如何检查方法是否不是 null？

Question

Since I am relatively new to programming I don't now how to check if a method is not null. In the following code I need to check if the answers.get method is not null, how can I achieve this?

public class SharedViewModel extends ViewModel {


private HashMap<Integer, MutableLiveData<String>> answers = new HashMap<>();

public MutableLiveData<String> getAnswer(int questionId) {
    return answers.get(questionId);
}

public void setAnswer(int questionId, String answer) {
    // TODO make sure that answers.get is not null

    answers.get(questionId).setValue(answer);
 }
}

Answer 1

Just add a null check on answers.get(questionId) whenever you are trying to access that Object or manipulate it.

public class SharedViewModel extends ViewModel {

  public void setAnswer(int questionId, String answer) {
    if(answers.get(questionId)!=null){
       answers.get(questionId).setValue(answer);
     }
   }
}

If you are using Java8 and above you can also use

//nonNull will return true if object is not null
if(Objects.nonNull(answers.get(questionId))){
   answers.get(questionId).setValue(answer);
} 

Answer 2

Just to add to existing answer and for your todo, you may want to use getOrDefault ,

answers.getOrDefault(questionId, answer);

Answer 3

Generally you could do this:

if(methodname(args)! =null ) {//statements} ;