The Cygnus Native Interface for C++/Java Integration
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Arrays

While in many ways Java is similar to C and C++, it is quite different in its treatment of arrays. C arrays are based on the idea of pointer arithmetic, which would be incompatible with Java's security requirements. Java arrays are true objects (array types inherit from java.lang.Object). An array-valued variable is one that contains a reference (pointer) to an array object.

Referencing a Java array in C++ code is done using the JArray template, which as defined as follows: 

class __JArray : public java::lang::Object
{
public:
  int length;
};

template<class T>
class JArray : public __JArray
{
  T data[0];
public:
  T& operator[](jint i) { return data[i]; }
};

template<class T> T *elements(JArray<T> &array);

This template function can be used to get a pointer to the elements of the array. For instance, you can fetch a pointer to the integers that make up an int[] like so: 
extern jintArray foo;
jint *intp = elements (foo);
The name of this function may change in the future.

There are a number of typedefs which correspond to typedefs from JNI. Each is the type of an array holding objects of the appropriate type: 

typedef __JArray *jarray;
typedef JArray<jobject> *jobjectArray;
typedef JArray<jboolean> *jbooleanArray;
typedef JArray<jbyte> *jbyteArray;
typedef JArray<jchar> *jcharArray;
typedef JArray<jshort> *jshortArray;
typedef JArray<jint> *jintArray;
typedef JArray<jlong> *jlongArray;
typedef JArray<jfloat> *jfloatArray;
typedef JArray<jdouble> *jdoubleArray;

You can create an array of objects using this function:

jobjectArray JvNewObjectArray(jint length, jclass klass, jobject init);

Here klass is the type of elements of the array; init is the initial value to be put into every slot in the array.

For each primitive type there is a function which can be used to create a new array holding that type. The name of the function is of the form `JvNew<Type>Array', where `<Type>' is the name of the primitive type, with its initial letter in upper-case. For instance, `JvNewBooleanArray' can be used to create a new array of booleans. Each such function follows this example:

jbooleanArray JvNewBooleanArray(jint length);

jsize JvGetArrayLength(jarray array);

Returns the length of array.

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