*; class Test { public static void main(String[] args) { // Creating a an ArrayList with String specified ArrayList<String> al = new ArrayList<String>(); al.add("Sachin"); al.add("Rahul"); // Typecasting is not needed String s1 = al.get(0); String s2 = al.get(1); } }

4. Generics Promotes Code Reusability: With the help of generics in Java, we can write code that will work with different types of data. For example,

Let’s say we want to Sort the array elements of various data types like int, char, String etc.

Basically we will be needing different functions for different data types.

For simplicity, we will be using Bubble sort.

But by using Generics, we can achieve the code reusability feature.

Java

public class GFG { public static void main(String[] args) { Integer[] a = { 100, 22, 58, 41, 6, 50 }; Character[] c = { 'v', 'g', 'a', 'c', 'x', 'd', 't' }; String[] s = { "Virat", "Rohit", "Abhinay", "Chandu","Sam", "Bharat", "Kalam" }; System.out.print("Sorted Integer array : "); sort_generics(a); System.out.print("Sorted Character array : "); sort_generics(c); System.out.print("Sorted String array : "); sort_generics(s); } public static <T extends Comparable<T> > void sort_generics(T[] a) { //As we are comparing the Non-primitive data types //we need to use Comparable class //Bubble Sort logic for (int i = 0; i < a.length - 1; i++) { for (int j = 0; j < a.length - i - 1; j++) { if (a[j].compareTo(a[j + 1]) > 0) { swap(j, j + 1, a); } } } // Printing the elements after sorted for (T i : a) { System.out.print(i + ", "); } System.out.println(); } public static <T> void swap(int i, int j, T[] a) { T t = a[i]; a[i] = a[j]; a[j] = t; } }

Output

Sorted Integer array :  6, 22, 41, 50, 58, 100, 
Sorted Character array :  a, c, d, g, t, v, x, 
Sorted String array :  Abhinay, Bharat, Chandu, Kalam, Rohit, Sam, Virat,

Here, we have created a generics method. This same method can be used to perform operations on integer data, string data, and so on.

5. Implementing Generic Algorithms: By using generics, we can implement algorithms that work on different types of objects, and at the same, they are type-safe too.

This article is contributed by Dharmesh Singh. If you like GeeksforGeeks and would like to contribute, you can also write an article and mail your article to [email protected]. See your article appearing on the GeeksforGeeks main page and help other Geeks. Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.

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https://docs.oracle.com/javase/tutorial/java/generics/types.html

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:

class name<T1, T2, ..., Tn> { /* ... */ }

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<T>". 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 <T> the type of the value being boxed
 */
public class Box<T> {
    // 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:

Box<Integer> integerBox;

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.

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<T> is a type parameter and the String in Foo<String> 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<Integer> 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 <Integer> between the class name and the parenthesis:

Box<Integer> integerBox = new Box<Integer>();

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<Integer> with the following statement:

Box<Integer> integerBox = new Box<>();

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<K, V> {
    public K getKey();
    public V getValue();
}

public class OrderedPair<K, V> implements Pair<K, V> {

    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<String, Integer> p1 = new OrderedPair<String, Integer>("Even", 8);
Pair<String, String>  p2 = new OrderedPair<String, String>("hello", "world");

The code, new OrderedPair<String, Integer>, 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<String, Integer>, these statements can be shortened using diamond notation:

OrderedPair<String, Integer> p1 = new OrderedPair<>("Even", 8);
OrderedPair<String, String>  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<String>). For example, using the OrderedPair<K, V> example:

OrderedPair<String, Box<Integer>> p = new OrderedPair<>("primes", new Box<Integer>(...));

标签：Java,String,generic,public,Generics,type,class
From： https://www.cnblogs.com/kungfupanda/p/17472466.html

Generics in Java

Generics in Java

Why Generics?

Types of Java Generics

Generic Class

Java

Java

Generic Functions:

Java

Generics Work Only with Reference Types:

Generic Types Differ Based on Their Type Arguments:

Java

Type Parameters in Java Generics

Advantages of Generics:

Java

How do Generics Solve this Problem?

Java

Java

Java

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

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