instanceof in Java

Question

import java.util.ArrayList; 

public class Folder extends AbstractFile 
{ 
//Replace previous ArrayLists with a single 
//ArrayList of AbstractFile references 
private ArrayList<AbstractFile> files = new ArrayList(); 
     AbstractFile abstractfile; 
/** 
* Constructor for objects of class Folder 
*/ 
public Folder(String name) 
{ 
    super(); 
    this.name = name; 
} 
// replace previous add methods 
// with a single add(AbstractFile fileObject) method 
public boolean add(AbstractFile fileObject) 
{ 
    return files.add(fileObject); 
} 
@Override 
public int size() 
{ 
    int size =0; // size holds the running total 
    for (AbstractFile file : files){ // for each AbsFile ref 
     size+=file.size(); //call size() and update the running total 
    } 
    return size; // return the final value 
} 


@Override 
public int getNumFiles(){ 
    int numFiles = 0; 
    for(AbstractFile file: files) 
    { 
     numFiles += file.getNumFiles(); 


    } 

    return numFiles;// default value 
} 
@Override 
public int getNumFolders(){ 
    int numFolders = 0; 

    // for(AbstractFile file: files) 
    // { 
     // if(file instanceof Folder) 
     // { 
     // numFolders += file.getNumFolders(); 
    // } 

    // } 
    // return numFolders;// default value 
    for (Object e : files) 
    { 
     if (e instanceof Folder) 
     { 
      numFolders += e.getNumFolders(); 
     } 
    } 
     return numFolders;// default value 
} 
@Override 
public AbstractFile find(String name){ 
    //TODO Later - not in mini assignment 
    return null; 
} 
} 

AbstractFile Klasse

public abstract class AbstractFile 
{ 
// instance variables - 
String name; 

public abstract int size(); 
public abstract int getNumFiles(); 
public abstract int getNumFolders(); 
public abstract AbstractFile find(String name); 

public String getName(){ 
    return name; 
} 

} 

Filesystem Class- für

Tests zählen mit

public class FileSystem 
{ 
    public static void main(String[] args) 
    { 
     FileSystem fileSystem = new FileSystem(); 
     fileSystem.fileTest1(); 

    } 

    public void fileTest1(){ 

     Folder documents = new Folder("Documents"); 
     Folder music = new Folder("Music"); 
     Folder photos = new Folder("Photos"); 
     documents.add(music); 
     documents.add(photos); 
     File assign1 = new File("assign1.doc"); 
     documents.add(assign1); 
     Folder dylan = new Folder("Dylan"); 
     music.add(dylan); 
     Folder band = new Folder("The Band"); 
     music.add(band); 
     File family = new File("family.jpg"); 
     photos.add(family); 
     File tambourine = new File("tambourine.mp3"); 
     dylan.add(tambourine); 
     File dixie = new File("dixie.mp3"); 
     band.add(dixie); 
     File weight = new File("weight.mp3"); 
     band.add(weight); 

     String contents1 = ("Hey, mister, can you tell me "); 
     String contents2 = ("Hey Mr Tambourine Man"); 
     String contents3 = ("The night they drove old dixie down"); 
     String contents4 = ("fee fi fo fum"); 

     weight.setContents(contents1); // add contents to each File 
     tambourine.setContents(contents2); 
     dixie.setContents(contents3); 
     assign1.setContents(contents4); 

     //********test for size()**************** 
     int expected = contents1.length() + contents2.length() + contents3.length() + contents4.length(); 
     int result = documents.size(); 

     if(result==expected){ // test fro equality 
      System.out.println("size() works"); 

     }else{ 
      System.out.println("size() doesn't work"); 
     } 

     //***************************************** 

     //*****************test for getNumFiles()****************** 

     expected =5;// what value should expected be set to? 
     result = documents.getNumFiles(); 

     if(result==expected){ // test fro equality 
      System.out.println("NumFiles() works"); 

     }else{ 
      System.out.println("NumFiles() doesn't work"); 

     } 

     //output the results of the test for equality 

     // ************************************** 

     //*****************test for getNumFiles()****************** 

     expected = 5; // what value should expected be set to? 
     result = documents.getNumFolders(); 



     if(result==expected){ // test fro equality 
      System.out.println("NumFolder() works"); 

     }else{ 
      System.out.println("NumFolder() doesn't work"); 
      System.out.printf("%d",result); 
     } 

     // ************************************** 


    } 
    } 

Datei-Klasse

public class File extends AbstractFile 
    { 
    private String contents; 
/** 
* Constructor for objects of class File 
*/ 
public File(String name) 
{ 
    super(); 
    this.name = name; 
} 

public String getContents(){ 
    return contents; 
} 

public void setContents(String contents){ 
    this.contents = contents; 
} 

@Override 
public int size() 
{ 
    if(contents==null){ //handle situation where contents may not have been set 
     return 0; 
    } 
    return contents.length(); 
} 
@Override 
public int getNumFiles(){ 
    return 1; // a File object just returns 1 (it contains one File - itself) 
} 
@Override 
public int getNumFolders(){ 
    //TODO 
    return 0; 
} 
@Override 
public AbstractFile find(String name){ 
    //TODO Later - not in mini assignment 
    return null; 
} 

}

Hallo, also im Grunde habe ich Probleme mit der getNumFolders(method) Ich habe eine Testklasse erstellt mit 5 Ordnern und diese sind in einer ArrayList vom Typ AbstractFile genannten Dateien gespeichert. Ich versuche, diese ArrayList Schleife durchlaufen, um die Ordner zu zählen, unter Berücksichtigung dieser ArrayList enthält auch Dateien, so dass sie nicht gezählt werden müssen. Ich würde jede Hilfe zu schätzen wissen. Vielen Dank!

Answer 1

Wenn Sie einfach die Anzahl der Ordner-Instanzen zählen möchten, dann sollten Sie unten die Arbeit erledigen.

public int getNumFolders() { 

int numFolders = 0; 

for(AbstractFile file: files) { 
    if(file instanceof Folder) { 
     numFolders++; 
    } 
} 
return numFolders; 
} 

und expectedValue sollten Sie mit Ihrem Testfall Accroding 2 hier. in fileTest1() -Methode des Testfall

expected = 2; // this should be two because there are two folders called music and photos in documents folder 
result = documents.getNumFolders();