How to call a method by name (String) in Java?

Question

类A<\/code>每个实例都有一个类B<\/code>的实例。 A<\/code>应该根据其成员变量method_num<\/code>在B<\/code>调用不同的方法。这是一个实现我想要的实现：

public class A {
    private B myB = new B();
    public int method_num = 1;
    public callBMethod() {
        if ( method_num == 1 )
            myB.method1();
        else
            myB.method2();
    }
}

public class B {
    public method1() { }
    public method2() { }
}

Answer 1

If you don't want to use reflection (and this is an excellent goal) then there are some neat features of enum s that allow you to set up an enum as a proxy.

public class A {
  private B myB = new B();
  public int method_num = 1;

  public void callBMethod() {
    // Could do it by name.
    BMethods.valueOf("method1").call(myB);
    // Or by number.
    BMethods.values()[method_num].call(myB);
  }

}

enum BMethods{
  method1 {
    @Override
    public void call(B b) {
      b.method1();
    }
  },
  method2 {
    @Override
    public void call(B b) {
      b.method2();
    }
  };

  public abstract void call (B b);
}

public class B {
  public void method1() {
  }

  public void method2() {
  }

}

Answer 2

You cannot. Java doesn't treat functions as first class objects, because it doesn't have functional features like Python or C#.

You can create a Command interface and pass that object reference:

public interface Command {
    void execute(Object [] args);
}

Answer 3

I think you can use reflection like this :

java.lang.reflect.Method method;
try {
  method = obj.getClass().getMethod(methodName, param1.class, param2.class, ..);
} catch (SecurityException e) {
  // ...
} catch (NoSuchMethodException e) {
  // ...
}  

try {
  method.invoke(obj, arg1, arg2,...);
} catch (IllegalArgumentException e) {  //do proper handling
} catch (IllegalAccessException e) {//do proper handling
} catch (InvocationTargetException e) {//do proper handling

Answer 4

Maybe with Runnable objects ? You can pass from B a runnable, and call .run() directly from A

Answer 5

Java reflection API provides you a way, where a Method type of object could be passed along with the target object and then the method could be invoked on the target object.

A sample example is here:

Method m; // The method to be invoked

  Object target; // The object to invoke it on

  Object[] args; // The arguments to pass to the method

  // An empty array; used for methods with no arguments at all.
  static final Object[] nullargs = new Object[] {};

  /** This constructor creates a Command object for a no-arg method */
  public Command(Object target, Method m) {
    this(target, m, nullargs);
  }

  /**
   * This constructor creates a Command object for a method that takes the
   * specified array of arguments. Note that the parse() method provides
   * another way to create a Command object
   */
  public Command(Object target, Method m, Object[] args) {
    this.target = target;
    this.m = m;
    this.args = args;
  }

  /**
   * Invoke the Command by calling the method on its target, and passing the
   * arguments. See also actionPerformed() which does not throw the checked
   * exceptions that this method does.
   */
  public void invoke() throws IllegalAccessException,
      InvocationTargetException {
    m.invoke(target, args); // Use reflection to invoke the method
  }