Continue with last article, we will try to write an identical application to use OO features including: encapsulation, inheritance, polymorphism, properties, meta info and event-driven mechanism. Java supports the 3 basic features in language level. It uses interfaces to implements event-driven. To implements properties and meta info, we have to write our own code. We want to implements API like someObject.setProperty(prop-name, prop-value). I write my own NewObject class:
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package my; import java.lang.reflect.Field; import java.lang.reflect.InvocationTargetException; import java.lang.reflect.Method; import my.annotation.ClassInfo; import my.annotation.ClassInfoList; import my.annotation.Property; import my.annotation.PropertyAccess; /** * We just set/get values as Object type. End users know the exact type of the * property, and they can do the conversion themselves. */ public class NewObject { private static String makeGetPropertyName(Field field) { String fieldName = field.getName(); if (fieldName == null || fieldName.equals("")) { return null; } return "get" + Character.toUpperCase(fieldName.charAt(0)) + fieldName.substring(1); } private static String makeSetPropertyName(Field field) { String fieldName = field.getName(); if (fieldName == null || fieldName.equals("")) { return null; } return "set" + Character.toUpperCase(fieldName.charAt(0)) + fieldName.substring(1); } /** * Get property * @param name * @return */ public Object getProperty(String name) { Class klass = this.getClass(); Field field = null; while (!klass.getName().equals(NewObject.class.getName())) { try { field = klass.getDeclaredField(name); break; // found } catch (NoSuchFieldException e) { // noop } klass = klass.getSuperclass(); } if (field != null) { Property property = field.getAnnotation(Property.class); if (property.value() == PropertyAccess.WRITE || property.value() == PropertyAccess.READWRITE) { String methodName = makeGetPropertyName(field); try { /** * We can also get the value directly as below, but this * bypass the getter function which is wrong. * <code> * field.setAccessible(true); * Object value = field.get(this); * field.setAccessible(false); * return value; * */ Method getMethod = klass.getMethod(methodName); return getMethod.invoke(this); } catch (IllegalAccessException e) { // noop } catch (IllegalArgumentException e) { // noop } catch (InvocationTargetException e) { // noop } catch (NoSuchMethodException e) { // noop } catch (SecurityException e) { // noop } } } return null; } /** * Set property * @param name * @param value */ public void setProperty(String name, Object value) { Class klass = this.getClass(); Field field = null; while (!klass.getName().equals(NewObject.class.getName())) { try { field = klass.getDeclaredField(name); break; // found } catch (NoSuchFieldException e) { // noop } klass = klass.getSuperclass(); } if (field != null) { Property property = field.getAnnotation(Property.class); if (property.value() == PropertyAccess.WRITE || property.value() == PropertyAccess.READWRITE) { String methodName = makeSetPropertyName(field); try { Method setMethod = klass.getMethod(methodName, field.getType()); setMethod.invoke(this, value); } catch (IllegalAccessException e) { // noop } catch (IllegalArgumentException e) { // noop } catch (InvocationTargetException e) { // noop } catch (NoSuchMethodException e) { // noop } catch (SecurityException e) { // noop } } } } /** * Dump class info by given class * @param klass */ public static void dumpClassInfo(Class klass) { System.out.println(klass.getCanonicalName() + "("); ClassInfo[] klassInfos = klass.getAnnotation(ClassInfoList.class).value(); for (int i = 0; i < klassInfos.length; i++) { System.out.println(klassInfos[i].name() + "=" + klassInfos[i].value()); } System.out.println(")"); } /** * Dump class info of current object */ public void dumpClassInfo() { Class klass = this.getClass(); System.out.println(klass.getCanonicalName() + "("); ClassInfo[] klassInfos = klass.getAnnotation(ClassInfoList.class).value(); for (int i = 0; i < klassInfos.length; i++) { System.out.println(klassInfos[i].name() + "=" + klassInfos[i].value()); } System.out.println(")"); } /** * Get class info by given name * @param name * @return */ public String getClassInfo(String name) { Class klass = this.getClass(); ClassInfo[] klassInfos = klass.getAnnotation(ClassInfoList.class).value(); for (int i = 0; i < klassInfos.length; i++) { if (klassInfos[i].name().equals(name)) { return klassInfos[i].value(); } } return null; } } |
To use our setProperty()/getProperty() method, all classes should derive from the NewObject class. To be consistent with the JavaBean convention, we assume that the getter/setter function to be “get”/”set” + capitalize_first_letter_of(member-variable-name).
Property annotation and PropertyAccess enum are defined to indicate properties:
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// PropertyAccess.java package my.annotation; public enum PropertyAccess { READ, WRITE, READWRITE } |
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// Property.java package my.annotation; import java.lang.annotation.ElementType; import java.lang.annotation.Retention; import java.lang.annotation.RetentionPolicy; import java.lang.annotation.Target; @Retention(RetentionPolicy.RUNTIME) @Target(ElementType.FIELD) public @interface Property { /* "value" seems to be a magic name */ public PropertyAccess value(); } |
ClassInfo and ClassInfoList annotation are defined to indicate class meta info:
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// ClassInfo.java package my.annotation; import java.lang.annotation.ElementType; import java.lang.annotation.Retention; import java.lang.annotation.RetentionPolicy; import java.lang.annotation.Target; @Retention(RetentionPolicy.RUNTIME) @Target(ElementType.TYPE) public @interface ClassInfo { public String name(); public String value(); } |
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// ClassInfoList.java package my.annotation; import java.lang.annotation.ElementType; import java.lang.annotation.Retention; import java.lang.annotation.RetentionPolicy; import java.lang.annotation.Target; @Retention(RetentionPolicy.RUNTIME) @Target(ElementType.TYPE) public @interface ClassInfoList { ClassInfo[] value(); } |
Let’s see how to use them, our Base is defined as:
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// Base.java package fake; import java.util.LinkedList; import java.util.List; import my.NewObject; import my.annotation.ClassInfo; import my.annotation.ClassInfoList; import my.annotation.Property; import my.annotation.PropertyAccess; @ClassInfoList({ @ClassInfo(name = "author", value = "gonwan"), @ClassInfo(name = "version", value = "1.0.0") }) public class Base extends NewObject { @Property(PropertyAccess.READWRITE) private int id; @Property(PropertyAccess.READWRITE) private String name; private List<IPrintInt> basePrintIntListeners; private List<IPrintString> basePrintStringListeners; public Base() { basePrintIntListeners = new LinkedList<IPrintInt>(); basePrintStringListeners = new LinkedList<IPrintString>(); } public int getId() { return id; } public void setId(int id) { this.id = id; } public String getName() { return name; } public void setName(String name) { this.name = name; } public void virtualDump() { System.out.printf("Base(virtual): id=%d, name=\"%s\"\n", getId(), getName()); } final public void nonvirtualDump() { System.out.printf("Base(nonvirtual): id=%d, name=\"%s\"\n", getId(), getName()); } public void addBasePrintIntListener(IPrintInt listener) { basePrintIntListeners.add(listener); } public void addBasePrintStringListener(IPrintString listener) { basePrintStringListeners.add(listener); } public void fireBasePrintIntEvent(int i) { for (IPrintInt listener : basePrintIntListeners) { listener.printInt(i); } } public void fireBasePrintStringEvent(String str) { for (IPrintString listener : basePrintStringListeners) { listener.PrintString(str); } } } |
Since our implementation of properties are simply methods, they can be inherited by subclasses. But the class meta info cannot be retrieved in subclasses. They just get their own.
I do not want to demo events/listeners code here, just find them in source code in my skydrive: http://cid-481cbe104492a3af.office.live.com/browse.aspx/share/dev/TestOO. In the TestJavaObject-{date}.zip file.