bounded wildcards

GenericsBoundedWildcards.java

package net.homecredit.mer.web;

import java.util.ArrayList;
import java.util.List;

/**
 * Created by Patrik.Mihalcin on 29.03.2018
 */
public class GenericsBoundedWildcards {

    // more restrictive than it need be
    public static <T> void copy1(List<T> src, List<T> dest) {  // uses no wildcards
        for (int i = 0; i < src.size(); i++)
            dest.set(i, src.get(i));
    }

    // any type without any restrictions
    // method takes one type of list as a source and copies the content into another - totally unrelated - type of destination list
    public static void copy2(List<?> src, List<?> dest) {  // uses unbounded wildcards

        // References pointing to objects of unknown type are usually expressed as a reference of type Object
        Object o = src.get(0);
//        dest.set(0, o);

//        for (int i = 0; i < src.size(); i++)
        // java: incompatible types: java.lang.Object cannot be converted to capture#1 of ?
//            dest.set(i, src.get(i));
    }

    // we require that a type T exists that is subtype of the output list's element type and supertype of the input list's element type
    // the input src list must have an element type with an upper bound T
    // the output dest list is required to have an element type with a lower bound T
    public static <T> void copy3(List<? extends T> src, List<? super T> dest) {  // uses bounded wildcards
        for (int i = 0; i < src.size(); i++)
            dest.set(i, src.get(i));
    }

    public static void main(String[] args) {
        List<Long> input = new ArrayList<>();
        List<Object> output = new ArrayList<>();

//        java: method copy1 in class net.homecredit.mer.web.Main cannot be applied to given types;
//        required: java.util.List<T>,java.util.List<T>
//        found: java.util.List<java.lang.Object>,java.util.List<java.lang.Long>
//        reason: inferred type does not conform to equality constraint(s)
//                inferred: java.lang.Long
//        equality constraints(s): java.lang.Long,java.lang.Object
//        copy1(output, input);

        // T needs to be a supertype of Long and a subtype of Object
        // T := Number & Serializable & Comparable
        copy3(input, output);

//        java: method copy3 in class net.homecredit.mer.web.Main cannot be applied to given types;
//        required: java.util.List<? extends T>,java.util.List<? super T>
//        found: java.util.List<java.lang.Long>,java.util.List<java.lang.String>
//        reason: inferred type does not conform to upper bound(s)
//                inferred: java.lang.Long
//        upper bound(s): java.lang.String,java.lang.Object

        List<Long> input2 = new ArrayList<>();
        List<String> output2 = new ArrayList<>();

        // compiler realizes that there is no type that is subtype of String and supertype of Long
        // T needs to be a supertype of Long and a subtype of String
//        copy3(input2, output2);
    }
}