Null Pointer Explanation

[Cyber_Star 0]

Does anyone know about null pointer in java. for example : why we use String id = se.request.getParameter("id");......................if (("").equals(id) || ("null").equals(id)) {//some codes here}instead of usingif(id.equals("") || id.equals("null")) {//some codes here}Anyway, i need some clear explanations here. Thanks for helping.

[cshao 0]

Does anyone know about null pointer in java. for example : why we use String id = se.request.getParameter("id");......................if (("").equals(id) || ("null").equals(id)) {//some codes here}instead of usingif(id.equals("") || id.equals("null")) {//some codes here}Anyway, i need some clear explanations here. Thanks for helping.

null does not have any method, including equals(), so if id is null, id.equals(...) will generate NullPointerException.

[Jack McKalling 2]

It is also possible to just check if something is null, or to use try..catch:

if (id == null || id.equals("")){ // if ID is null, the .equals() will not be compiled}try{ if (id.equals("")) { /* id is an empty string */ }}catch (Exception e){ // id might be null, or some other error occured}

Edited September 1, 2009 by Jack McKalling