Java invoke class to call method inside

Question

I have a problem. In my code I have the following line:

HashMap<String, String> strategyResponse = strategy_005.run(runDateTimeLocal);

This function is inside the strategy_005 class:

public class strategy_005 {

    public static HashMap<String, String> run(Integer i) {

        HashMap<String, String> output = new HashMap<>();
        output.put("Response", i.toString());
        return output;

    }

}

I am not calling the function in the strategy_005 class, but in my MainClass. The problem I have is that the 005 part in the class is dynamic, so I have multiple classes from strategy_001 to strategy_015.

Here is the code from my MainClass:

public class MainClass {

    public static void main(String[] args) {

        for (int i = 1; i <= 15; i++) {
            
            // Call every "strategy_0(i)" run() method
            HashMap<String, String> strategyResponse = strategy_005.run(i);
            System.out.println(strategyResponse.get("Response"));

        }

    }

}

I know how to invoke methods from a class by name, but I don't know how to invoke a class and then call the method that I do know. The only thing I found that is close to what I want is this: Creating an instance using the class name and calling constructor

Unfortunatly this topic is about calling the constructor, but I want to call a custom method. Please let me know how I can achieve this!

If you want the methods to be dynamically dispatched, why make them static? Create an interface and have a list of 15 instances of different classes (or anonymous classes, or lambdas, or whatever) which implement that interface. — kaya3, May 04 '21 at 23:00
Does this do what you need? https://stackoverflow.com/questions/11594797/dynamically-load-a-class-and-invoke-a-method-in-java — Jerry Jeremiah, May 04 '21 at 23:01
Yeah, but the 15 can be expanded. So I am looking for a solution where I don't have to add a new instance manually — A. Vreeswijk, May 04 '21 at 23:01
`clazz.getMethod` reflection method works the same way as `clazz.getConstructor` like in the post you found — OneCricketeer, May 04 '21 at 23:06

MattDog_222 Overwatch · Answer 1 · 2021-05-04T23:24:02.597

So what I would do is try making a Strategy abstract class, then making all 0 through 15 extend from Strategy. Then make a method in Strategy.java

public static HashMap<String, String> run( ? extends Strategy, Integer i ) {
    Strategy.getResponse() ...
}

? extends Strategy means any subclass of Strategy (in this case Strats 0-15) Then you would construct it in the main and call

Strategy.run( strategyResponse, i)

I tried doing what I said here and I can't get it to compile correctly. Instead I made a dummy class to play around with:

public class Strategies< T extends AbstractStrategy >
{
    ArrayList<? extends AbstractStrategy> li = new ArrayList<>() ;
    
    public HashMap<String,String> run( Strategies<?> strat ) 
    {
        HashMap<String,String> map = new HashMap<>() ;
        return map ;
    }
    
}

both of those compile so you might be able to play around with the syntax a bit to get it to your liking and usage.

Your initial suggestion won't work for static methods. You need an instance of the class in order to dispatch to the correct implementation. — something, May 04 '21 at 23:56

score 0 · Accepted Answer · answered May 04 '21 at 23:15

you can invoke methods using reflection:

public HashMap<String, String> invoke(int i, int arg0){
    Class<?> clz = Class.forName("package_name.strategy_" + i);
    Method method = clz.getDeclaredMethod("run", int.class);
    return (HashMap<String, String>) method.invoke(null, arg0);
}

for(int i = 0; i < 15; i++){
    HashMap<String, String> output = invoke(i, runDateTimeLocal);
}

the name of class for Class.forName("package_name.strategy_" + i); must be fully quallified with package name

Java invoke class to call method inside

2 Answers2