What is this syntax called? Can you provide me with tutorial/documentation?

[gentle adder]

this.addSlot(new Slot(this.container, 0, 15, 15){
    public boolean mayPlace(ItemStack stack) {
        return stack.is(Items.FILLED_MAP);
    }
});

It's the first time I've ever seen a syntax like this before.

[waxen cove]

<@&987246399047479336> please have a look, thanks.

[waxen cove]

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[glossy heart]

anonymous class

[gentle adder]

Thank you!

[glossy heart]

same as doing

class SlotImpl extends Slot {
    public boolean mayPlace(ItemStack stack) {
        return stack.is(Items.FILLED_MAP);
    }
}
this.addSlot(new SlotImpl(this.container, 0, 15, 15));