Answers for "what are generics in java"

java generic type method

// 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>

java generics

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));
  }
}

java generic type method

// 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>

generic argument java

static void fromArrayToCollection(Object[] a, Collection<?> c) {
    for (Object o : a) { 
        c.add(o); // compile-time error
    }
}

java generics

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));
  }
}

generics in java

import java.util.Scanner;
import reader.*;

public class Main {
    public static void main(String[] args) throws Exception {
        Scanner scanner = new Scanner(System.in);
        String filePath = scanner.nextLine().trim();
        System.out.println("The file path is " + filePath);
        String type = filePath.substring(filePath.lastIndexOf('.') + 1).replaceAll("\\W", "");
        System.out.println("The reader type is " + type);
        AbstractReaderService reader = getReaderService(type);
        // reader.read(filePath);
        // reader.printContent();

    }

    private static AbstractReaderService getReaderService (String type) throws Exceptions.TypeNotFoundException {
        switch (type) {
            case "txt": return new TxtReaderService();
            case "json": return new JsonReaderService();
            default: throw new Exceptions().new TypeNotFoundException("There is no \"" + type + "\" type of reader.");
        }
    }
}

generic argument java

static void fromArrayToCollection(Object[] a, Collection<?> c) {
    for (Object o : a) { 
        c.add(o); // compile-time error
    }
}

generics in java

import java.util.Scanner;
import reader.*;

public class Main {
    public static void main(String[] args) throws Exception {
        Scanner scanner = new Scanner(System.in);
        String filePath = scanner.nextLine().trim();
        System.out.println("The file path is " + filePath);
        String type = filePath.substring(filePath.lastIndexOf('.') + 1).replaceAll("\\W", "");
        System.out.println("The reader type is " + type);
        AbstractReaderService reader = getReaderService(type);
        // reader.read(filePath);
        // reader.printContent();

    }

    private static AbstractReaderService getReaderService (String type) throws Exceptions.TypeNotFoundException {
        switch (type) {
            case "txt": return new TxtReaderService();
            case "json": return new JsonReaderService();
            default: throw new Exceptions().new TypeNotFoundException("There is no \"" + type + "\" type of reader.");
        }
    }
}