Java - Use Return Value as Parameter in another Method - Memory Management

Question

Considering internal memory usage

input = web_service.FullCompanyListChunksGet(x, ChunkSize);
ReadXML(input);

should take the same memory as

ReadXML(web_service.FullCompanyListChunksGet(x, ChunkSize));

Doesn't it? Do both samples pass just the reference to the ReadXML Method?

Please help me understanding the difference. Thanks

Answer 1

I'd expect the JITted code to be pretty much the same. In the bytecode there's an extra store/load for the first form, but I'd be very surprised to see that make a real difference. The advantage of the first form is that it can make debugging easier - but it makes the code more verbose at the same time.

EDIT: I originally wrote that the two create the same bytecode, but that's not true:

public class Test {
  static void separate() {
    String x = "hello";
    String y = x.toString();
    foo(y);
  }

  static void combined() {
    String x = "hello";
    foo(x.toString());
  }

  static void foo(String x) {
  }
}

Compiles to (just the separate and combined methods):

static void combined();
  Code:
   0:   ldc #2; //String hello
   2:   astore_0
   3:   aload_0
   4:   invokevirtual #3; //Method java/lang/String.toString:()Ljava/lang/String;
   7:   invokestatic #4; //Method foo:(Ljava/lang/String;)V
   10:  return

static void separate();
  Code:
   0:   ldc #2; //String hello
   2:   astore_0
   3:   aload_0
   4:   invokevirtual #3; //Method java/lang/String.toString:()Ljava/lang/String;
   7:   astore_1
   8:   aload_1
   9:   invokestatic #4; //Method foo:(Ljava/lang/String;)V

Answer 2

In your first example

input = web_service.FullCompanyListChunksGet(x, ChunkSize);
ReadXML(input);

the variable input remains in scope until the end of the block that contains this code. It's not eligible for GC until the exit of that block. In this example

ReadXML(web_service.FullCompanyListChunksGet(x, ChunkSize));

the variable goes out of scope at the function return, so is eligible for garbage collection earlier.