Restrictions while declaring a generic (type) in Java

Generics is a concept in Java where you can enable a class, interface and, method, accept all (reference) types as parameters. In other words it is the concept which enables the users to choose the reference type that a method, constructor of a class accepts, dynamically. By defining a class as generic you are making it type-safe i.e. it can act up on any datatype.

Restrictions on generics

You cannot use generics in certain ways and in certain scenarios as listed below −

• You cannot use primitive datatypes with generics.

class Student<T>{    T age;    Student(T age){       this.age = age;    } } public class GenericsExample {    public static void main(String args[]) {       Student<Float> std1 = new Student<Float>(25.5f);       Student<String> std2 = new Student<String>("25");       Student<int> std3 = new Student<int>(25);    } }

Compile time error

GenericsExample.java:11: error: unexpected type       Student<int> std3 = new Student<int>(25);                ^    required: reference    found: int GenericsExample.java:11: error: unexpected type       Student<int> std3 = new Student<int>(25);                                        ^ required: reference found: int 2 errors

• You cannot instantiate the generic parameters.

class Student<T>{    T age;    Student(T age){       this.age = age;    }    public void display() {       System.out.println("Value of age: "+this.age);    } } public class GenericsExample {    public static void main(String args[]) {       Student<Float> std1 = new Student<Float>(25.5f);       std1.display();       T obj = new T();    } }

Compile time error

GenericsExample.java:15: error: cannot find symbol       T obj = new T();       ^    symbol: class T    location: class GenericsExample GenericsExample.java:15: error: cannot find symbol       T obj = new T();                   ^    symbol: class T    location: class GenericsExample 2 errors

• The generic type parameter cannot be static.

class Student<T>{    static T age;    Student(T age){       this.age = age;    }    public void display() {       System.out.println("Value of age: "+this.age);    } } public class GenericsExample {    public static void main(String args[]) {       Student<Float> std1 = new Student<Float>(25.5f);       std1.display();    } }

Compile time error

GenericsExample.java:3: error: non-static type variable T cannot be referenced from a static context    static T age;          ^ 1 error

• You cannot cast parameterized type of one datatype to other.

import java.util.ArrayList; public class Generics {    public static void main(String args[]) {       ArrayList<Integer> list1 = new ArrayList<Integer>();       list1.add(25);       list1.add(26);       ArrayList<Number> list2 = list1;    } }

Compile time error:

Generics.java:8: error: incompatible types: ArrayList<Integer> cannot be converted to ArrayList<Number>    ArrayList<Number> list2 = list1;             ^ 1 error

• You cannot create an array of generic type objects.

class Student<T>{    T age;    Student(T age){       this.age = age;    }    public void display() {       System.out.println("Value of age: "+this.age);    } } public class GenericsExample {    public static void main(String args[]) {       Student<Float>[] std1 = new Student<Float>[5];    } }

Output

GenericsExample.java:12: error: generic array creation    Student<Float>[] std1 = new Student<Float>[5];                            ^ 1 error

• A generic type class cannot extend the throwable class therefore, you cannot catch or throw these objects.

class Student<T>extends Throwable{    T age;    Student(T age){       this.age = age;    }    public void display() {       System.out.println("Value of age: "+this.age);    } } public class GenericsExample {    public static void main(String args[]) {    } }

Compile time error

GenericsExample.java:1: error: a generic class may not extend java.lang.Throwable class Student<T>extends Throwable{                         ^ 1 error