java generics

// generic methods

public <T> List<T> fromArrayToList(T[] a) {   
	    return Arrays.stream(a).collect(Collectors.toList());
	}

public static <T, G> List<G> fromArrayToList(T[] a, Function<T, G> mapperFunction) {
	    return Arrays.stream(a)
	      .map(mapperFunction)
	      .collect(Collectors.toList());
	}

// bounded generics

public <T extends Number> List<T> fromArrayToList(T[] a) {
	    ...
	}

//multiple bounds

<T extends Number & Comparable>

// upper bound wildcards

public static void paintAllBuildings(List<? extends Building> buildings) {
	    ...
	}
    
// lower bound wildcard

<? super T>

// A Simple 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(); 
    } 
}

// A Simple 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()); 
    } 
}