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A few document clarifications for Java & Internals.

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Change-Id: I770b53cf7d82c860422c1fe6193fb597d9c9495c
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Wouter van Oortmerssen
2014-07-31 11:39:40 -07:00
parent 8f80fecc44
commit d8a173ddc7
5 changed files with 27 additions and 14 deletions
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22
docs/source/JavaUsage.md
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@@ -1,11 +1,11 @@
# Use in Java
There's experimental support for reading FlatBuffers in Java. Generate code
FlatBuffers supports reading and writing binary FlatBuffers in Java. Generate code
for Java with the `-j` option to `flatc`.
See `javaTest.java` for an example. Essentially, you read a FlatBuffer binary
file into a `byte[]`, which you then turn into a `ByteBuffer`, which you pass to
the `getRootAsMonster` function:
the `getRootAsMyRootType` function:
ByteBuffer bb = ByteBuffer.wrap(data);
Monster monster = Monster.getRootAsMonster(bb);
@@ -33,9 +33,9 @@ since FlatBuffer's UTF-8 strings can't be read in-place by Java.
Vector access is also a bit different from C++: you pass an extra index
to the vector field accessor. Then a second method with the same name
suffixed by `_length` let's you know the number of elements you can access:
suffixed by `Length` let's you know the number of elements you can access:
for (int i = 0; i < monster.inventory_length(); i++)
for (int i = 0; i < monster.inventoryLength(); i++)
monster.inventory(i); // do something here
You can also construct these buffers in Java using the static methods found
@@ -61,11 +61,19 @@ Create a table with a struct contained therein:
As you can see, the Java code for tables does not use a convenient
`createMonster` call like the C++ code. This is to create the buffer without
using temporary object allocation (since the `Vec3` is an inline component of
`Monster`, it has to be created right where it is added, whereas the name and
the inventory are not inline).
using temporary object allocation.
It's important to understand that fields that are structs are inline (like
`Vec3` above), and MUST thus be created between the start and end calls of
a table. Everything else (other tables, strings, vectors) MUST be created
before the start of the table they are referenced in.
Structs do have convenient methods that even have arguments for nested structs.
As you can see, references to other objects (e.g. the string above) are simple
ints, and thus do not have the type-safety of the Offset type in C++. Extra
case must thus be taken that you set the right offset on the right field.
Vectors also use this start/end pattern to allow vectors of both scalar types
and structs: