Java : Load class based on user input

Question

I have to get the class name from user and load it dynamically.

  public class sample{
    public static void main(String[] args) {
    if(args.length < 1)
    {
        print_usage();
    }
    else{
        Class inputClass = null;
        try {
            inputClass = Class.forName(args[0]);
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        }
    }
  }

I have classes named Sample2, Sample3:

public class Sample2 {
    private String name;
}

public class Sample3 {
    private int value;
}

I want to load class based on user input, either Sample2/Sample3 class.

I have the files in the same directory, but I get the java.lang.ClassNotFoundException error. How do I fix this error?

Your classes are in package? You have added the directory to the classpath? — Jens
– Jens, Commented Oct 15, 2015 at 5:54

mid · Accepted Answer · 2015-10-15 06:10:35Z

2

Please be more clear on your question, I assume you are typing in the class names without the package.
If they're in a package, you must use Class.forName by that.

Class inputClass = null;
try {
    inputClass = Class.forName("my.package.MyClass");
} catch (ClassNotFoundException e) {
    e.printStackTrace();
}

answered Oct 15, 2015 at 6:10

mid

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5 Comments

user2761431 Over a year ago

Thanks! That was the problem. I was not giving the package name.

user2761431 Over a year ago

Is it possible to call the getter/setter/normal methods that are defined in the inputClass? I am only able to get the defined methods using getDeclaredMethods(), but I am not able to define an instance of the inputClass and call the methods passing arguments.

mid Over a year ago

You could try to use newInstance(). But it's really evil.

user2761431 Over a year ago

Method[] methods = inputClass.getMethods(); methods[count].invoke(inputClass.newInstance(),inputString); I get java.lang.InstantiationException. Am I invoking the method right?

user2761431 Over a year ago

I figured what the problem was. I had defined a custom constructor and it seems newInstance() works only if no constructor is defined for the inputClass

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