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Java Java Objects Meet Objects Final

Sean Loke
Sean Loke
14,659 Points

SwapHead by Chris - do not understand it

class PezDispenser { private String characterName;

public PezDispenser(String characterName) { this.characterName = characterName; }

public String getCharacterName() { return characterName; }

public String swapHead(String characterName) { String originalCharacterName = this.characterName; this.characterName = characterName; return originalCharacterName; } }

public class Example {

public static void main(String[] args) {

System.out.println("We are making a new PEZ Dispenser"); PezDispenser dispenser = new PezDispenser("Donatello"); System.out.printf("The dispenser is %s %n", dispenser.getCharacterName() ); String before = dispenser.swapHead("Darth Vader"); System.out.printf("It was %s but Chris switched it to %s %n", before, dispenser.getCharacterName()); }

}

Anyone can help me break down step-by-step the swapHead part by chris? Thanks!

2 Answers

andren
andren
28,520 Points

Sure

String originalCharacterName = this.characterName;

This creates a new variable called originalCharacterName that stores what is currently stored in the characterName field/member variable ("Donatello" in the example above) that is defined at the top of the class. The this keyword has to be used to help Java distinguish the field/member variable characterName from the characterName parameter that is passed in to the function.

this.characterName = characterName;

This sets the field/member variable characterName to the value of the characterName parameter, the characterName parameter will contain whatever string you passed in as the first argument to the method. Which is "Darth Vader" in the example above.

return originalCharacterName; 

This simply returns the value stored in the originalCharacterName variable that was created earlier.

So to summarize, the method creates a variable to hold the current character name, then it changes the character name to one you supplied to the method, then it returns the old name.

If you have any further questions or feel confused by a part of my explanation then feel free to reply to this post with your questions, I'll answer anything I can.

Sean Loke
Sean Loke
14,659 Points

Thank you! Could you explain this bit as well in the example.java file?

String before = dispenser.swapHead("Darth Vader"); System.out.printf("It was %s but Chris switched it to %s %n", before, dispenser.getCharacterName()); }

andren
andren
28,520 Points
String before = dispenser.swapHead("Darth Vader"); 

This line calls the dispenser.swapHead method and then stores the value it returns into the before variable. Since the dispenser.swapHead returns the name the dispenser had before it was swapped you end up with the before variable containing the old name of the dispenser.

So the line both runs dispenser.swapHead (which means that the characterName within the dispenser changes) and assigns the value it returns to the before variable.

System.out.printf("It was %s but Chris switched it to %s %n", before, dispenser.getCharacterName()); }

This prints out a formatted string. The %s symbols are replaced by the string values you provide to the printf method, in this case the first %s is replaced by the contents of the before variable and the second %s is replaced by the value that dispenser.getCharacterName() returns.

So you end up with the first %s being replaced by the old dispenser name since that was stored in the before variable on the line above, and the second %s ends up containing the current dispenser name since that is what the dispenser.getCharacterName() returns.

Cassi Emerson
Cassi Emerson
1,910 Points

Hi Andren - is it because String before = dispenser.swapHead("Darth Vader"); is creating a new instance of the variable dispenser?

andren
andren
28,520 Points

No, that line does not create a new instance. All it does is run the swapHead method within the dispenser object and then assign the result to the before variable. It does not do anything beyond that.

The swapHead method changes the characterName field on the dispenser to whatever name you pass into it and then returns the old name. That means that dispenser.characterName is now equal to whatever you passed in to the swapHead method, and before is equal to whatever the characterName was before the name change.