Creating objects from a string

Question

I know if I were in PHP I would be able to create an object as following:

// class Name
$var = "Name";
$my_name = new $var("John", "Doe");

Now, I wanted to adopt similar approach in Java. I have a file which I need to parse and produce objects depending on the line read from the file. For example:

2235:org.powertac.common.TariffSpecification::8::addRate::9
2235:org.powertac.common.Tariff::8::new::8
2235:org.powertac.common.TariffSpecification::10::new::1::PRODUCTION
2236:org.powertac.common.Rate::11::new
2236:org.powertac.common.Rate::11::withValue::0.015
2236:org.powertac.common.Rate::11::setTariffId::10
2236:org.powertac.common.TariffSpecification::10::addRate::11
2236:org.powertac.common.Tariff::10::new::10
2247:org.powertac.common.RandomSeed::12::init::DistributionUtilityService::0::model::-7651755067434358
2257:org.powertac.genco.Genco::13::new::nsp1a::true::true
2258:org.powertac.genco.Genco::13::new::nsp1a
2258:org.powertac.genco.Genco::15::new::nsp1b::true::true

then I should create an object of the type Rate, (new), with default values. Therefore, parsing that line would yield me: "org.powertac.common.Rate"(String) and I wanted to create a org.powertac.common.Rate object. How could I do it? I know it's a lazy evaluation ... how can I create something similar in Java?

You're looking for reflection.

SLaks
– SLaks

2012-08-27 19:13:20 +00:00
Commented Aug 27, 2012 at 19:13 — SLaks
– SLaks, Commented Aug 27, 2012 at 19:13

João Silva · Accepted Answer · 2012-08-27 19:32:07Z

3

Use the Java Reflection API:

String className = "org.powertac.common.Rate";
String ctorArg = "8";
Class<Rate> clazz = (Class<Rate>) Class.forName(className);
Constructor<Rate> ctor = clazz.getConstructor(String.class);
Rate rate = ctor.newInstance(ctorArg);

Then, if you want to invoke a method using Reflection:

String methodName = "setTariffId";
String methodArg = "1";
Method setTariffId = clazz.getMethod(methodName, String.class);
setTariffId.invoke(rate, methodArg);

edited Aug 27, 2012 at 19:32

answered Aug 27, 2012 at 19:15

João Silva

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5 Comments

João Silva Over a year ago

@philippe: But don't you have the name in a variable after parsing the line?

cybertextron Over a year ago

I do ... but how could I create for example: Rate rate = (Rate) Class.forName("org.powertac.common.Rate").newInstance();

João Silva Over a year ago

Instead of "org.powertac.common.Rate" use the name of the variable which holds that value.

cybertextron Over a year ago

For example, org.powertac.common.Tariff::8::new::8 has: org.powertac.common.Tariff is the class name, 8 is the order it is being executed, and 8 is the argument to the constructor, how could I create a Tariff object with that argument?

João Silva Over a year ago

@philippe: I've put up an example.

John Ericksen · Accepted Answer · 2012-08-27 19:28:11Z

1

It may be easier to use some preexisting serialization / deserialization solution in Java, but here's something that would work for your String based approach:

Class<?> rateClass = class.getClassLoader().loadClass("org.powertac.common.Rate");
Constructor<?> constructor = rateClass.getConstructor(String.class);

Rate rate = (Rate)constructor.newInstance("Value")

So in this case:

2258:org.powertac.genco.Genco::15::new::nsp1b::true::true

you would do the following:

String className = "org.powertac.genco.Genco";
Object[] args = {"nsp1b", true, true};

Class<?> rateClass = getClass().getClassLoader().loadClass(className);
Constructor<?> constructor = rateClass.getConstructor(String.class, boolean.class, boolean.class);

Object deseraliazed = constructor.newInstance(args);

You may want to store the type along with the constructor values.

edited Aug 27, 2012 at 19:28

answered Aug 27, 2012 at 19:16

John Ericksen

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2 Comments

cybertextron Over a year ago

Johncarl I've updated my question ... There is also the arguments for the constructor, which goes after the new keyword

John Ericksen Over a year ago

Right, the constructor arguments are represented by the class type in the getConstructor() method, and the value in-order in the newInstance() method.

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