One annoyance in Java is having to do instanceof checks on multiple lines. e.g.<\/p>\n
\r\nif (object instanceof Foo) {\r\n Foo foo = (Foo)object;\r\n foo.foo();\r\n}\r\n<\/pre>\nWhile this is often a sign of a design failure, there are times when instanceof checks are required often due to framework constraints etc. The above is quite ugly and involves using 3 lines instead of 1 for a single method call. If foo() is a void method we can’t even use the ternary ? : operator to make it more concise. <\/p>\n
In c# with Lambdas and Extension methods one can create something like : <\/p>\n
\r\n object.When(foo => foo.Foo());\r\n<\/pre>\nWhere “When” is an extension method added to all objects which takes a type parameter and a lambda to execute if the type parameter matches. (Implementation available if anyone cares).<\/p>\n
Since Java doesn’t have Extension methods or Lambdas yet (Oracle minus minus) we can’t do this. We can however always use reflection and get something like:<\/p>\n
\r\npackage scratchpad;\r\n\r\nimport java.lang.reflect.Method;\r\nimport net.sf.cglib.proxy.Enhancer;\r\nimport net.sf.cglib.proxy.MethodInterceptor;\r\nimport net.sf.cglib.proxy.MethodProxy;\r\n\r\nimport static scratchpad.When.when;\r\n\r\npublic class Let {\r\n\tpublic static void main(String... args) {\r\n\t\tObject a = new Foo();\r\n\t\tObject b = \"hello world\";\r\n\t\t\r\n\t\twhen(Foo.class).isTypeOf(a).foo(); \/\/ Prints \"foo\"\r\n\t\twhen(Foo.class).isTypeOf(b).foo(); \/\/ Does nothing at all\r\n\t}\r\n}\r\n\r\nclass Foo {\r\n\tpublic void foo() {\r\n\t\tSystem.out.println(\"foo\");\r\n\t}\r\n}\r\n\r\nclass When {\r\n\tpublic static Is when(final Class cls) {\r\n\t\treturn new Is() {\r\n\t\t\t@SuppressWarnings(\"unchecked\")\r\n\t\t\tpublic T isTypeOf(Object o) {\r\n\t\t\t\treturn o != null && cls.isAssignableFrom(o.getClass()) \r\n\t\t\t\t\t? (T)o\r\n\t\t\t\t\t: NullObject.create(cls);\r\n\t\t\t}\r\n\r\n\t\t};\r\n\t}\r\n\t\t\r\n\tpublic interface Is {\r\n\t\tpublic T isTypeOf(Object o);\r\n\t}\r\n}\r\n\r\nclass NullObject implements MethodInterceptor {\r\n\t\r\n\t@SuppressWarnings(\"unchecked\")\r\n\tpublic static T create(Class cls){\r\n\t\tEnhancer enhancer = new Enhancer();\r\n\t\tenhancer.setSuperclass(cls);\r\n\t\tenhancer.setCallback(new NullObject());\r\n\t\treturn (T)enhancer.create();\r\n\t}\r\n\r\n\tpublic Object intercept(Object o, Method method, Object[] os, MethodProxy mp) throws Throwable {\r\n\t\treturn null;\r\n\t}\r\n}\r\n<\/pre>\n","protected":false},"excerpt":{"rendered":"