Java – generic method call

I have this code from "Java - Beginner's Guide - Schildt", Chapter 13:

package com.chapter.thirteen;

public class GenericMethodDemo {
static <T extends Comparable<T>,V extends T> boolean arraysEqual(T[] x,V[] y){
    if(x.length != y.length) return false;

    for(int i = 0; i < x.length; i++)
        if(!x[i].equals(y[i])) return false;

    return true;
}

public static void main(String args[]){

    Integer [] nums  = { 1,3,4,6 };
    Integer [] nums2 = { 1,6 };
    Integer [] nums3 = { 1,6 };
    Integer [] nums4 = { 1,6,7};
    Double [] dVals = {1.1,2.2,3.3,4.4};

    if(arraysEqual(nums,nums))
        System.out.println("nums equal nums");

    if(arraysEqual(nums,nums2))
        System.out.println("nums equal nums2");

    if(arraysEqual(nums,nums3))
        System.out.println("nums equal nums3");

    if(arraysEqual(nums,nums4))
        System.out.println("nums equal nums4");

    //Edit:Removed the comments from the below two lines.

    if(arraysEqual(nums,dVals))
        System.out.println("Nums equal dVals");

    }
}

Compilation fails with message – "error: (39,12) Java: arraysequal.genericmethoddemo in method class com.chapter.thirten cannot be applied to the given type; required: T [], v [] found: java.lang.integer [], java.lang. Double [] reason: inference variable t has incompatible boundary equality constraints: java.lang.integer lower bound: V, java.lang.double, java.lang.integer "This is expected

However, when I miss adding parameters to comparable (as shown in the following code), the code compiles and generates the correct results

package com.chapter.thirteen;


public class GenericMethodDemo {
    static <T extends Comparable,nums4))
        System.out.println("nums equal nums4");

    if(arraysEqual(nums,dVals))
        System.out.println("Nums equal dVals");
   }
}

Can anyone explain why compilation does not fail in the second case? I once expected the compiler to complain that t extends comparable, and V extends t in the second instance?

What's going on?

Solution

The reason is because of PECS rules

When you do this

static <T extends Comparable,V[] y)

You are basically stating that T and V are subtypes of comparable This means that calling arraysequal (integer [], double []) should work because both integer and double implement comparable

However, when you add a generic type to comparable, the contract will be lost,

static <T extends Comparable<T>,V[] y)

Here, double does not implement comparable < integer >, which is the reason for the compiler error

Editor: if your question is why rawtype comparable doesn't give compiler errors, the answer is how generics work

You can also try using number,

static <T extends Number,V[] y)

Rawtypes are not involved here. You can call arrayequals (integer [], double []), which works normally because they are all numbers