java - Java - java.lang.NullPointerException (put data into List<>)

Question

I would like to know why I have got this error. Have you got idea and do you know how to fix it?

Error(I get this error in put()):

Exception in thread "main" java.lang.NullPointerException
at nacs.put(nacs.put:36)
at Program.main(Program.java:69)

Program: (Main)

Nac nacs = new Nac();
nacs.put(new Intsult(1));
nacs.put(new Intsult(2));

Class:

public class Nac implement IPoraw
{
public List<IAbstrsUlt> abs;
public void put(IAbstrsUlt value)
{
abs.add(value);
}
}

Answer 1

Initialize list.

private List<IAbstrsUlt> abs = new ArrayList<IAbstrsUlt>();

Note: You should always try to declare your members more restrictive. Default value for Object is null. Refer Oracle tutorial to know default values.

Answer 2

变量 abs 永远不会初始化。

尝试这个

public List<IAbstrsUlt> abs = new ArrayList<>();

Answer 3

public List<IAbstrsUlt> abs;

-在上面的语句中，您只是简单地声明了一个List Reference Variable名为abstype的名称IAbstrsUlt，并且由于Object Reference Variable它的默认值为，null所以在这种情况下也是如此。

-你必须初始化它。

public List<IAbstrsUlt> abs = new List<IAbstrsUlt>();