为什么 JAVA 源代码中的方法调用另一个方法，即使调用方法只是做一个调用工作？

Question

When I was reading source code of LinkedList, I find a strange phenomenon(Maybe starnge just to me....) that many public method do nothing else but only call another private method like below.

public void addFirst(E e) {
    linkFirst(e);
}
public void addLast(E e) {
    linkLast(e);
}

Thus, could anybody tell me why method should be written like this, thanks a lot

Answer 1

As Jeroen comments, this is often done to avoid writing multiple methods that do the same thing when a class is implementing (directly or indirectly) multiple interfaces, and the interfaces methods have overlapping semantics.

But in this case, linkFirst and linkLast are internal methods. Here, the reason appears to be that the code has been reorganized, and this is consequence of that reorganization. The Java 6 source code implements these methods as follows:

public void addFirst(E e) {
    addBefore(e, header.next);
}

public void addLast(E e) {
    addBefore(e, header);
}

Does it matter? Is it inefficient?

Probably not. The JIT compiler is likely inline the addFirst and addLast methods wherever they are called. But even if it doesn't the overhead of the unnecessary call will be tiny.

Answer 2

Well, the Methods addFirst(E e) & addLast(E e) are both required, as java.util.LinkedList implements the interface java.util.Deque where these 2 methods are specified.

Nowadays, you would use the @Override annotation on those 2 implementations to indicate you know that to the compiler, but back then when much of this code was written annotations weren't that widespread (or even invented?). (Annotations first became publicly available in JDK 1.5)

Now, linkFirst(E e) is private & only called from addFirst(E e) , so maybe you could argue it could be inlined, but maybe someone decided it would be prudent to protect that logic as addFirst(E e) is public & may be subclassed.

linkLast(E e) has default visibility & is called from many places, so it may be invoked from other members in the java.util package, but again it may have been deemed prudent to not expose this logic as addLast(E e) may be subclassed.