Java instanceof 在我的情况下不起作用

Question

我有以下类继承结构：

public class Document {}
public class AuditProgramDocument extends Document {}
public class CaseFilePart {
    private Document document;
    public Document getDocument() {
        return document;
    }
}
public class MyClass {
    public boolean canDeleteDocument(CaseFilePart selectedCFP) {
        ...//somelogic
        if (selectedCFP.getDocument() instanceof AuditProgramDocument) {
            System.out.println("instance");
        }
    }
}

在调试器中，我清楚地看到，selectedCFP.getDocument() returns AuditProgramDocument。但是 If 语句没有被评估。

我的测试：

System.out.println("2.1 " + selectedCFP.getDocument().getClassName());
System.out.println("2.2 " + selectedCFP.getDocument().getClass().getName());
System.out.println("2.3 " + selectedCFP.getDocument().getClass().getCanonicalName());
System.out.println("2.4 " + selectedCFP.getDocument().getClass().getSimpleName());

输出结果：
2.1 auditProgramDocument
2.2 eu.erp.crams.cmg.bom.document.Document$$_javassist_79_
2.3 eu.erp.crams.cmg.bom.document.Document$$_javassist_79_
2.4 文档$$_javassist_79_

Answer 1

1)-:在侧 CaseFilePart class 中，您正在创建文档 class 的引用变量，而不是文档 class.

的对象

您应该 return 文档对象 class 而不是引用。

2)-:AuditProgramDocument class 是文档 class 的子 class 我们不能向下转换它。

3)-:这就是为什么 selectedCFP.getDocument() 不是 instanceof AuditProgramDocument

Answer 2

所以，为了确保我们有以下规则：

AuditProgramDocument 是 Document

CaseFilePart 有 Document

当我们写代码时：

Document document = new AuditProgramDocument();
if (document instanceof Document) //true
if (document instanceof AuditProgramDocument) //true

但是，当我们编写代码时：

Document document = someVar.getDocument();
if (document instanceof Document) //true
if (document instanceof AuditProgramDocuemtn) //false

最后一条语句是false因为Document不是AuditProgramDocument。因为 getDocument() 方法总是 returns 我们引用 Document，而不是 subtype

Answer 3

尝试此代码...（已注释掉，但包括您的原始测试）

public class MyClass {
    public boolean canDeleteDocument(Document doc) {
        return (doc instanceof AuditProgramDocument);
    }

    public static void main(String[] args) {
        //System.out.println("2.1 " + selectedCFP.getDocument().getClassName());//DOES NOT COMPILE
        //Not sure about these tests....
        //System.out.println("2.2 " + selectedCFP.getDocument().getClass().getName());
        //System.out.println("2.3 " + selectedCFP.getDocument().getClass().getCanonicalName());
        //System.out.println("2.4 " + selectedCFP.getDocument().getClass().getSimpleName());

        MyClass mc = new MyClass();
        CaseFilePart cfp = new CaseFilePart();

        //TEST 1
        cfp.setDocument(new AuditProgramDocument());
        boolean canDelete = mc.canDeleteDocument(cfp.getDocument());
        System.out.println("Can we delete " + cfp.getDocument().getClass() + " " + canDelete);

        //TEST 2
        cfp.setDocument(new SomeOtherDocument());
        canDelete = mc.canDeleteDocument(cfp.getDocument());
        System.out.println("Can we delete " + cfp.getDocument().getClass() + " " + canDelete);
    }

}

//Put all these in the same file to simplify the answer
class Document {}
class AuditProgramDocument extends Document {}
//Created 2nd Document implementation for illustration
class SomeOtherDocument extends Document {}
class CaseFilePart {
    private Document document;
    public Document getDocument() {
        return document;
    }

    public void setDocument(Document document) {
        this.document = document;
    }
}

Java instanceof 在我的情况下不起作用

Java instanceof doesn't work in my case

java

instanceof