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    <p>Is this way comfortable for you ?</p> <pre><code>public class AreaCalculator { private int length = 10; private int width = 10; private static final AreaCalculator areaCalculator = new AreaCalculator(); private AreaCalculator() { // TODO Auto-generated constructor stub } public static AreaCalculator getInstance() { return areaCalculator; } public AreaCalculator fillLength(int length) { this.length = length; return getInstance(); } public AreaCalculator fillWidth(int width) { this.width = width; return getInstance(); } public AreaCalculator fillAll(int length, int width) { this.length = length; this.width = width; return getInstance(); } public int calculateArea() { if (!isInit()) { throw new RuntimeException("Should Init first!"); } return length * width; } public boolean isInit() { // add something return true; } </code></pre> <p>Test Code</p> <pre><code>public class TestMain { public static void main(String[] args) { int result = AreaCalculator.getInstance().fillWidth(20).calculateArea(); System.out.println("10*20=" + result); result = AreaCalculator.getInstance().fillWidth(10).fillLength(10).calculateArea(); System.out.println("10*10=" + result); result = AreaCalculator.getInstance().fillAll(10, 30).calculateArea(); System.out.println("10*30=" + result); } </code></pre> <p>**</p> <blockquote> <p>Using Config interface.</p> </blockquote> <p>**</p> <p>ConfigInf(lib side)</p> <pre><code>public interface ConfigInf { public static String F_LENGTH = "length"; public static String F_WIDTH = "width"; public String getProperty(String key); } </code></pre> <p>ConfigFactory(lib side)</p> <pre><code>public class ConfigFactory { private static ConfigInf config = null; public static void init(ConfigInf config) { ConfigFactory.config = config; } public static ConfigInf getConfig() { if (config == null) { throw new NullPointerException("Config should be init first"); } return config; } } </code></pre> <p>ClientConfig (client side)</p> <pre><code>public class ClientConfig implements ConfigInf { String value = "10"; @Override public String getProperty(String key) { // read configFile if (ConfigInf.F_LENGTH == key) { return value; } else { return "100"; } } public void update() { value = "100"; } } </code></pre> <p>AreaCalculatorWithConfig(lib side)</p> <pre><code>public class AreaCalculatorWithConfig { private int length = 10; private int width = 10; // private static final AreaCalculatorWithConfig areaCalculator = new AreaCalculatorWithConfig(); private AreaCalculatorWithConfig() { // TODO Auto-generated constructor stub } public static AreaCalculatorWithConfig getInstance() { return areaCalculator; } public int calculateArea() { ConfigInf config = ConfigFactory.getConfig(); length = getInt(config.getProperty(ConfigInf.F_LENGTH)); width = getInt(config.getProperty(ConfigInf.F_WIDTH)); return length * width; } public static int getInt(String value) { return Integer.parseInt(value); } } </code></pre> <p>TestMainWithConfig (client side)</p> <pre><code>public class TestMainWithConfig { public static void main(String[] args) { // init; ClientConfig config = new ClientConfig(); ConfigFactory.init(config); // System.out.println("Before update:" + AreaCalculatorWithConfig.getInstance().calculateArea()); config.update(); System.out.println("After update:" + AreaCalculatorWithConfig.getInstance().calculateArea()); } } </code></pre>
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    1. PODesign Pattern for a "library" in Java?
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    1. CONo, because If somebody calls `AreaCalculator.getInstance().calculateArea();` before anything else, on my case it would fail, because I can't define default values for the class variables.
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    2. COIf you use static variable(in your first way), you will also get error when you run method without initialize. You can throw an exception if the data are not prepared. I updated my code with exception managed. I also file a demo for config file which mentioned between you and @jolivier. Hope that helps...
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