generics in java method code example

Example 1: 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>

Example 2: generics Interface in java

/*We can define our own classes with generics type. A generic type is a class or interface that is parameterized over types. We use angle brackets (<>) to specify the type parameter.

To understand the benefit, let’s say we have a simple class as:*/
package com.journaldev.generics;

public class GenericsTypeOld {

	private Object t;

	public Object get() {
		return t;
	}

	public void set(Object t) {
		this.t = t;
	}

        public static void main(String args[]){
		GenericsTypeOld type = new GenericsTypeOld();
		type.set("Pankaj"); 
		String str = (String) type.get(); //type casting, error prone and can cause ClassCastException
	}
}