Changing parameters of method in functional interface may break the code if the interface is used as parameter of another method and a method reference is passed instead of instance.

Example:
    interface FIface {    
        boolean test(int a);
    }
                    
    public boolean anotherMethod(int x) {
        return true;
    }
    
    public void use() {
        method(this::anotherMethod);                
    }
    
    public void method(FIface in) {        
    }


adding parameter to method FIface.test() breaks line method(this::anotherMethod); 


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