The Heap

A Simple Class

We will use the simple class below for our examples.

public class simple
{
  private int num;
  public simple(int a) { num = a; }
  public void set(int a) { num = a;}
  public int get() { return num; }
}
  • One variable num
  • One constructor
  • One set method
  • One get method

The Setup

Code Starts


main();
  • Calling a method adds a frame to the stack
java heap example

Creating a primitive


public main()
{
  int i_prim = 3;
}
  • Primitive variables are stored in a frame
java heap example

Creating a object


public main()
{
  int i_prim = 3;
  simple s_obj = new simple(5);
}
  • Objects are stored in the Heap
  • A link to the object is stored in the frame
  • A reference
java heap example

Objects in memory

Objects are stored on the heap. When an object is created two memory locations are set up

  • In the heap the object is created
  • In the current frame the memory address of that object is stored
  • simple q = new simple(9);
  • q stores the memory address, in the heap, of an object of type simple that stores the value 9

Creating an object


public main()
{
  int i_prim = 3;
  simple s_obj = new simple(5);
}
  • Objects are stored in the Heap
  • A link to the object is stored in the frame
  • A reference
java heap example

Changing an object


public main()
{
  int i_prim = 3;
  simple s_obj = new simple(5);
  s_obj.set(10);
}
  • Changes to the object are changes to the heap
  • The frame is unchanged
java heap example

Passing objects as parameters


public main()
{
  int i_prim = 3;
  simple s_obj = new simple(5);
  s_obj.set(10);
  test(i_prim, s_obj);
}
public test(int a, simple b)
{
   a = 6;
   b.set(15);
}
  • Objects can be passed as parameters
java heap example 

public main()
{
  int i_prim = 3;
  simple s_obj = new simple(5);
  s_obj.set(10);
  test(i_prim, s_obj);
}
public test(int a, simple b)
{
   a = 6;
   b.set(15);
}
  • Objects can be passed as parameters
  • Remember though that it is the reference we are passing
  • The link to the heap
java heap example

Primitives are passed by value

so only the local value changes


public main()
{
  int i_prim = 3;
  simple s_obj = new simple(5);
  s_obj.set(10);
  test(i_prim, s_obj);
}
public test(int a, simple b)
{
   a = 6;
   b.set(15);
}
  • Changing the primitive parameter changes only the local variable
java heap example

Objects are passed by reference

so only the local value changes


public main()
{
  int i_prim = 3;
  simple s_obj = new simple(5);
  s_obj.set(10);
  test(i_prim, s_obj);
}
public test(int a, simple b)
{
   a = 6;
   b.set(15);
}
  • The object changes on the heap so the object changes everywhere
java heap example

Objects changes are persistent

They stay changed after the method exits.


public main()
{
  int i_prim = 3;
  simple s_obj = new simple(5);
  s_obj.set(10);
  test(i_prim, s_obj);
}
public test(int a, simple b)
{
   a = 6;
   b.set(15);
}
java heap example

Objects as parameters

Objects themselves are not actually passed as parameters. Instead the memory address of the object is passed. The memory address is passed like a primitive variable.

Object reference is a local primitive variable


public main()
{
  int i_prim = 3;
  simple s_obj = new simple(5);
  s_obj.set(10);
  test(i_prim, s_obj);
}
public test(int a, simple b)
{
   a = 6;
   b.set(15)
   b = new simple(8);
}
java heap example

Object reference is a local primitive variable


public main()
{
  int i_prim = 3;
  simple s_obj = new simple(5);
  s_obj.set(10);
  test(i_prim, s_obj);
}
public test(int a, simple b)
{
   a = 6;
   b.set(15)
   b = new simple(8);
}
  • This creates a new object on the heap
  • It does not change the original object, nor does it affect the original link
java heap example

Object scope


public main()
{
  int i_prim = 3;
  simple s_obj = new simple(5);
  s_obj.set(10);
  test(i_prim, s_obj);
}
public test(int a, simple b)
{
   a = 6;
   b.set(15)
   b = new simple(8);
}
  • When test() exits, the second object is deleted
  • Objects are deleted when they have no handles to them
java heap example

Garbage collection


public main()
{
  int i_prim = 3;
  simple s_obj = new simple(5);
  s_obj.set(10);
  test(i_prim, s_obj);
}
public test(int a, simple b)
{
   a = 6;
   b.set(15)
   b = new simple(8);
}
  • Technically the object still exists but we have no way to access it
  • Java’s garbage collection will eventually notice we have no link and delete the object
java heap example

Garbage collection

Some Time Later


public main()
{
  int i_prim = 3;
  simple s_obj = new simple(5);
  s_obj.set(10);
  test(i_prim, s_obj);
}
public test(int a, simple b)
{
   a = 6;
   b.set(15)
   b = new simple(8);
}
  • Technically the object still exists but we have no way to access it
  • Java’s garbage collection will eventually notice we have no link and delete the object
java heap example