添加到列表时出现Nullpointer异常

Question

I am trying to iterate through a list of objects to figure out which ones are fighting each other. I am using the checkedGladiators list as something to compare against to prevent it from checking gladiators already assigned to a combat, as each s that isn't assigned already will build their entire combat around them. Currently I get a NullPointerException , so I used some test text to figure that it was happening right at listContains(checkedGladiators,s) . I added the part before it. NOW the problem is happening between "Null" and "Null Changed" as well, which makes no sense to me.

for (Gladiator s : gladiators){
    if (checkedGladiators == null) {
        System.out.println("Null");
        combat1.add(s);
        checkedGladiators.add(s);   
        System.out.println("Null Changed"); 
    }
    if (listContains(checkedGladiators,s)) { 
        // if gladiator is already in a combat do nothing
    } else { // if he isn't

    }

}

listContains class:

public boolean listContains(List<Gladiator> List, Gladiator search) {
    for (Gladiator p : List) {
        if (p.equals(search)) {
        return true;
        }
    }
    return false;

}

Does anyone know why this would occur? Thanks

Edit1:

public class Ai {
private List<Gladiator> gladiators;
private List<List<Gladiator>> combatsList;
private List<Gladiator> checkedGladiators;
private List<Gladiator> combat1;
private List<Gladiator> combat2;
private List<Gladiator> combat3;
private List<Gladiator> combat4;
private List<Gladiator> combat5;
private List<Gladiator> combat6;    
private List<Gladiator> combat7;    
private List<Gladiator> combat8;    
private List<Gladiator> guardList;
private List<Gladiator> advanceList;
private List<Gladiator> retreatList;
int totalCombats = 0; // total combats going on

I have initialized the list variable in the class already.

Answer 1

You are forgetting to create the checkedGladiators object.

So, create the object before your loop as:

List<Gladiators> checkGladiators = new ArrayList<Gladiators>();

Then, in your loop, rather than testing for checkGladiators == null...

test for checkGladiators.isEmpty().

Answer 2

if(checkedGladiators ==null) is true and you are adding something to it ,

it will definitely throw a NullPointerException , because you are operating on null

Thanks Abhi

Answer 3

Why do you need to do all this? Why isn't this sufficient?

// This has your data in it.
List<Gladiators> gladiators = new ArrayList<Gladiators>();
// Obviously some attributes, including a unique key or name.  
// MUST override equals and hashcode properly
Gladiator g = new Gladiator();
if (gladiators.contains(g)) {
  // do something here.
}

NullPointerException is one of the easiest problems to fix. Run your code in an IDE with debugging turned on and put a breakpoint where the stack trace says the exception occurred. You'll figure out quickly why something you assumed should not be null has violated your assumptions.

Answer 4

When checkedGladiators is null you try to add to it (as if it were a List/Collection/etc).

...
if (checkedGladiators == null) {
    ...
    checkedGladiators.add(s);   // <-- You handle 'checkedGladiators'
                                // <-- as an instaciated, when it is 'null'

Do this instead:

...
if (checkedGladiators == null) {
    ...
    checkedGladiators = new ArrayList<...>();   // <-- instanciate appropriately
    checkedGladiators.add(s);  
    ...

Answer 5

Instead of checking with System.out.println , use StackTrace or a debugger like eclipse . I will pin-point clearly.

  if (checkedGladiators == null) { 
            System.out.println("Null");
            combat1.add(s);
            checkedGladiators.add(s);  --> checkedGladiators is null here. Here null pointer exception will occur. 
            System.out.println("Null Changed"); 
        }

Answer 6

you check if checkedGladiators is null and then call a method on it:

if (checkedGladiators == null) { // <-- null!!
   checkedGladiators.add(s); // <-- null pointer exception.
}