How much should a function trust another function

Question

Consider the following two functions in a class called Graph  The entire source code is found here  http   www keithschwarz com interesting code  dir dijkstra    public void addNode T node            mGraph put node  new HashMap lt T  Double gt             public void addEdge T start  T dest  double length            mGraph get start  put dest  length           Here  the addEdge method is blindly trusting addNode method that it has added hashmap to mGraph   Is it a common practice to trust that other methods in class are doing their job correctly   Or is it recommended that a method be skeptical of everything and do a check something like    public void addEdge T start  T dest  double length                Map m   mGraph get start              if   m     null                        Once again  I am interested in knowing whats commonly done and whats ideally recommended

User · Answer

Typically this is bad practice  Since it is possible to call addEdge before addNode and have a NullPointerException  NPE  thrown  addEdge should check if the result is null and throw a more descriptive Exception  In my opinion  the only time it is acceptable not to check for nulls is when you expect the result to never be null  in which case  an NPE is plenty descriptive

User · Answer

The addEdge is trusting more than the correction of the addNode method  It s also trusting that the addNode method has been invoked by other method  I d recommend to include check if m is not null

User · Answer

If it is in the same class it is fine to trust the method   It is very common to do this  It is good practice to check null values in constructor s and method s arguments to make sure that nobody is passing null values into them  if it is not allowed   Then if you implement your methods in a way that they never set the  start  graph to null  don t check for nulls there    It is also good practice to implement unit tests for your methods and make sure that they are correctly implemented  so you can trust them

User · Answer

That s where constructors come into play  If you have a default constructor  eg  with no parameters  that always creates a new Map  then you re sure that every instance of this class will always have an already instantiated Map

User · Answer

Such debugging is part of the development process and should not be the issue at runtime   Methods don t trust other methods  They all trust you  That is the process of developing  Fix all bugs  Then methods don t have to  trust   There should be no doubt   So  write it as it should be  Do not make methods check wether other methods are working correctly  That should be tested by the developer when they wrote that function  If you suspect a method to be not doing what you want  debug it

User · Answer

My 2 cents   This is a loaded question imho  A rule of thumb I use to is see how this function will be called   If the caller is something I have control over then   its ok to assume that it will be called with the right parameters and with proper initialization    On the other hand if its some client I don t control then it is a good idea to do thorough error checking

[java] How much should a function trust another function

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