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Java: check if a class exists and call a specific method if it exists

Is there a way to do the following? Check if a class exists (in the same package) and if it does exist, check if a particular method exists, and if so, calling it?
Say that I have class X. In some method of class X, I want to do the following:
if (class Y exists) { //Maybe use Class.forName("Y")?
if ( Y has method a(String, String) ) {
call Y.a("hello", "world");
}
}
Is such a thing possible? And is doing such a thing reasonable? Thanks.

Is such a thing possible? And is doing such a thing reasonable?
Thanks.
Of course it is possible.
If you develop a program or a library that has to discover dynamically some classes, it is a very reasonable thing.
If it is not the case, it could not be.
If your need makes sense, you should ask you an additional question : should you invoke a static or instance method ?
Here is a sample example with both solutions :
ReflectionClass that contains the logic using reflection :
import java.lang.reflect.Method;
public class ReflectionCalls {
public static void main(String[] args) {
new ReflectionCalls();
}
public ReflectionCalls() {
callMethod(true);
callMethod(false);
}
private void callMethod(boolean isInstanceMethod) {
String className = "DiscoveredClass";
String staticMethodName = "methodStatic";
String instanceMethodName = "methodInstance";
Class<?>[] formalParameters = { int.class, String.class };
Object[] effectiveParameters = new Object[] { 5, "hello" };
String packageName = getClass().getPackage().getName();
try {
Class<?> clazz = Class.forName(packageName + "." + className);
if (!isInstanceMethod) {
Method method = clazz.getMethod(staticMethodName, formalParameters);
method.invoke(null, effectiveParameters);
}
else {
Method method = clazz.getMethod(instanceMethodName, formalParameters);
Object newInstance = clazz.newInstance();
method.invoke(newInstance, effectiveParameters);
}
} catch (Exception e) {
e.printStackTrace();
}
}
}
DiscoveredClass (the class we manipulate in the example)
package reflectionexp;
public class DiscoveredClass {
public static void methodStatic(int x, String string) {
System.out.println("static method with " + x + " and " + string);
}
public void methodInstance(int x, String string) {
System.out.println("instance method with " + x + " and " + string);
}
}
Output :
instance method with 5 and hello
static method with 5 and hello

Yes, this can be done. I've created a Test class in the same Package as the current class.
import java.lang.reflect.Method;
public class Sample {
public static void main(String[] args) {
Class<?> clazz = null;
try {
clazz = Class.forName("Test");
} catch (ClassNotFoundException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
if (clazz == null) {
System.out.println("class not found. Go eat some waffles and correct the name");
return;
}
Method m = null;
try {
m = clazz.getMethod("foo", null);
} catch (NoSuchMethodException | SecurityException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
if (m == null) {
System.out.println("method not found. Go eat some waffles and correct the name");
return;
}
Test t;
try {
t = (Test) clazz.newInstance();
m.invoke(t, null);
} catch (Exception e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
}
public class Test {
static {
System.out.println("test...");
}
public void foo() {
System.out.println("foo");
}
}
O/P :
test...
foo

You can use Class.forName:
try {
Class yourClass = Class.forName( "classname" );
Object o = yourClass.newInstance();
} catch( ClassNotFoundException e ) {
//Throw error or whatever
}
To check if a method exists you could use the NoSuchMethodError e in a try/catch

You can do this using reflection, however it isnt really practical unless you are trying to access classes that potentially will not be present at runtime or if you are trying to access private or hidden fields. Example below.
public static void reflectionDemo(){
//Here we attempt to get the common URI class
//If it is found, we attempt to get the create method
//We then invoke the create method and print the class name of the result.
try {
Class<?> uriClass = Class.forName("java.net.URI");
//getMethod(String name, Class<?>... args);
java.lang.reflect.Method create = uriClass.getMethod("create", String.class);
//The first parameter is null because this is a static method.
//invoke(Object target, Object... args);
System.out.println(create.invoke(null, "some/uri").getClass());
//Will print class java.net.URI
} catch (ClassNotFoundException e) {
// If class doesnt exist
e.printStackTrace();
} catch (NoSuchMethodException e) {
// If method doesnt exist
e.printStackTrace();
} catch (SecurityException e) {
// See Javadoc
e.printStackTrace();
} catch (IllegalAccessException e) {
// From invoke
e.printStackTrace();
} catch (IllegalArgumentException e) {
// From invoke
e.printStackTrace();
} catch (java.lang.reflect.InvocationTargetException e) {
// From invoke
e.printStackTrace();
}
}

To find whether a class exists, you can use the forName() method on Class.
To find whether a method exists, you can use the getMethod() method on Class.
Documentation here:
https://docs.oracle.com/javase/7/docs/api/java/lang/Class.html#forName(java.lang.String)
https://docs.oracle.com/javase/7/docs/api/java/lang/Class.html#getMethod(java.lang.String,%20java.lang.Class...)
For your class problem, you'd want to use code like:
try {
Class.forName("Y");
}
catch (ClassNotFoundException e) {
}
For your method problem, you'd want to use code like:
try {
Class.getMethod(a);
}
catch (NoSuchMethodException e) {
}

You can check if the Class exists with Class.forName("classname");
See this SO question: Check if class exists somewhere in package
If a method exists can be catched with NoSuchMethodError in your try/catch.
See this SO question: Check if method exists at Runtime in Java
try {
Object object = Class.forName("Y").newInstance();
object.a(String, String);
} catch( ClassNotFoundException | NoSuchMethodError ex) {
//do Something else
}

I want to extract a string from the current exception thrown in a java program

I am calling a method from a third party jars whose class file is not accessible to me. So for some cases it throws exception logs and I want to extract string from the current log dynamically.
This is the java program method which throws exception
public String runLayoutTest(final String xmlFile){
try{
String gettingValue = "novalue";
boolean errorFlag = perform("runLayoutTest", new Reporter.Reportable() {
#Override
public boolean run() throws Exception {
String layoutXml = null;
//current directory
Path currentRelativePath = Paths.get("");
String currentProjectPath = currentRelativePath.toAbsolutePath().toString();
System.out.println("Current relative path is: " + currentProjectPath);
System.out.println("layoutXml "+xmlFile);
String x = client.runLayoutTest(currentProjectPath+"\\Excel\\"+xmlFile);
System.out.println("******* x ********"+x);
setX(x);
return true;
}
});
gettingValue = getX();
return gettingValue;
}catch(Exception e){
System.out.println("**Any one rule in Layout is failed**");
//System.out.println(e.getMessage());
return getX();
}
}
Here the client is a object of the third party jar file , And that is throwing me the exception on some odd cases .
The Exception logs are
com.experitest.client.InternalException: Exception caught while executing runLayoutTest: {"rule_1":{"Exists":true},"rule_2":{"Exists":true,"EqualHeight":false},"rule_3":{"AlignedLeft":false}}
at com.experitest.client.JavaClientInternals.executeInternally(JavaClientInternals.java:234)
at com.experitest.client.Client.execute(Client.java:237)
at com.experitest.client.Client.runLayoutTest(Client.java:1475)
at com.igate.framework.NativeDriver$79.run(NativeDriver.java:2753)
at com.igate.framework.Reporter.action(Reporter.java:81)........
From this exception I want to extract
runLayoutTest: {"rule_1":{"Exists":true},"rule_2":{"Exists":true,"EqualHeight":false},"rule_3":{"AlignedLeft":false}}
as a String.
Hence is there any method with which I can dynamically extract such String whenever it occurs.
And I still don't know the reason why my catch method is not getting called.

In your case you can just use Exception.getMessage() which will result in
Exception caught while executing runLayoutTest:
{"rule_1":{"Exists":true},"rule_2":{"Exists":true,"EqualHeight":false},"rule_3":{"AlignedLeft":false}}
Below can be used to get the json string. You can then parse the json to get desired data from it
String message = e.getMessage();
int colonIndex = message.indexOf(":");
String json = null;
if (colonIndex != -1) {
json = message.substring(colonIndex + 1);
}
Please note that this solution will not work in case of wrapped exception

I had to implement this code to attack directly to throwable detailMessage. I had to ignore all the hierarchy of classes that extended Exception because someone implemented a horrible getMessage method that was being overlapped with web components. In JUnit was no way to define unit test cases because framework's exceptions always tried to translate its error code into the 'session' (web) language...
public static Object getThrowableDetailMessage(Throwable throwable) {
try {
// Tiramos la puerta abajo y pedimos a Throwable que nos pase el código de error
Field f = Throwable.class.getDeclaredField("detailMessage");
f.setAccessible(true);
Object detailMessageFound = f.get(throwable);
return detailMessageFound;
} catch (SecurityException ex) {
//LOG
return null;
} catch (NoSuchFieldException e) {
//LOG
return null;
} catch (IllegalArgumentException e) {
//LOG
return null;
} catch (IllegalAccessException e) {
//LOG
return null;
}
}
You also can try to replace Throwable by InternalException and keep the reflection on this level of the hierachy. But like someone is already pointing. Make sure your code is being affected by the Exception you want to inspect.
I have implemented all the catches possible because I wanted to trace a different log for everyone of them. (For debugging and testing overall)
Note: My code has been implemented under JDK 1.6_18

Can I pass parameters to my dynamically created classes?

I am successfully loading a class from an external MyClass.class file at runtime, and am able to call it's methods assuming I know the name of them, which I do.
The problem I'm having is I cannot figure out how to pass a parameter to the constructor of the class I am loading.
How can I modify this to pass a parameter to MyClass's constructor? Also, how can I access MyClass's public variable: infoToAccess?
Here are the files I'm working with. (Keep in mind, ClassLoaderExample.pde was written for Processing, but other than sketchPath("") and the lack of a main function, it is identical.
ClassLoaderExample.pde: The main file that loads the class:
import java.io.File;
import java.net.URL;
import java.net.URLClassLoader;
import java.net.MalformedURLException;
import java.lang.ClassLoader;
import java.lang.reflect.Method;
import java.lang.reflect.InvocationTargetException;
/////// FILE STRUCTURE /////
// ClassLoaderExample //
// --this file //
// --tmp //
// ----MyClass.class //
////////////////////////////
// TODO
// Send parameter to MyClass constructor (by reference)
// Get public int number from MyClass
void setup()
{
String className = "MyClass";
Object instance;
Method updateMethod;
// sketchPath("") returns the directory this file is in
File file = new File(sketchPath(""));
try
{
URL url = file.toURL();
URL[] urls = new URL[]{url};
ClassLoader classLoader = new URLClassLoader(urls);
Class<?> loadedClass = classLoader.loadClass("tmp."+className);
try
{
instance = loadedClass.newInstance();
updateMethod = loadedClass.getDeclaredMethod("update");
// Calls MyClass's update() method
try
{
updateMethod.invoke(instance);
}
catch (InvocationTargetException e) {System.out.println(e);}
catch (IllegalAccessException e) {System.out.println(e);}
}
catch (InstantiationException e) {System.out.println(e);}
catch (IllegalAccessException e) {System.out.println(e);}
catch (NoSuchMethodException e) {System.out.println(e);}
}
catch (MalformedURLException e) {System.out.println(e);}
catch (ClassNotFoundException e) {System.out.println(e);}
}
MyClass.java: The class I am loading:
package tmp;
public class MyClass
{
public int infoToAccess = 1337;
public MyClass(int i)
{
System.out.println("MyClass constructor was called with numebr: " + i);
}
public void update()
{
System.out.println("Update was called.");
}
}
Thanks for any help with this!

you can use the class you loaded to get hold of the constructor
you need an use that to create a new instance.
import java.lang.reflect.Constructor;
...
Class<?> loadedClass = classLoader.loadClass("tmp."+className);
try {
Constructor<?> ctor=loadedClass.getConstructor(int.class);
ctor.newInstance(42);
...
}
...

You need to use reflection for both of the things you asked about:
Class<?> loadedClass = classLoader.loadClass("tmp." + className);
Object oInstance = null;
try {
Constructor<?> c = loadedClass.getConstructor(Integer.class);
oInstance = c.newInstance(10);
} catch (InstantiationException | IllegalAccessException | InvocationTargetException | NoSuchMethodException e) {
//Handle the possibility that you cannot access this constructor
}
To access the public field of your object:
try {
Field field = loadedClass.getField("infoToAccess");
int fieldValue = field.getInt(oInstance);
} catch (NoSuchFieldException | IllegalAccessException e) {
//Handle the possibility that you cannot access this field
}

As mentioned in the link I wrote in the comments, you can pass parameters in this call
updateMethod = loadedClass.getDeclaredMethod("update", parameters go here);
then you should be able to invoke the method with a parameter(s)

If you want to pass arguments to the constructor you cannot use getInstance(), you have to get the constructor object and pass the parameters there.
Example:
Constructor constructor = aClass.getConstructor(new Class[]{String.class});
MyObject myObject = (MyObject) constructor.newInstance("constructor-arg1");
See tutorial here: Java Reflection - Constructors

Finding the cluster that an instance got assigned to with Weka

Im using an EM clusterer with an AddCluster Filter in order to see what instances are getting assigned to the different clusters after training. Below is the code that I'm using. I'm faily sure that I am applying the filter correctly but once I have the new Instances I still dont know how to get the cluster info from them. Im sure its just a simple getBlah() call but I'm just not locating it. Thanks in advance.
public Cluster()
{
clusterer = new EM();
filter = new AddCluster();
try
{
clusterer.setMaxIterations(100);
clusterer.setNumClusters(20);
filter.setClusterer(clusterer);
}
catch (Exception e)
{
e.printStackTrace();
}
}
public void buildCluster(String fileName)
{
try
{
DataSource source = new DataSource(fileName);
inst = source.getDataSet();
filter.setInputFormat(inst);
inst = AddCluster.useFilter(inst, filter);
}
catch (Exception e)
{
e.printStackTrace();
}
}

I think you should use "Dictionary" Class. Here is my example code:
Enumeration clusteredInst = data_to_use.enumerateInstances();<br>
Dictionary<Integer, ArrayList<Instance>> clusteredSamples = new ashtable<>();
while (clusteredInst.hasMoreElements()) {<br>
Instance ins = (Instance) clusteredInst.nextElement();<br>
int clusterNumb = em.clusterInstance(ins);<br>
ArrayList<Instance> cls = null;<br>
cls = clusteredSamples.get(clusterNumb);<br>
if (cls != null) {<br>
cls.add(ins);<br>
} else {<br>
cls = new ArrayList<>();<br>
cls.add(ins);<br>
//you add elements to dictionary using put method<br>
//put(key, value)<br>
clusteredSamples.put(clusterNumb, cls);<br>
}
}
And you also can retrieval your data from dictionary by call it's Key.

How to get an instanced Object from an incomplete class name?

world! I need to instantiate an Object from the name of its Class. I know that it is possible to do it, this way
MyObject myObject = null;
try {
Constructor constructor = Class.forName( "fully.qualified.class.name" ).getConstructor(); // Get the constructor without parameters
myObject = (MyObject) constructor.newInstance();
} catch (Exception e) {
e.printStackTrace();
}
The problem is that the name of my class is not fully qualified. Is there a way to get the complete name by only knowing the short name?

MyObject myObject = null;
for (Package p : Package.getPackages()) {
try {
myObject = Class.forName(p.getName() + "." + className).newInstance();
break;
} catch (ClassNotFoundException ex) {
// ignore
}
}
The Package.getPackages() call will give you every package known to the current classes ClassLoader and its ancestors.
Warning: this will be expensive because you are repeatedly throwing and catching exceptions. It may be possible to speed it up by testing:
this.getClass().getClassLoader().findResource(binaryClassName) != null
before calling Class.forName(...) or the equivalent.

Try this repeatedly for a package search path. ;)
String[] packages = ...;
String className = ...;
MyObject myObject = null;
for(String p : packages)
try {
myObject = Class.forName(p + '.' + className).newInstance();
break;
} catch (Exception ignored) {
}