Debugging... help please.

Question

hello. i have made a simple java program called 'library management'
it has 2 java files. i have copy pasted the contents of both the files. while the files compile properly i get the following runtime error in command prompt...
"Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException" 5
at l.lib.org(lib.java:137)
at l.lib.issue(lib.java:86)
at m.mem2.main(mem2.java:10)
the source code is as follows...
/*lib.java*/
package l;
public class lib
    private static char book[][]=new char[5][5];
    private static int lmt=1;
    private static int a=0;
    private static int b=0;
    public static void build()
        if(lmt==1)
            char ch='a';
            for(int i=0;i<5;i++)
                for(int j=0;j<5;j++)
                    book[i][j]=ch;
                    ch++;
            lmt++;
        else
            System.out.println("Cannot build more than once.");
    public static void crntlst()
        System.out.println("Books currently available");
        for(int i=0;i<5;i++)
            for(int j=0;j<5;j++)
                System.out.print(book[i][j]+", ");
     System.out.println("");
    private static int chk(char ch)
        int val=1;
        for(int i=0;i<5;i++)
            for(int j=0;j<5;j++)
                if(book[i][j]==ch)
                    val=0;
                    a=i;
                    b=j;
                    break;
                else
                    continue;
        return(val);
    public static void issue(char ch)
        if(ch=='z')
            System.out.println("Invalid entry.");
        else
            int x=0;
            x=chk(ch);
            if(x==1)
                System.out.println("Book already issued.");
            else
                System.out.println("Book issued.");
                book[a]='z';
org();
crntlst();
public static void ret(char ch)
if(ch=='z')
System.out.println("Invalid entry.");
else
int x=0;
x=chk(ch);
if(x==0)
System.out.println("Book already present in library.");
else
for(int i=0;i<5;i++)
for(int j=0;j<5;j++)
if(book[i][j]=='z')
book[i][j]=ch;
break;
else
continue;
org();
crntlst();
private static void org()
char t;
for(int i=0;i<5;i++)
for(int j=0;j<5;j++)
if(book[i][j]=='z')
if(j==4)
t='z';
book[i][j]=book[i+1][0];
book[i+1][0]=t;
else
t='z';
book[i][j]=book[i][j+1];
book[i][j+1]=t;
/*mem2.java*/
package m;
class mem2
    public static void main(String args[])
        l.lib.build();
        l.lib.crntlst();
        l.lib.issue('z');
        l.lib.issue('d');
        l.lib.issue('k');
        l.lib.issue('a');
        l.lib.ret('k');
        l.lib.issue('r');
        l.lib.ret('z');
        l.lib.ret('d');
        l.lib.ret('a');
}any help is appreciated.
thankyou.

Answer 1

I can understand that you have a class with a library and a square matrix, but you do not instance class, it has not instance variables (only static variables, as global variables or procedural languages), you repeat 5 until satiety, however you should declare a constant or use .length property of matrix, and properties methods have not significance names. You use lmt as a boolean in place of Factory method getInstance(), you don't use a and b properties, ....
// Class name must start upper case.
public class Library{
   private char[] books;
   private int numberBooks;
   private Library instance;
   private Library(int numberBooks){
       this.numberBooks=numberBooks;
       books=new char[numberBooks*numberBooks]
   public static Library getInstance(){
       if(instance==null)
          instance=new Library)(5);
       return instance;
   public int numberBooks(){
      return numberBooks;
   public String toString(){ // to print Library matrix in place of crntlst
       return Arrays.toString(books);
   // Linear position or -1 if book is not found.
   public int getPosition(char book){
     int pos=books.length;
     while(pos>0&&books[--pos]!=book);
     return pos; 
   public int getX(int pos){
       return pos/numberBooks;
   public int getY(int pos){
      return pos%numberBooks;
}Probably there is some methods or variables that can be defined better than my example, but Singleton pattern is as I define. If you want lets multiple access for different threads, you must do synchronized methods, otherwise, it is not recommendable.