Pointer in Java C/C++ conversion

Question

I'm trying to convert library from C++ to Java. I have a problem with pointer..

The code from C++ is:

int DECL2 daveSetBit(daveConnection * dc,int area, int DB, int byteAdr, int bitAdr) {
    int a=1;
    return daveWriteBits(dc, area, DB, 8*byteAdr+bitAdr, 1, &a);
}

and daveWriteBits declaration is:

 daveWriteBits(daveConnection * dc,int area, int DB, int start, int len, void * buffer)

How can I convert this into java? I don't know how to convert pointer &a into java language..

public int SetBits(
        int area,
        int DBnum,
        int byteAdr,
        int bitAdr) {       
    int a=1;        
    return writeBits(area, DBnum, 8*byteAdr+bitAdr, 1, ??? )            
}

UPDATE:

I forgot to add part of WriteBits function in Java..

public int writeBits(
        int area,
        int DBnum,
        int start,
        int len,
        byte[] buffer) {
        int errorState = 0;
        semaphore.enter();
        PDU p1 = new PDU(msgOut, PDUstartOut);

        p1.prepareWriteRequest();
        p1.addBitVarToWriteRequest(area, DBnum, start, len, buffer);

        errorState = exchange(p1);

        if (errorState == 0) {
            PDU p2 = new PDU(msgIn, PDUstartIn);
            p2.setupReceivedPDU();

            if (p2.mem[p2.param + 0] == PDU.FUNC_WRITE) {
                if (p2.mem[p2.data + 0] == (byte) 0xFF) {
                    if ((Nodave.Debug & Nodave.DEBUG_CONN) != 0)
                        System.out.println("writeBytes: success");
                    semaphore.leave();
                    return 0;
                }
            } else {
                errorState |= 4096;
            }
        }
        semaphore.leave();
        return errorState;
    }

... or you can just use the JNI home.pacifier.com/~mmead/jni/cs510ajp/index.html — user827992
– user827992, Commented Aug 9, 2012 at 14:36

Peter Lawrey · Accepted Answer · 2012-08-09 14:32:35Z

1

Since the variable is ignored I would delete it.

However if it were not ignored you could sue

int[] a = { 1 };
writeBits(area, DBnum, 8*byteAdr+bitAdr, 1, a )  ;
// do something with a[0];

answered Aug 9, 2012 at 14:32

Peter Lawrey

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Comments

Gir · Accepted Answer · 2012-08-09 14:34:14Z

0

Java doesn't have pointers. It has references:

c++:

Object *a=new Object();

java:

Object a=new Object();

answered Aug 9, 2012 at 14:34

Gir

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Comments

Nivas · Accepted Answer · 2012-08-09 14:34:37Z

0

It depends on how daveWriteBits uses buffer. If it is just treated as a simple integer, and if daveSetBit does not use the value of a after this call, just make it an int. No pointers/references.

If daveSetBit needs the value of a (after any potential modifications in daveWriteBits, then making a an Integer may help.

If there is some really fancy pointer stuff going on (treating &a itself as an address etc) then you may have to do a bit more design.

answered Aug 9, 2012 at 14:34

Nivas

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Comments

john · Accepted Answer · 2012-08-09 14:35:06Z

0

Are you more familar with C++ or Java? The C++ code is treating an integer as an array of bytes. You can't do that directly in Java (I think) but you can do this

ByteArrayOutputStream buf = new ByteArrayOutputStream();
new DataOutputStream(buf).write(a);
return writeBits(area, DBnum, 8*byteAdr+bitAdr, 1, buf );

Converts an integer to an array of bytes which you can then pass to writeBits. Untested code.

answered Aug 9, 2012 at 14:35

john

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3 Comments

obataku Over a year ago

probably better to manually split the integer than to use DataOutputStream... that or use ByteBuffer the whole time.

john Over a year ago

@veer: Sure, the important point is that the equivalent in Java is a conversion to byte[]

obataku Over a year ago

@Elfoc new byte[] { 0, 0, 0, 1 } assuming it expects it big-endian like Java is

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