Java Singleton Method always nullpointer exception

Question

I have a class called HtmlConnect.java. I declare the variable log as follows:

public Log log = Log.getInstance();

The Log.java file looks like this:

public class Log {

    private static Log instance = null;
    private String log;

    private Log() {

    }

    public static Log getInstance() {
        if (instance == null) {
            instance = new Log();
        }
        return instance;
    }

    public String getLog() {
        return log;
    }

    public void appendLog(String message) {
        this.log.concat(message+"\n");       
    }


    }

So when I call

log.appendLog("TestLog");

I always get a nullpointer exception. Why is taht?

Maybe you are trying to appendLog without first have instantiate the log. — giannis christofakis
– giannis christofakis, Commented Jan 4, 2014 at 20:09
Use a logging framework, never try and implement logging yourself. This just leads to grief for everyone involved. — Boris the Spider
– Boris the Spider, Commented Jan 4, 2014 at 20:15
This this.log.concat(message+"\n"); returns the result. Discarding the result won't do anything. — Peter Lawrey
– Peter Lawrey, Commented Jan 4, 2014 at 21:07

Idan Arye · Accepted Answer · 2014-01-04 20:10:02Z

7

You forgot to initialize the log member field. You need to do it in the constructor.

answered Jan 4, 2014 at 20:10

Idan Arye

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Comments

StarsSky · Accepted Answer · 2014-01-04 20:10:17Z

5

You need to initialize the log variable inside the constructor.

    private Log() {
      log = "";
    }

answered Jan 4, 2014 at 20:10

StarsSky

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Comments

Prashant Thakkar · Accepted Answer · 2014-01-04 20:11:23Z

1

Internally log.appendLog("TestLog"); uses log which is String and is not initialized.

Replace

private String log;

With

private String log = new String();

answered Jan 4, 2014 at 20:11

Prashant Thakkar

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1 Comment

Peter Lawrey Over a year ago

Instead of new String() use just "";

Peter Lawrey · Accepted Answer · 2014-01-04 21:09:39Z

0

A simpler solution would be like this.

public enum Log {;

    private static final StringBuilder LOG = new StringBuilder();

    public static synchronized void append(String message) {
        LOG.append(message).append("\n");       
    }

    public static synchronized String getString() {
        return LOG.toString();
    }

    public static synchronized String getStringAndClear() {
        String s = LOG.toString();
        LOG.setLength(0);
        return s;
    }
}

which you can call with

Log.append("Hello");
Log.append("World");

String lines = Log.getStringAndClear();

answered Jan 4, 2014 at 21:09

Peter Lawrey

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