checking wether a hashmap has items or not?

Question

What is wrong with the statement if (!adjacentNodes.get(nodeId).isEmpty()) in the below code?

Such statement leads to the following message:

java.lang.NullPointerException

package IN.company;

import java.util.HashMap;
import java.util.HashSet;

public class DiffusionModels {
    public int ICModel(int[] seedSet, HashMap<Integer, HashSet<Integer>> adjacentNodes, double p) {
        HashSet<Integer> infected = new HashSet();
        HashSet<Integer> recentlyInfected = new HashSet();
...

        while (recentlyInfected.size() > 0) {
            HashSet<Integer> nextActivator = new HashSet<>();
            if (!recentlyInfected.isEmpty()) {
                for (Integer nodeId : recentlyInfected) {
                        if (!adjacentNodes.get(nodeId).isEmpty()) {
                            HashSet<Integer> neighborsOfNode =adjacentNodes.get(nodeId) ;
                            for (Integer nodeNeighbor : neighborsOfNode) {
                                if ((Math.random() <= p)&& !(infected.contains(nodeNeighbor))) {
                                    nextActivator.add(nodeNeighbor);
                                }
                            }
                        }
                }
            }
            infected.addAll(recentlyInfected);
            recentlyInfected.clear();
            recentlyInfected.addAll(nextActivator);
        }
        return infected.size();
    }
}

If the expression you designate generates a `NullPointerException` then it is surely because `adjacentNodes.get(nodeId)` returns `null`. Note in particular that `null` is an altogether different beast from a non-null reference to an empty `Map`. — John Bollinger, Mar 16 '17 at 13:23
Thanks, but how can I fix it? I check this issue with isEmpty function, is it wrong? — JavaDev, Mar 16 '17 at 13:26
@Timothy Truckle, thank you, but that topic would not be useful for me. — JavaDev, Mar 16 '17 at 13:28

score 1 · Accepted Answer · answered Mar 16 '17 at 13:30

1

This beacause adjacentNodes is null,which result to NPE.Before that line try to print the values of adjacentNodes.Hope this helps.

answered Mar 16 '17 at 13:30

Porkko M

307
2
10

Thank you, I know it but how can I fix it? – JavaDev Mar 16 '17 at 13:31
!adjacentNodes.isEmpty() or != null – Berkley Lamb Mar 16 '17 at 13:36

score 1 · Answer 2 · answered Mar 16 '17 at 13:37

1

Try this if (adjacentNodes.get(nodeId) != null && !adjacentNodes.get(nodeId).isEmpty()) {

answered Mar 16 '17 at 13:37

jr593

287
1
8

score 0 · Answer 3 · edited Jun 20 '20 at 09:12

I would be grateful if anyone help me and explain what is the problem of this statement:
if (!adjacentNodes.get(nodeId).isEmpty()) 
This statement leads to "java.lang.NullPointerException"

According to the code you posted either adjacentNodes is null or it does not contain the key currently referenced by nodeId.

You have to check that adjacentNodes is not null before that line.

If adjacentNodes is not null you should use the getOrDefault() method of Java8's Map class (https://docs.oracle.com/javase/8/docs/api/java/util/Map.html#getOrDefault-java.lang.Object-V-)

if(!adjacentNodes.getOrDefault(nodeId, Collections.emptySet()).isEmpty())){ //...

checking wether a hashmap has items or not?

3 Answers3