Array type expected [compile-error]

Question

Im working with the following method:

public void m(List<? extends Object[]> objs){
    objs.stream()
        .map(oa -> oa[0])   //compile error
                            //array type expected
        .forEach(System.out::println); 

}

DEMO

Why doesn't it work? I thought everything that extends array can be viewed as an array. Actually I can get length from the array.

Downvoter, would you be so kind and explain in comments why doesn't it work? Why do I need to cast explicitly to Object[]? — St.Antario
– St.Antario, Commented May 11, 2017 at 17:38
If you change m(List<? extends Object[]> objs) to m(List<Object[]> objs) it compiles, but I'm not answering because I don't understand the implications of this change or what's wrong with your original. — Stephen P
– Stephen P, Commented May 11, 2017 at 17:45

Bohemian · Accepted Answer · 2017-05-11 17:48:43Z

5

There is actually no such class that extends Object[]; each array has a fixed type and its own class, eg MyClass[].class

You should use a typed method:

public <T> void m(List<T[]> objs){
    objs.stream()
            .map(oa -> oa[0])   // no compile error
            .forEach(System.out::println);

}

answered May 11, 2017 at 17:48

Bohemian♦

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4 Comments

St.Antario Over a year ago

Never thought abt it, but indeed class MyClass extends Object[]{ } does not compile.

Bohemian Over a year ago

@St.Antario arrays are a native impl - you can't do anything with them

Andy Turner Over a year ago

@Bohemian nit it's not an implementation reason that precludes it, the forbidden by specification. (Failing to find the relevant section of JLS on my phone...)

Hulk Over a year ago

@AndyTurner: I would say it follows from JLS8 - 10.8: "Although an array type is not a class, the Class object of every array acts as if: * The direct superclass of every array type is Object."

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