Question

I am having a strange issue with the followin syntax:

The following code block doesn’t throw null pointer exception

Map<String, String> requestHeaderMap = FacesContext.getCurrentInstance().getExternalContext().getRequestHeaderMap();

if(!"partial/ajax".equals(requestHeaderMap.get("faces-request")))    
{
    System.out.println("This works");
}

But this one throws null pointer exception:

FacesContext.getCurrentInstance().getExternalContext().getRequestHeaderMap().get("faces-request").equals("partial/ajax")

I just couldn’t figure out whats wrong here. I don’t see any difference between two call except for readability.

Answer 1

The two do things in different orders. It’s like the difference between x.equals(y) and y.equals(x). If y is null, only the second one of those will throw an NPE. In your case, requestHeaderMap.get("faces-request") is null.

This doesn’t throw the exception because *.equals(null) is perfectly fine:

"partial/ajax".equals(requestHeaderMap.get("faces-request")

On the other hand, this is trying to dereference null:

getRequestHeaderMap().get("faces-request").equals("partial/ajax")