CSC301: Variables and fields [2017/01/17-22]

CSC301: Storage: Variables and fields [2/8]

Storage

Local variable (including method parameters): stored with record for each method call. (In reality, determined by compiler. The compiler freely introduces and removes local variables. They may be stored in register or not stored at all.)
Object variable (field): stored with object.
Class variable (static field): stored with class.

Sharing

Local variable: separate for each method call, even on the same object.
Object variable (field): shared by method calls on the same object.
Class variable (static field): shared by all objects.

Initialization

Local variable: initialized when method called.
Object variable (field): initialized when object is first created.
Class variable (static field): initialized when class is loaded (before any object created).

file:XFields.java [source] [doc-public] [doc-private]

package algs12;

import stdlib.*;

public class XFields {
  public static int classVariable = 42;

  public int objectVariable = classVariable++;

  public void f(int localVariable1) {
    int localVariable2 = StdRandom.uniform (100);
    if (localVariable1 > 0)
      f (localVariable1 - 1);
  }

  public static void main (String[] args) {
    Trace.drawSteps ();
    Trace.run ();

    XFields a = new XFields ();
    XFields b = new XFields ();

    a.f(1);
    b.f(1);
  }

}

Here is the state just before the main function ends:

(This example revealed a bug in my trace software. You can download a corrected copy here: file:Trace.java)