Exception in Thread(java.nullPointerException)

Question

public class Printer {
    static Printer obj =null;
    private Printer(){}
    public static Printer getInstance(){
        if(obj==null){
            Printer pr=new Printer();
        }
        return obj;  
    }
    void print(String msg){
        System.out.println("["+msg);
        try {
            Thread.sleep(1000);
        } catch(InterruptedException c) {
            c.printStackTrace();
        }
        System.out.println("]");
    }
}

class PrintThread implements Runnable {
    Printer p;
    String msg;
    PrintThread(Printer p,String msg) {
        this.p=p;
        this.msg=msg;
    }
    public void run() {
        p.print(msg);//Getting error in this line
    }
}

//Deploying main class
public class Execution {
    public static void main(String[] args) {
        Printer pr=Printer.getInstance();
    Thread t1=new Thread(new PrintThread(pr,"java"));
    t1.start();
    PrintThread r=new PrintThread(pr,"javadeveloper");
    Thread t2=new Thread(r);
    t2.start();
    }
}

Hi, I have written this program to understand that how the thread works. Here i made Printer class as singleton and tried to implement thread in second class PrintThread by implementing Runnable. Here i overrided the Run(){} method but at the the time of execution the jvm is throwing an error saying that there is a

Exception in "Thread-0"(java.nullPointerException) at PrintThread.run(Printer.java:31).

I tried to google it and also read other related question but still i am not able to rectify the problem

Jeroen Vannevel · Accepted Answer · 2013-08-19 09:18:44Z

5

static Printer obj =null;
    private Printer(){}
     public static Printer getInstance(){
         if(obj==null){
             Printer pr=new Printer();
         }
     return obj;  
     }

You create a new Printer object, but return the null value.

Correct:

static Printer obj = null;
private Printer(){}

public static Printer getInstance(){
 if(obj == null){
  obj = new Printer();
 }
 return obj;
}

answered Aug 19, 2013 at 9:18

Jeroen Vannevel

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Comments

Azodious · Accepted Answer · 2013-08-19 09:18:50Z

3

Correct your getInstance implementation:

public static Printer getInstance() {
    if(obj==null) {
         obj = new Printer(); // don't create local variable
    }
return obj;  
}

answered Aug 19, 2013 at 9:18

Azodious

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Comments

ihsan kocak · Accepted Answer · 2013-08-19 09:19:47Z

3

Printer pr=new Printer();

change this to

  obj=new Printer();

answered Aug 19, 2013 at 9:19

ihsan kocak

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Comments

LaurentG · Accepted Answer · 2013-08-19 12:45:24Z

3

You are not correctly initializing the Printer variable: Printer.getInstance() always return null, what leads to a NullPointerException. Do this instead:

public static Printer getInstance() {
     if (obj == null) {
         obj = new Printer();
     }
     return obj;
}

edited Aug 19, 2013 at 12:45

answered Aug 19, 2013 at 9:21

LaurentG

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Comments

Sushil Deshmukh · Accepted Answer · 2013-08-19 09:28:34Z

1

Printer pr=new Printer();

this code create new Object of Printer but no use of this and obj object is have null for his lifetime scope

suggested code for this

public static Printer getInstance(){
     if(obj==null){
    obj=new Printer();            // Printer pr=new Printer();  /// here it's wrong
     }
 return obj;  
 }

answered Aug 19, 2013 at 9:28

Sushil Deshmukh

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Comments

Sreenivas M · Accepted Answer · 2013-08-19 09:48:32Z

0

You need to correctly implement a singleton class. Declare a constructor as a private method. Implement a static method which returns the instance of the same class. Also implement the creation of the instance in thread safe way. like

Printer p= new Printer();

answered Aug 19, 2013 at 9:48

Sreenivas M

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