Parsing Objects from String in Java

问题:

I am trying to write a general method to parse objects from strings. To be clear, I have the following not-so-elegant implementation:

public static Object parseObjectFromString(String s, Class class) throws Exception {
  String className = class.getSimpleName();
  if(className.equals("Integer")) {
    return Integer.parseInt(s);
  }
  else if(className.equals("Float")) {
    return Float.parseFloat(s);
  }
  else if ...

}

Is there a better way to implement this?

回答1:

Your method can have a single line of code:

public static <T> T parseObjectFromString(String s, Class<T> clazz) throws Exception {
    return clazz.getConstructor(new Class[] {String.class }).newInstance(s);
}

Testing with different classes:

Object obj1 = parseObjectFromString("123", Integer.class);
System.out.println("Obj: " + obj1.toString() + "; type: " + obj1.getClass().getSimpleName());
BigDecimal obj2 = parseObjectFromString("123", BigDecimal.class);
System.out.println("Obj: " + obj2.toString() + "; type: " + obj2.getClass().getSimpleName());
Object obj3 = parseObjectFromString("str", String.class);
System.out.println("Obj: " + obj3.toString() + "; type: " + obj3.getClass().getSimpleName());
Object obj4 = parseObjectFromString("yyyy", SimpleDateFormat.class);
System.out.println("Obj: " + obj4.toString() + "; type: " + obj4.getClass().getSimpleName());

The output:

Obj: 123; type: Integer
Obj: str; type: String
Obj: 123; type: BigDecimal
Obj: java.text.SimpleDateFormat@38d640; type: SimpleDateFormat

回答2:

I'm not sure what you're trying to do. Here's a few different guesses:

• You want to be able to convert an object to a string, and vice-versa.

You should look into serialization. I use XStream, but writeObject and java.beans.XMLEncoder also works.

• The user enters text, and you want to coerce it to the "right" type, of which there are many.

Usually, this means a problem with the user specification. What are you receiving from the user, and why would it be able to be so many different kinds?

In general, you will want the type to be as broad as possible: use double if it's a number, and String for almost everything else. Then build other things from that variable. But don't pass in the type: usually, the type should be very obvious.

回答3:

How about enums?

  public enum Types {

     INTEGER {
        @Override
        public Object parse(String s) { return Integer.parseInt(s); }
     },
     FLOAT {
        @Override
        public Object parse(String s) { return Float.parseFloat(s); } 
     }
     ...
     ;

     public abstract Object parse(String s);

     public static Object parseObjectFromString(String s, Class<?> cls)  {
        return valueOf(cls.getSimpleName().toUpperCase()).parse(s);
     }
  }

  public static void main(String[] args) {
     System.out.println(Types.parseObjectFromString("5", Integer.class));
  }

回答4:

NumberUtils.createNumber(str) (from apache commons-lang)

It decides what type of number to create, so you don't pass the class.

回答5:

If those are your only types of examples, then you can also do something like:

Constructor ctor = Class.getConstructor( String.class ); 
return ctor.newInstance( s );

More information may provide for better answers. I'd be very much surprised if there isn't already some utility code out there that meets your specifications. There are a few ways to tackle this problem depending on the actual requirements.

回答6:

To convert from an Object (here a String) to another one, you can use transmorph :

Transmorph transmorph = new Transmorph(new DefaultConverters());
float myFloat = transmorph.convert("55.2",Float.TYPE);

long[] longsArray = transmorph.convert(new Integer[] { 1, 2, 3, 4 }, long[].class);