Java 与 generics 的接口是否可以强制执行通用类型？

Question

Consider this example

    public interface Equatable<T> {
        public boolean equal(T t1, T t2);
    }

    public class Square implements Equatable<Square> {
        public boolean equal(Square t1, Square t2) {
            return false;
        }
    }

Is it possible for the interface Equatable to enforce Square to implement an equal function that takes in two Squares, and not just two of any types (two Strings etc)?

Answer 1

Yes, you didn't define what is "T" in your interface, you can limit it only to classes which implements Equatable, so String will not be allowed:

public interface Equatable<T extends Equatable<T>>
{ 
     //...
}

Works Good:

public class Square implements Equatable<Square>
{
    public boolean equal(Square t1, Square t2)
    {
        return false;
    }
}

Compilation Error:

public class Circle implements Equatable<String>
{
    public boolean equal(String t1, String t2)
    {
        return false;
    }
}

Note
Pay attention, it's your responsibility to put in the brackets the same Type name, because below case will not be covered by compiler.
I guess you want to disallow to use equal method for different type.

public class Square implements Equatable<Circle>
{
    public boolean equal(Circle t1, Circle t2)
    {
        return false;
    }
}

Answer 2

You can enforce this to an extent with a recursive generic type.

public interface Equatable<T extends Equatable<T>> {
    public boolean equal(T t1, T t2);
}

related:Java Enum definition