1. Methods Again Parameter Passage
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2. The Handle Scheme Objects are referred to by a handle
They are not valid until a value is assigned to them
Either from
An existing object
A use of new
The use of an existing object makes another handle that points at the same object
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3. Recall Triangle The last version of the triangle had two turtles
They moved independently
Consider the next screens
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4. Example static void triangle( Turtle t, int len){ t.forward(len);
t.turn(120); }
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5. Called by public static void main( String [] a){
World w = new World(400,240);
Turtle yertle = new Turtle(w);
yertle.setPenWidth(3);
triangle(yertle,80);
Turtle sam=new Turtle(100,100,w);
sam.setPenWidth(4);
triangle(sam,40); }
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6. Gives Results
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7. Independence
The two turtles move independently since they are two objects
We will get a different result if we assign one to the other instead of creating a second
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8. Called by public static void main( String [] a){
World w = new World(400,240);
Turtle yertle = new Turtle(w);
yertle.setPenWidth(3);
triangle(yertle,80);
Turtle sam = yertle;
sam.setPenWidth(4);
triangle(sam,40); }
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9. Gives Results
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10. What was changed?
The middle statement Turtle sam=new Turtle(100,100,w);
Was changed to Turtle sam = yertle;
Instead of sam being independent, a new turtle, it is a handle to the same old turtle
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11. The difference Turtle yertle = new Turtle(w); Turtle instance yertle
Turtle sam = new Turtle(100,100,w);
sam
Turtle instance
Turtle yertle = new Turtle(w);
Turtle instance
yertle
Turtle sam = yertle;
sam
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12. Another Example public static void main( String [] a){
World w = new World(400,240);
Turtle yertle = new Turtle(w);
yertle.setPenWidth(3);
Turtle sam = yertle;
sam.forward(50);
yertle.turn(90);
sam.turn(45);
yertle.forward(90); }
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13. Results
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14. Commentary It is just one turtle in this case
yertle and sam are synonyms or aliases for exactly the same object
Any method on sam affects the common object which yertle also points at
Once we understand this we can understand parameter passage
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15. Parameter Passage Mechanisms
How are parameters passed in Java?
In general there are several ways to do this
Java always passes parameters by value
This just means a copy is made and given to the method
This has a different effect for primitives and objects
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16. Passing Primitives
Call by value is the way a primitive is passed to a method
A copy of the primitive is made and given to the method
If the method changes the primitive, the change is made to the copy, not to the original
Thus the actual parameter is never changed
Consider triangle again
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17. Triangle Changed static void triangle( Turtle t, int len){
t.forward(len);
t.turn(120);
len = 10; // Set len }
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18. Called by public static void main(String [] a){
World w = new World(400,240);
Turtle yertle = new Turtle(w);
yertle.setPenWidth(3);
Turtle sam = new Turtle(100,100,w);
sam.setPenWidth(4);
int dist = 80;
triangle(yertle,dist);
triangle(sam,dist);
System.out.println(dist); }
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19. Gives Results
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20. Commentary
Even though triangle changed the value of its parameter it had no effect on the outside
It changed a copy, rather than the original
It works a little different with objects
As we have and will see
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21. Passing Objects
When an object is passed as a parameter a copy of the handle is passed
As we saw with assignment we then have two handles pointing at the same object
These are synonyms or aliases
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22. Triangle revisited
When the triangle method was called it moved the turtle and changed its direction
The copy of the handle still pointed at the same object
Unlike passing a primitive (like int) we can actually change the object
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23. Finally
The net effect is that we can change the characteristics of a passed object
In the turtle: direction, position etc.
A copy of a handle still points at the object
A primitive cannot be changed in any significant way
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