1. Example with Static Variable
Start
class X { static int count = 0;
private int data; public X(int n) { data = n; } // end int constructor public int getData() { return data; } // end getData } // end class X
public class Z extends X { private Y a = new Y(3); public Z(int x, int y) { super(x); a.setValue(y); } // end int, int constructor public String toString () { return "From Z: " + a.getValue(); } // end toString public static void main(String[] args) { Z myZ = new Z (5, 7); Z myY = new Z (12, 14); System.out.println("value myZ: " + myZ); System.out.println("value myY: " + myY.a); } // end main } // end class Z
Class Hierarchy – in memory
Object X
count Z Y
Memory Map – instance vars
class Y { private int value; public Y(int n) { value = n; } // end int constructor public void setValue(int z) {
value = z; } // end method getValue
public int getValue() {
return value; } // end method getValue public String toString () { return "From Y: " + value; } // end toString } // end class Y
mvZ
X.data 5 a 7
mvY
Y.value 12 a 14

2. Example with Static Variable - 1
Initial
class X { static int count = 0;
private int data; public X(int n) { data = n; } // end int constructor public int getData() { return data; } // end getData } // end class X
public class Z extends X { private Y a = new Y(3); public Z(int x, int y) { super(x); a.setValue(y); } // end int, int constructor public String toString () { return "From Z: " + a.getValue(); } // end toString public static void main(String[] args) { Z myZ = new Z (5, 7); Z myY = new Z (12, 14); System.out.println("value myZ: " + myZ); System.out.println("value myY: " + myY.a); } // end main } // end class Z
Class Hierarchy – in memory
Object X
count Z Y
Memory Map – instance vars
class Y { private int value; public Y(int n) { value = n; } // end int constructor public void setValue(int z) {
value = z; } // end method getValue
public int getValue() {
return value; } // end method getValue public String toString () { return "From Y: " + value; } // end toString } // end class Y
Comments:
Before the program starts, but after it’s loaded into memory, the class hierarchy is loaded and the static variables are instantiated as part of the class load.

3. Example with Static Variable - 2
One
class X { static int count = 0;
private int data; public X(int n) { data = n; } // end int constructor public int getData() { return data; } // end getData } // end class X
public class Z extends X { private Y a = new Y(3); public Z(int x, int y) { super(x); a.setValue(y); } // end int, int constructor public String toString () { return "From Z: " + a.getValue(); } // end toString public static void main(String[] args) { Z myZ = new Z (5, 7); Z myY = new Z (12, 14); System.out.println("value myZ: " + myZ); System.out.println("value myY: " + myY.a); } // end main } // end class Z
Class Hierarchy – in memory
Object X
count Z Y
Memory Map – instance vars
class Y { private int value; public Y(int n) { value = n; } // end int constructor public void setValue(int z) {
value = z; } // end method getValue
public int getValue() {
return value; } // end method getValue public String toString () { return "From Y: " + value; } // end toString } // end class Y
mvZ
X.data 5 a 7
Comments:
The line just executed accomplished the following – all the code shown in red is involved:
Reserved memory for the mvZ handle
Reserved memory for a Z instance
Reserved memory for a Y instance
Inited X.data to 5 in this instance
Inited Y.value to 7 for this Y instance
Returned handle to new Y instance, setting value Z.a
Same for Z and mvZ
Y.value

4. Example with Static Variable
Two
class X { static int count = 0;
private int data; public X(int n) { data = n; } // end int constructor public int getData() { return data; } // end getData } // end class X
public class Z extends X { private Y a = new Y(3); public Z(int x, int y) { super(x); a.setValue(y); } // end int, int constructor public String toString () { return "From Z: " + a.getValue(); } // end toString public static void main(String[] args) { Z myZ = new Z (5, 7); Z myY = new Z (12, 14); System.out.println("value myZ: " + myZ); System.out.println("value myY: " + myY.a); } // end main } // end class Z
Class Hierarchy – in memory
Object X
count Z Y
Memory Map – instance vars
class Y { private int value; public Y(int n) { value = n; } // end int constructor public void setValue(int z) {
value = z; } // end method getValue
public int getValue() {
return value; } // end method getValue public String toString () { return "From Y: " + value; } // end toString } // end class Y
mvZ
X.data 5 a 7
Comments:
The line just executed accomplished the following – all the code shown in red is involved
As before for mvY
mvY
Y.value 12 a 14

5. Example with Static Variable
Three
class X { static int count = 0;
private int data; public X(int n) { data = n; } // end int constructor public int getData() { return data; } // end getData } // end class X
public class Z extends X { private Y a = new Y(3); public Z(int x, int y) { super(x); a.setValue(y); } // end int, int constructor public String toString () { return "From Z: " + a.getValue(); } // end toString public static void main(String[] args) { Z myZ = new Z (5, 7); Z myY = new Z (12, 14); System.out.println("value myZ: " + myZ); System.out.println("value myY: " + myY.a); } // end main } // end class Z
Class Hierarchy – in memory
Object X
count Z Y
Memory Map – instance vars
class Y { private int value; public Y(int n) { value = n; } // end int constructor public void setValue(int z) {
value = z; } // end method getValue
public int getValue() {
return value; } // end method getValue public String toString () { return "From Y: " + value; } // end toString } // end class Y
mvZ
X.data 5 a 7
Comments:
The line just executed accomplished the following – all the code shown in red is involved
This line of code uses toString from Z, which then calls getValue in Y
mvY
Y.value 12 a 14

6. Example with Static Variable
Four
class X { static int count = 0;
private int data; public X(int n) { data = n; } // end int constructor public int getData() { return data; } // end getData } // end class X
public class Z extends X { private Y a = new Y(3); public Z(int x, int y) { super(x); a.setValue(y); } // end int, int constructor public String toString () { return "From Z: " + a.getValue(); } // end toString public static void main(String[] args) { Z myZ = new Z (5, 7); Z myY = new Z (12, 14); System.out.println("value myZ: " + myZ); System.out.println("value myY: " + myY.a); } // end main } // end class Z
Class Hierarchy – in memory
Object X
count Z Y
Memory Map – instance vars
class Y { private int value; public Y(int n) { value = n; } // end int constructor public void setValue(int z) {
value = z; } // end method getValue
public int getValue() {
return value; } // end method getValue public String toString () { return "From Y: " + value; } // end toString } // end class Y
mvZ
X.data 5 a 7
Comments:
The line just executed accomplished the following – all the code shown in red is involved
Here the call is to the toString method in Y
mvY
Y.value 12 a 14