Asked  2 Years ago    Answers:  5   Viewed   108 times

Interfaces allow you to create code which defines the methods of classes that implement it. You cannot however add any code to those methods.

Abstract classes allow you to do the same thing, along with adding code to the method.

Now if you can achieve the same goal with abstract classes, why do we even need the concept of interfaces?

I've been told that it has to do with OO theory from C++ to Java, which is what PHP's OO stuff is based on. Is the concept useful in Java but not in PHP? Is it just a way to keep from having placeholders littered in the abstract class? Am I missing something?

 Answers

4

The entire point of interfaces is to give you the flexibility to have your class be forced to implement multiple interfaces, but still not allow multiple inheritance. The issues with inheriting from multiple classes are many and varied and the wikipedia page on it sums them up pretty well.

Interfaces are a compromise. Most of the problems with multiple inheritance don't apply to abstract base classes, so most modern languages these days disable multiple inheritance yet call abstract base classes interfaces and allows a class to "implement" as many of those as they want.

Thursday, December 8, 2022
2

An Abstract Class allows for "partial implementation" (see the template method pattern), but in this case, if all methods are abstract, you don't see that benefit. One other thing you can do is include fields, you're not just limited to methods.

Remember, there's a conceptual difference between an "abstract method" and the contract defined by an interface. An abstract method has to be overridden by a subclass which is done through inheritence implementation. Any polymorphic calls (downcasting) will require one superclass per class or it would hit the diamond inheritance problem. This kind of inheritence based tree structure is typical of OO design.

As a contrast, an interface provides a signature of a contract to fulfil. You can fulfil many interface's needs as long as you retain the signature as there is no question of going back up the class hierarchy to find other implementations. Interfaces don't really rely on polymorphism to do this, it's based on a contract.

The other thing of note is you may have "protected" abstract methods, it makes no sense to do such a thing in an interface (in fact it's illegal to do so).

Monday, September 26, 2022
 
samt
 
5

Like a dict, a DataFrame's index is backed by a hash table. Looking up rows based on index values is like looking up dict values based on a key.

In contrast, the values in a column are like values in a list.

Looking up rows based on index values is faster than looking up rows based on column values.

For example, consider

df = pd.DataFrame({'foo':np.random.random(), 'index':range(10000)})
df_with_index = df.set_index(['index'])

Here is how you could look up any row where the df['index'] column equals 999. Pandas has to loop through every value in the column to find the ones equal to 999.

df[df['index'] == 999]

#           foo  index
# 999  0.375489    999

Here is how you could lookup any row where the index equals 999. With an index, Pandas uses the hash value to find the rows:

df_with_index.loc[999]
# foo        0.375489
# index    999.000000
# Name: 999, dtype: float64

Looking up rows by index is much faster than looking up rows by column value:

In [254]: %timeit df[df['index'] == 999]
1000 loops, best of 3: 368 µs per loop

In [255]: %timeit df_with_index.loc[999]
10000 loops, best of 3: 57.7 µs per loop

Note however, it takes time to build the index:

In [220]: %timeit df.set_index(['index'])
1000 loops, best of 3: 330 µs per loop

So having the index is only advantageous when you have many lookups of this type to perform.

Sometimes the index plays a role in reshaping the DataFrame. Many functions, such as set_index, stack, unstack, pivot, pivot_table, melt, lreshape, and crosstab, all use or manipulate the index. Sometimes we want the DataFrame in a different shape for presentation purposes, or for join, merge or groupby operations. (As you note joining can also be done based on column values, but joining based on the index is faster.) Behind the scenes, join, merge and groupby take advantage of fast index lookups when possible.

Time series have resample, asfreq and interpolate methods whose underlying implementations take advantage of fast index lookups too.

So in the end, I think the origin of the index's usefulness, why it shows up in so many functions, is due to its ability to perform fast hash lookups.

Monday, October 10, 2022
 
bhuang3
 
2

require_once() basically relies on the physical file to determine whether or not it's been included. So it's not so much the context that you're calling require_once() in, it's whether or not that physical file has previously been required.

In your code above, your foo() function would not re-parse baz.php, since it is going to be the same file as was previously included at the top.

However, you will get different results based on whether you included it inside foo(), or included it at the top, as the scoping will apply when require_once() does succeed.

Thursday, November 24, 2022
 
1

A big reason these terms are confusing is because they are keywords in Java and they have meaning in a general, language-agnostic context. What the terms mean in the general context of programming is slightly different from what the specific keywords mean in Java.

In general, the term interface is synonymous with abstraction. When you read, "Program to an interface" it means program to an abstraction. In Java, the interface keyword indicates a specific type of stateless abstraction, which is contrasted with abstract class. In general, both of these are abstractions and thus both are programming interfaces.

In general, the term public indicates any part of code that is visible to and used by clients outside of the development team. In Java, the public keyword indicates a specific access level between classes and members. Since clients may view and use protected members as well, these may also be considered public in the general context of programming. Effective Java uses the term exported to combine public and protected members. Note that in Java 9+ modules can be used to control access, so even public classes may not be exported and thus may not be public in the general sense.

The term API is not a Java keyword, so at least we have no language-specific ambiguity to contend with. It is typically used to mean public API, which is synonymous with exported API. It is the sum of everything that you (the developer) allow your clients to view and use. Occasionally, the term internal API is used to indicate the opposite: classes and members that you do not allow your clients to view or use.

Sunday, December 25, 2022
 
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