call a method using string in Java

Question

How can I call a method using String in Java. For example,

    String a = "call";
    int b = a();

    private static int call() {
         return 1;
    }

I know that it's got something to do with reflection, but I am not sure how to get it to work.

Don’t you think it’s more easy to have a method that receives a String and using a switch case call a specific method? — Jacobo, Apr 01 '18 at 06:26
user7 i know its invalid, that's why I am asking if there is a way — newguuy, Apr 01 '18 at 06:28
See this [SO post](https://stackoverflow.com/questions/160970/how-do-i-invoke-a-java-method-when-given-the-method-name-as-a-string). There are lots of examples of reflection in SO. — DevilsHnd, Apr 01 '18 at 06:31
Here's documentation on how to do this: https://docs.oracle.com/javase/tutorial/reflect/member/methodInvocation.html — EJoshuaS - Reinstate Monica, Apr 01 '18 at 06:31
There is a way, reflection, but in 99% of the cases, there is a much better, safer, faster way of solving the problem. Why do you want to do that? — JB Nizet, Apr 01 '18 at 06:31

score 0 · Answer 1 · answered Apr 01 '18 at 06:30

0

You can do this with reflection. First get a reference to the Class, use that to obtain the Method by-name. Then you can invoke that method. For example,

public class Test {    
    public static int call() {
        return 42;
    }

    public static void main(String[] args) throws Exception {
        Method m = Test.class.getMethod("call", new Class[0]);
        Object o = m.invoke(null, new Object[0]);
        System.out.println(o);
    }
}

Outputs the meaning of life, the universe and everything.

answered Apr 01 '18 at 06:30

Elliott Frisch

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**The meaning of life**? (I'm gonna try this thing. Maybe I can solve that String Theory thingy). – DevilsHnd Apr 01 '18 at 06:36
@DevilsHnd [The old](https://youtu.be/5ZLtcTZP2js) and [the modern](https://youtu.be/aboZctrHfK8). – Elliott Frisch Apr 01 '18 at 06:48

Johannes Kuhn · Answer 2 · 2018-04-01T07:56:59.980

Reflection. Or Method Handles.

Reflection:

 import java.lang.Reflect;

 public class SomeClass {
     public void someMethod() {
         String a = "call";

         try {
             Method m = SomeClass.class.getDeclaredMethod(a);
             m.setAccessible(true);
             m.invoke(null);
         } catch (Exception e) {
             throw new RuntimeException(e);
         }
    }
}

Without Reflection, using MethodHandles:

import java.lang.invoke.MethodHandle;
import java.lang.invoke.MethodHandles;
import java.lang.invoke.MethodType;

public class Example {
    public static void main(String[] args) {
        String a = "call";
        try {
            MethodHandle h = MethodHandles.lookup()
                .findStatic(Example.class, a, MethodType.methodType(Integer.TYPE));
            int result = (int) h.invokeExact();
            System.out.println(result);
        } catch (Throwable e) {
            throw new RuntimeException(e);
        }
    }

    @SuppressWarnings("unused")
    private static int call() {
        return 42;
    }
}

MethodHandles are said to be faster, but they have some additional limitations, like you can only use them to call code that you can call in the current context.

This limitation works for us in this case, because call is private, so we can call it. But if we try to do that from a different class, we will get an exception. (We can pass the result of lookup() or findStatic(), and others can call it for us.)

Also, the MethodHandles example works if there is a SecurityManager.

call a method using string in Java

2 Answers2