将方法内部生成的本体用于java中的另一个方法

First I was generated an ontology inside a method1. it is successful. Then inside the second method I need to use this generated ontology for another process. I used following code. It gave an error:

首先，我在method1中生成了一个本体。它很成功。然后在第二种方法中，我需要将这个生成的本体用于另一个进程。我使用了以下代码。它给出了一个错误：

Exception in thread "main" java.lang.NullPointerException.

线程“main”java.lang.NullPointerException中的异常。

Where is the problem? How I need to take generated ontology in method1 to method2?

哪里有问题？我如何将方法1中的生成本体带到方法2？

     public class OntologyCreation {

        public static void main (String args[]) {

        //main
        OntologyCreation ont = new OntologyCreation(); 
        OntModel m = null;
        String ontoClass = null;
        ont.method2(ontoClass, m);


        public  void method1(OntModel m) {     //for ontologyCreation
        m = ModelFactory.createOntologyModel(OntModelSpec.OWL_MEM);
        -----
        -----
        }


        public ArrayList<String> method2(String ontoClass, OntModel m) {   // 2nd method for use generated ontology to another process
        method1(m);
        m.read("http://localhost/myont/ont.owl");      ????????
        ExtendedIterator<OntClass> classes = ((OntModel) m).listClasses(); 
        ----------
        ----------               
         }
        return xx; 
        }

4 个解决方案

#1

If you use the OntModel more than once with the OntologyCreation instance, you could use a field to store the variable:

如果您使用OntologyCreation实例多次使用OntModel，则可以使用字段来存储变量：

private OntModel m;

public void method1() {
    this.m = ModelFactory.createOntologyModel(OntModelSpec.OWL_MEM);
}

public ArrayList<String> method2(String ontoClass) {
    method1();
    this.m.read("http://localhost/myont/ont.owl"); // use field value

The alternative is returning the created OntModel as described in the other answers.

另一种方法是返回创建的OntModel，如其他答案中所述。

Note: Java is always pass-by-value, which is why the value in the method2 method remains null, see Is Java "pass-by-reference" or "pass-by-value"?

注意：Java总是按值传递，这就是为什么method2方法中的值保持为null，请参阅Java是“传递引用”还是“按值传递”？

#2

You cannot change variables outside of a function by assigning to the name of the parameter inside the function. The only thing this does is change the local variable in question. So the assignment to m in method1 sets the local variable m correctly, but that does not effect the variable m in method2.

您不能通过指定函数内部参数的名称来更改函数外部的变量。这样做的唯一方法是更改有问题的局部变量。因此，在method1中对m的赋值可以正确设置局部变量m，但这不会影响method2中的变量m。

The solution is to make method1 return the object and then assigning it to the local variable: m = method1()

解决方案是让method1返回对象，然后将其分配给局部变量：m = method1（）

#3

Return m in method1:

在method1中返回m：

public OntModel method1(OntModel m) {     //for ontologyCreation
    m = ModelFactory.createOntologyModel(OntModelSpec.OWL_MEM);
    /*-----*/
    return m;
}

And in method2, the call should be:

在方法2中，调用应该是：

m = method1(m);

Java passes references by value, meaning method1 receives an adress of object m. However, m = something means you change that address value inside method1. Outside, it is still the same and still points towards a null.

Java按值传递引用，这意味着method1接收对象m的地址。但是，m =某事意味着您在method1中更改了该地址值。在外面，它仍然是相同的，仍然指向null。

#4

You m variable isn't being instantiated before method2is called.

在调用method2之前，没有实例化变量。

Therefore it has the value null associated.

因此它具有null关联值。

Even though you're calling method1, that attribution will only be valid on the method's scope.

即使您正在调用method1，该归因也只对方法的范围有效。

You ought to do OntModel m = ModelFactory.createOntologyModel(OntModelSpec.OWL_MEM); before ont.method2(ontoClass, m); in order to be able to use m on method2.

你应该做OntModel m = ModelFactory.createOntologyModel（OntModelSpec.OWL_MEM）;在ont.method2之前（ontoClass，m）;为了能够在方法2上使用m。

#1