Type inference for generic parameters when calling a method

I was making a class with builder-type setters for generic member variables and I noticed something when I tried to pass it to a function accepting an object with a specialized generic argument for that type via new. Mainly this:

    public static void bar(Foo<Integer> f){}
    public static class Foo<T>{
        Foo<T> self(){
            return this;
        }
        
    }
    public static void main(String[] args) {
        bar(new Foo<>().self()); //gives an error, because new Foo<>().self() returns Foo<Object>
    }

    public static void bar(Foo<Integer> f){}
    public static class Foo<T>{
        Foo<T> self(){
            return this;
        }
        
    }
    public static void main(String[] args) {
        bar(new Foo<>().self()); //gives an error, because new Foo<>().self() returns Foo<Object>
    }

It appears that the compiler cannot infer the type of

new Foo<>

new Foo<>

unless it's specifically told that it's

new Foo<Integer>

new Foo<Integer>

first off...why? Is the java compiler stupid or something?
second off is there a way around this? Because in my case the generic argument is a little long and looks very ugly.

JC|HCL

Java Community | Help. Code. Learn.•15mo ago•

12 replies

nikcho-kouhai

Type inference for generic parameters when calling a method

    public static void bar(Foo<Integer> f){}
    public static class Foo<T>{
        Foo<T> self(){
            return this;
        }
        
    }
    public static void main(String[] args) {
        bar(new Foo<>().self()); //gives an error, because new Foo<>().self() returns Foo<Object>
    }

    public static void bar(Foo<Integer> f){}
    public static class Foo<T>{
        Foo<T> self(){
            return this;
        }
        
    }
    public static void main(String[] args) {
        bar(new Foo<>().self()); //gives an error, because new Foo<>().self() returns Foo<Object>
    }

It appears that the compiler cannot infer the type of

new Foo<>

new Foo<>

unless it's specifically told that it's

new Foo<Integer>

new Foo<Integer>

first off...why? Is the java compiler stupid or something?
second off is there a way around this? Because in my case the generic argument is a little long and looks very ugly.

Type inference for generic parameters when calling a method

Type inference for generic parameters when calling a method

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