如何“动态地”将对象类型的实例转换为其特定的数据类型？ [英] How to "dynamically" cast an instance of Object type into its specific data type?

问题描述

public Object foo(int opt){
  if (opt == 0) return new String();
  else if (opt == 1) return new Integer(1);
  else if (opt == 2) return new Double(1);
  else if ...
  .. and many more
}

public void doSomething(String s){..}
public void doSomething(Integer i){..}
public void doSomething(Double d){..}
... and many more doSomething method

public static void main(String[] args){
  ...
  Object o = foo(x); //x is a value obtained during runtime, e.g. from user input

  //now I want to call doSomething method
  // (1)
  if (o instanceof String) doSomething((String) o);
  else if (o instanceof Integer) doSomething((Integer) o);
  else if (o instanceof Double) doSomething((Double) o);
  ...
  // (2)
}

有没有更好的方法来简化由（1）...（2）？括起的语句？

Java反射帮助？

Is there any better way to simplify statements enclosed by (1) ... (2)?
Does Java Reflection help?

推荐答案

有效率和干净地处理这个问题的最佳方法是让foo为对象返回一个持有者类。

The best way to handle this efficiently and cleanly is to have foo return a holder class for the object.

abstract class Holder<T> {
    private final T object;

    protected Holder(T object) { this.object = object; }
    public T get() { return object; }
    public abstract void doSomething();
}

public Holder foo(int opt) {
    if (opt == 0) return new Holder<String>("") {
        public void doSomething() { }
    };
    else if (opt == 1) return new Holder<Integer>(1) {
        public void doSomething() { }
    };
    else if (opt == 2) return new Holder<Double>(1.0) {
        public void doSomething() { }
    };
    // many more
}

public static void main(String... args) throws IOException {
    Holder h  = foo(x); //x is a value obtained during runtime, e.g. from user input

    //now I want to call doSomething method
    h.doSomething();
}

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