接口类型变量可以从没有实现它的 class 中的该接口调用方法吗？

Question

I'm trying to understand this just for my own knowledge. Here is a broken down version of my code:

interface ILightningPhone {
    void recharge();
    void useLightning();
}

interface IMicroUsbPhone {
    void recharge();
    void useMicroUsb();
}

class LightningToMicroUsbAdapter implements IMicroUsbPhone {
    private final ILightningPhone lightningPhone;

    public LightningToMicroUsbAdapter(ILightningPhone lightningPhone) {
        this.lightningPhone = lightningPhone;
    }

    @Override
    public void useMicroUsb() {
        System.out.println("MicroUsb connected");
        lightningPhone.useLightning();
    }

    @Override
    public void recharge() {
        lightningPhone.recharge();
    }
}

I'm trying to understand interfaces and I just need an explanation on the following:

1. I notice that a variable private final ILightningPhone lightningPhone; is declared within a class of type ILightningPhone interface. but this class doesn't implement that interface. How is it able to do this?

2. Next I notice that in the following method:

    public void useMicroUsb() { System.out.println("MicroUsb connected"); lightningPhone.useLightning();

   the variable lightningPhone is able to call the useLightning() method from the ILightningPhone interface. Even though the ILightningPhone interface is not implemented in the class LightningToMicroUsbAdapter . Why is it able to do this?

Overall, I'm just trying to understand why we are able to create a variable of a interface type in a class that doesn't implement that interface? And why when declaring that variable of that interface type are we able to use the methods from that interface in a class that doesn't implement that interface?

Here is the full code if needed: https://en.wikipedia.org/wiki/Adapter_pattern

Answer 1

The LightningToMicroUsbAdapter class (or any class for that matter) can have any type of member - be it a String , a List , or even a ILightningPhone . Having a members is a "hasA" relationship, not an "isA" relationship.

Since the lightningPhone is an instance of ILightningPhone , it can call the useLightning method defined by that interface.