Generic Method invocation

Question

I have this piece of code from "Java - A 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, 3, 4, 6 };
    Integer [] nums2 = { 1, 3, 3, 4, 6 };
    Integer [] nums3 = { 1, 3, 3, 4, 6 };
    Integer [] nums4 = { 1, 3, 3, 4, 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, 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");

    }
}

The compilation fails with the message - "Error:(39, 12) java: method arraysEqual in class com.chapter.thirteen.GenericMethodDemo cannot be applied to given types; required: T[],V[] found: java.lang.Integer[],java.lang.Double[] reason: inference variable T has incompatible bounds equality constraints: java.lang.Integer lower bounds: V,java.lang.Double,java.lang.Integer", which is expected.

However, when I missed adding the parameter to Comparable (as shown in the code below), the code compiles and produces the correct result.

package com.chapter.thirteen;


public class GenericMethodDemo {
    static <T extends Comparable, 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, 3, 4, 6 };
    Integer [] nums2 = { 1, 3, 3, 4, 6 };
    Integer [] nums3 = { 1, 3, 3, 4, 6 };
    Integer [] nums4 = { 1, 3, 3, 4, 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, 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");

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

Can someone please explain why the compilation does not fail in the second instance? I had expected the compiler to complain about T extends Comparable, V extends T in the second instance?

What's going on?

Answer 1

The reason is because of the rules of PECS.

When you do,

static <T extends Comparable, V extends T> boolean arraysEqual(T[] x, V[] y)

You are basically stating, both T and V are subtype of Comparable. Which means calling arraysEqual(Integer[], Double[]) should work because both Integer and Double implement Comparable.

But when you add generic type to Comparable, the contract is lost,

static <T extends Comparable<T>, V extends T> boolean arraysEqual(T[] x, V[] y)

In this, Double is not implementing Comparable<Integer>, which is why the compiler error.

EDIT: If your question is why the rawtype Comparable is not giving compiler error, the answer is that's how generics work...

You can try with Number too,

static <T extends Number, V extends T> boolean arraysEqual(T[] x, V[] y)

No rawtypes are involved in this, and you can call arrayEquals(Integer[], Double[]) to this and it will work fine because both are Number.