Lectures / lecture3_1
.pdf
References to Different Object Types
The reference |
The object |
type is Shirt. |
type is Shirt. |
Shirt myShirt = new Shirt(); myShirt.display();
Trousers myTrousers = new Trousers(); myTrousers.display();
The reference |
The object type is |
type is Trousers. |
Trousers. |
References and Objects In Memory
int counter = 10;
Shirt myShirt = new Shirt(); Shirt yourShirt = new Shirt();
0x034009
Stack |
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12 |
shirtID |
Heap |
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15.99 |
price |
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B |
colorCode |
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counter |
10 |
0x99f311 |
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myShirt |
0x034009 |
12 |
shirtID |
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15.99 |
price |
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yourShirt |
0x99f311 |
B |
colorCode |
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Assigning a Reference to Another Reference
myShirt = yourShirt;
12
15.99
B
counter |
10 |
0x99f311 |
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myShirt |
0x99f311 |
12 |
shirtID |
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15.99 |
price |
yourShirt |
0x99f311 |
B |
colorCode |
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Two References, One Object
Code fragment:
Shirt myShirt = new Shirt();
Shirt yourShirt = new Shirt();
myShirt = yourShirt;
myShirt.colorCode = 'R'; yourShirt.colorCode = 'G';
System.out.println("Shirt color: " + myShirt.colorCode);
Output from code fragment:
Shirt color: G
Assigning a Reference to Another Reference
myShirt.colorCode = 'R'; yourShirt.colorCode = 'G';
12
15.99
B
counter |
10 |
0x99f311 |
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myShirt |
0x99f311 |
12 |
shirtID |
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15.99 |
price |
yourShirt |
0x99f311 |
G |
colorCode |
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Quiz
Which of the following lines of code instantiates a Boat object and assigns it to a sailBoat object reference?
a.Boat sailBoat = new Boat();
b.Boat sailBoat;
c.Boat = new Boat()
d.Boat sailBoat = Boat();
Java Class Design
Using Access Control
You have seen the keywords public and private. There are four access levels that can be applied to data fields and methods. The following table illustrates access to a field or method marked with the access modifier in the left column.
Modifier |
Same Class |
Same |
Subclass in |
Universe |
(keyword) |
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Package |
Another |
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Package |
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private |
Yes |
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default |
Yes |
Yes |
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protected |
Yes |
Yes |
Yes * |
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public |
Yes |
Yes |
Yes |
Yes |
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Classes can be default (no modifier) or public.
Protected Access Control: Example
1 |
package demo; |
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2 |
public class Foo { |
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3 |
protected int result = 20; |
subclass-friendly |
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int other = 25; |
declaration |
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5 |
} |
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1package test;
2import demo.Foo;
3public class Bar extends Foo {
4private int sum = 10;
5public void reportSum () {
6 |
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sum += result; |
7 |
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sum += other; |
8 |
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} |
compiler error |
9 |
} |
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Field Shadowing: Example
1package demo;
2public class Foo2 {
3protected int result = 20;
4}
1package test;
2import demo.Foo2;
3public class Bar2 extends Foo2 {
4private int sum = 10;
5private int result = 30;
6public void reportSum() {
7 sum += result;
8}
9}
result field shadows the parent's field.