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Understanding Java Class Hierarchies with Static Variables

This example demonstrates how to work with Java class hierarchies, particularly focusing on the handling of static variables. The code consists of two main classes, X and Z, where Z extends X. The program illustrates how to instantiate objects from both classes, manipulate data, and incorporate instance variables. By examining the relationships between the classes and their properties, you can gain insights into object-oriented programming principles in Java, emphasizing memory management and static context.

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Understanding Java Class Hierarchies with Static Variables

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  1. Start 14 7 12 5 a a Example with Static Variable 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 mvY Y.value

  2. Initial Example with Static Variable - 1 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. One 7 5 a Example with Static Variable - 2 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 • 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. Two 14 7 12 5 a a Example with Static Variable 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 Comments: The line just executed accomplished the following – all the code shown in red is involved As before for mvY mvY Y.value

  5. Three 14 7 12 5 a a Example with Static Variable 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 constructorpublic 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 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

  6. Four 14 7 12 5 a a Example with Static Variable 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 getValuepublic String toString () { return "From Y: " + value; } // end toString} // end class Y mvZ X.data 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

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