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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
Updated on: 06-Sep-2019
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