Null for Optional

None.java

public final class None<T> extends Option<T> {
    
    public static class NoneHasNoValue extends RuntimeException {}
    
    public None() {}
    
    public boolean hasValue() { return false; }
    
    public T get() { throw new NoneHasNoValue(); }
    
    @Override
    public String toString() { return "None"; }
    
    @Override
    public boolean equals(Object other) {
        return (other == null || other.getClass() != None.class) ? false : true;
    }
    
    @Override
    public int hashCode() { return -1; }
}