Is it possible to use struct-like constructs in Java?

Question

I'm considering using Java for a large project but I haven't been able to find anything that remotely represented structures in Java. I need to be able to convert network packets to structures/classes that can be used in the application.

I know that it is possible to use RandomAccessFile but this way is NOT acceptable. So I'm curious if it is possible to "cast" a set of bytes to a structure like I could do in C. If this is not possible then I cannot use Java.

So the question I'm asking is if it is possible to cast aligned data to a class without any extra effort beyond specifying the alignment and data types?

dacwe · Accepted Answer · 2011-08-01 06:26:54Z

17

No. You cannot cast a array of bytes to a class object.

That being said, you can use a java.nio.Buffer and easily extract the fields you need to an object like this:

class Packet {

    private final int type;
    private final float data1;
    private final short data2;

    public Packet(byte[] bytes) {
        ByteBuffer bb = ByteBuffer.wrap(bytes);
        bb.order(ByteOrder.BIG_ENDIAN); // or LITTLE_ENDIAN

        type = bb.getInt();
        data1 = bb.getFloat();
        data2 = bb.getShort();
    }
}

edited Aug 1, 2011 at 6:26

answered Jul 31, 2011 at 19:13

dacwe

43.6k12 gold badges118 silver badges142 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Confusion · Accepted Answer · 2011-07-31 19:18:56Z

9

You're basically asking whether you can use a C-specific solution to a problem in another language. The answer is, predictably, 'no'.

However, it is perfectly possible to construct a class that takes a set of bytes in its constructor and constructs an appropriate instance.

class Foo {

  int someField;
  String anotherField;

  public Foo(byte[] bytes) {
    someField = someFieldFromBytes(bytes);
    anotherField = anotherFieldFromBytes(bytes);
    etc.
 }
}

You can ensure there is a one-to-one mapping of class instances to byte arrays. Add a toBytes() method to serialize an instance into bytes.

answered Jul 31, 2011 at 19:18

Confusion

17k8 gold badges51 silver badges73 bronze badges

Comments

Cameron Skinner · Accepted Answer · 2011-07-31 19:19:27Z

No, you cannot do that. Java simply doesn't have the same concepts as C.

You can create a class that behaves much like a struct:

public class Structure {
    public int field1;
    public String field2;
}

and you can have a constructor that takes an array or bytes or a DataInput to read the bytes:

public class Structure {
    ...
    public Structure(byte[] data) {
        this(new DataInputStream(new ByteArrayInputStream(data)));
    }

    public Structure(DataInput in) {
        field1 = in.readInt();
        field2 = in.readUTF();
    }
}

then read bytes off the wire and pump them into Structures:

byte[] bytes = network.read();
DataInputStream stream = new DataInputStream(new ByteArrayInputStream(bytes));
Structure structure1 = new Structure(stream);
Structure structure2 = new Structure(stream);
...

It's not as concise as C but it's pretty close. Note that the DataInput interface cleanly removes any mucking around with endianness on your behalf, so that's definitely a benefit over C.

Ray Toal · Accepted Answer · 2011-07-31 19:20:05Z

2

As Joshua says, serialization is the typical way to do these kinds of things. However you there are other binary protocols like MessagePack, ProtocolBuffers, and AvRO.

If you want to play with the bytecode structures, look at ASM and CGLIB; these are very common in Java applications.

answered Jul 31, 2011 at 19:20

Ray Toal

88.7k20 gold badges186 silver badges245 bronze badges

Comments

Joshua Kaplan · Accepted Answer · 2011-07-31 19:13:29Z

1

There is nothing which matches your description.

The closest thing to a struct in Java is a simple class which holds values either accessible through it's fields or set/get methods.

The typical means to convert between Java class instances and on-the-wire representations is Java serialization which can be heavily customized as need be. It is what is used by Java's Remote Method Invocation API and works extremely well.

answered Jul 31, 2011 at 19:13

Joshua Kaplan

8935 silver badges9 bronze badges

3 Comments

krb Over a year ago

What do most Java developers use to cast bytes to classes?

Joshua Kaplan Over a year ago

I think you are asking to an instance of a class. In that a case you can use an ObjectInputStream.

Perception Over a year ago

You never cast bytes to classes. But you can map bytes to classes in an implementation specific manner. Which is what @Joshua Kaplan is referring to in his response. Please note that you will have to be careful with encoding, and also, Java is much more strongly typed than C, so things like uint16_t, size_t etc do not map over.

Vlad · Accepted Answer · 2011-07-31 19:20:43Z

1

ByteBuffer.wrap(new byte[] {}).getDouble();

answered Jul 31, 2011 at 19:20

Vlad

10.8k3 gold badges40 silver badges38 bronze badges

Comments

Mike Baranczak · Accepted Answer · 2011-07-31 19:20:12Z

0

No, this is not possible. You're trying to use Java like C, which is bound to cause complications. Either learn to do things the Java way, or go back to C.

In this case, the Java way would probably involve DataInputStream and/or DataOutputStream.

answered Jul 31, 2011 at 19:20

Mike Baranczak

8,4178 gold badges52 silver badges73 bronze badges

Comments

AlexR · Accepted Answer · 2011-07-31 19:23:33Z

-1

You cannot cast array of bytes to instance of class. But you can do much much more with java. Java has internal, very strong and very flexible mechanism of serialization. This is what you need. You can read and write object to/from stream.

If both sides are written in java, there are no problem at all. If one of sides is not java you can customeze your serialization. Start from reading javadoc of java.util.Serializable.

answered Jul 31, 2011 at 19:23

AlexR

116k16 gold badges137 silver badges216 bronze badges

1 Comment

Aleksandr Kravets Over a year ago

Java's serialization is considered dangerous and should not be used in any new project according to Josh Block's Effective Java 3rd edition

Collectives™ on Stack Overflow

Is it possible to use struct-like constructs in Java?

8 Answers 8

Comments

Comments

Comments

Comments

3 Comments

Comments

Comments

1 Comment

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

8 Answers 8

Comments

Comments

Comments

Comments

3 Comments

Comments

Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Linked

Related