<commit_before>package org.zeropage.log;

import java.util.ArrayList;

/**
 * A Logger object can log messages with specific level.
 */
public class Logger {
    private volatile static Logger instance;
    private ArrayList<LogListener> listeners;

    /**
     * Private method to construct a logger
     */
    private Logger() {
        listeners = new ArrayList<>();
    }

    /**
     * Returns object of logger.
     *
     * @return object of logger
     */
    public static Logger getInstance() {
        if (instance == null) {
            synchronized (Logger.class) {
                if (instance == null) {
                    instance = new Logger();
                }
            }
        }
        return instance;
    }
<commit_msg>docs(LogListener): modified JavaDoc<commit_after>package org.zeropage.log;

import java.util.ArrayList;

/**
 * A Logger object can log messages with specific level.
 */
public class Logger {
    private volatile static Logger instance;
    private ArrayList<LogListener> listeners;
    private Logger() {
        listeners = new ArrayList<>();
    }

    /**
     * Returns object of logger.
     *
     * @return object of logger
     */
    public static Logger getInstance() {
        if (instance == null) {
            synchronized (Logger.class) {
                if (instance == null) {
                    instance = new Logger();
                }
            }
        }
        return instance;
    }

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