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Michael
Java null pointer exception
2 Answer(s)      3 years and a month ago
Posted in : Java Beginners


Hi I've made an array of Book objects that I made, the array is called library and it can hold 100 books, but currently there are 6. The Book objects contain three private strings; an author, a title and a short summary. A Book object also contains the appropriate constructor and getters and setters. Now I've made a method to search for a given string in a given field and it returns a string called results, but when I try run it, it gives me a NullPointerException. The code is shown below, please help:

protected static Book[] library=new Book[100];

public String searchLibrary(int field,String param){

    String result = "";
    String strfield = "";
    int counter=0;

    switch(field){
    case 1: strfield=library[counter].getAuthor();
    break;
    case 2: strfield=library[counter].getTitle();
    break;
    case 3: strfield=library[counter].getDdc();
    break;
    }

    while(library[counter]!=null){
        if(strfield.contains(param)){
            result+=library[counter].toString()+"\n";
        }
        counter++;
    }
    return result;
}

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View Answers

June 16, 2011 at 5:03 PM


Ah I just figured out my error, sorry guys. What a simple mistake, I feel retarded.



June 16, 2011 at 5:58 PM


You can also try the following code:

import java.util.*;

 class Book{
    String author;
    String title;
    String summary;
    public Book(String author,String title,String summary){
    this.author=author;
    this.title=title;
    this.summary=summary;
    } 
    public String getTitle(){
        return title;
    }
    public String getAuthor(){
        return author;
    }
    public String getSummary(){
        return summary;
    }
}
public class BookInformation{
    public static void main(String[]args){
        Scanner input=new Scanner(System.in);
        Book myLibrary[]=new Book[6];
        for(int i=0;i<myLibrary.length;i++){
        System.out.print("Author: ");
        String author=input.nextLine();
        System.out.print("Title: ");
        String title=input.nextLine();
        System.out.print("Summary: ");
        String summary=input.nextLine();
        myLibrary[i]=new Book(author,title,summary);
        System.out.println();
        }
        System.out.println();
        System.out.println("Book Information");

        for (int i = 0; i < myLibrary.length; i++){
            Book library = myLibrary[i];
            System.out.println(library.getAuthor() + "\t "+ library.getTitle() + "\t "+ library.getSummary() + "\t ");
        }
        System.out.println();
        System.out.print("Enter title to search infomation: ");
        String tit=input.nextLine();
        System.out.println();
        for(int i = 0; i <  myLibrary.length; i++) {
            if(tit.equals(myLibrary[i].getTitle())) {
                System.out.println("Author is: "+myLibrary[i].getAuthor());
                System.out.println("Summary is: "+myLibrary[i].getSummary());
            }
        }
     }   
}


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