java generics example

public class Box<T> {
    private T content;
    
    public void setContent(T content) {
        this.content = content;
    }
    
    public T getContent() {
        return content;
    }
}Java Generic Type Naming convention helps us understanding code easily and having a naming convention is one of the best practices of Java programming language. So generics also comes with its own naming conventions. Usually, type parameter names are single, uppercase letters to make it easily distinguishable from java variables. The most commonly used type parameter names are:

E – Element (used extensively by the Java Collections Framework, for example ArrayList, Set etc.)
K – Key (Used in Map)
N – Number
T – Type
V – Value (Used in Map)
S,U,V etc. – 2nd, 3rd, 4th types// Generic method example 1
<T> void fromArrayToCollection(T[] a, Collection<T> c){
    for (T o: a){
        c.add(o);
    }
}

// Generic method example 2
public static <T extends SomeClass & SomeInterface> methodName(T o){
    o.setterMethod(123);
}

// Generic method example 3
public static <T> Stack <T> loadFromArray(Object[] arr, Class<T> type){
    Stack <T> stack = new StackArray<>(arr.length);
    for (Object o:arr){
        if (type.isInstance(o)){
            stack.push( (T) o); // type checking with "isInstance" and casting. "instanceof" will not work here.
        }
    }
}
<Car> c = loadFromArray(object_arr_of_cars, Car.class);
// 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>public class Tuple <T> {
  // the T is a placeholder for any datatype
  public T leftValue;
  public T rightValue;
  
  public Tuple(T leftValue, T rightValue){
    // again, T is being used as a placeholder for any type
    this.leftValue = leftValue;
    this.rightValue = rightValue;
}

public class Program{
  public static void main (String args){
    // And upon using Tuples we can fill in the T from the Tuple class with actual datatypes
    Tuple <int> intTuple = new Tuple <int>(5, 500)
    Tuple <String> stringTuple = new Tuple <String> ("Hello", "World")

    // we can even put Tuples inside of Tuples!
    Tuple<Tuple<int>> metaIntTuple = new Tuple <Tuple <int>> (intTuple, new Tuple <int> (456, 0));
  }
}public <T> List<T> fromArrayToList(T[] a) {   
    return Arrays.stream(a).collect(Collectors.toList());
}public class GenericFunction {
    public static void main(String[] args) {
        Integer[] intArray  = {1,2,3,4,5,6,7};
        Double[] doubleArray = {1.1,2.2,3.3,4.4,5.0};
        Character[] charArray = {'a' , 'b' , 'c' , 'd' , 'e'};
        System.out.println("Integer Array :");
        genericFunction(intArray);
        System.out.println();
        System.out.println("Double Array : ");
        genericFunction(doubleArray);
        System.out.println();
        System.out.println("Character Array : ");
        genericFunction(charArray);

    }
	// Generic Function In Java
    public static <T> void genericFunction(T[] nums){
        for(T element : nums){
            System.out.print(element + " ");
        }
    }
}
// Generic interface
interface BoxInterface<T> {
    void setValue(T value);
    T getValue();
}

// Generic class implementing the BoxInterface with T
class Box<T> implements BoxInterface<T> {
    private T value;

    @Override
    public void setValue(T value) {
        this.value = value;
    }

    @Override
    public T getValue() {
        return value;
    }
}

public class Main {
    public static void main(String[] args) {
        // Using T 
        Box<String> stringBox = new Box<>();
        stringBox.setValue("Hello, Generics!");
        String stringValue = stringBox.getValue();

        // Using ? (wildcard) with Interface
        BoxInterface<?> wildcardBox = stringBox;
        // wildcardBox.setValue("This won't compile"); // We cannot set value of unknown type
        Object wildcardValue = wildcardBox.getValue(); // We can only cast value to Object type

        // Using ? extends T (upper bounded wildcard) with Interface
        // Focus type : the type on the right side of extend
        BoxInterface<? extends String> upperBoundedWildcardBox = stringBox;
        // upperBoundedWildcardBox.setValue("This won't compile"); // We cannot set value of unknown type
        String upperBoundedWildcardValue = upperBoundedWildcardBox.getValue(); // We can only cast value to Parent type, String in this case

        // Using ? super T (loower bounded wildcard) with Interface
        // Focus type : the type on the left side of super
        BoxInterface<? super String> lowerBoundedWildcardBox = new Box<>();
        lowerBoundedWildcardBox.setValue("Lower Bounded Wildcard");  // We can set value to parent type, String in this case
        Object lowerBoundedWildcardValue = lowerBoundedWildcardBox.getValue(); // We can only cast value to Parent type, Object in this case
    }
}