Question

maybe it’s dumb but is there a difference between

new Something().method();

and

Something tmpSomething = new Something();
tmpSomething.method(); 

as I was calling only one method on this particular object, I chose the first solution but I’m not sure that this is exactly the same behavior…

I just want to mention the constructor initializes a Writer and method writes in this file…

Answer 1

I did a quick test. This is the extremely trivial test code:

public class TestLiveness
{
    public static void test1()
    {
        System.out.println(new Square(4).square());
        count();
    }

    public static void test2()
    {
        Square t = new Square(4);
        System.out.println(t.square());
        count();
    }

    private static void count()
    {
        for(int i=0; i<1000000; i++)
            System.out.println(i);
    }

    static class Square
    {
        private int val;

        Square(int val)
        {
            this.val = val;
        }

        int square()
        {
            return val * val;
        }
    }
}

Javap shows that the two methods are compiled differently; chaining doesn’t touch the local variable table whereas the temporary variable does indeed stick around until the method returns. However, the VM/JIT compiler may still perform liveness analysis and allow the instance to be garbage collected before the method returns.

public static void test1();
  Code:
   0:   getstatic       #2; //Field java/lang/System.out:Ljava/io/PrintStream;
   3:   new     #3; //class TestLiveness$Square
   6:   dup
   7:   iconst_4
   8:   invokespecial   #4; //Method TestLiveness$Square."<init>":(I)V
   11:  invokevirtual   #5; //Method TestLiveness$Square.square:()I
   14:  invokevirtual   #6; //Method java/io/PrintStream.println:(I)V
   17:  invokestatic    #7; //Method count:()V
   20:  return

public static void test2();
  Code:
   0:   new     #3; //class TestLiveness$Square
   3:   dup
   4:   iconst_4
   5:   invokespecial   #4; //Method TestLiveness$Square."<init>":(I)V
   8:   astore_0
   9:   getstatic       #2; //Field java/lang/System.out:Ljava/io/PrintStream;
   12:  aload_0
   13:  invokevirtual   #5; //Method TestLiveness$Square.square:()I
   16:  invokevirtual   #6; //Method java/io/PrintStream.println:(I)V
   19:  invokestatic    #7; //Method count:()V
   22:  return