AP CSA 3.8 Exercise 1: Same Name, Different Variable

Unit 3: Class Creation · Lesson 3.8 · Exercise 1

Same Name, Different Variable

Scope and Access. Write real Java, run it, and submit it to be graded against hidden test cases.

Back to the Lesson 3.8 page All Unit 3 lessons

Why this one is worth doing

A local variable with the same name as an instance variable does not touch it. This class has both, on purpose, and the only way to pass is to know which one each line is talking about.

What to write

Write a class named Vault with a private instance variable named balance and a private count of deposits.
previewDeposit must declare a LOCAL variable also named balance, set it to the instance balance plus the amount, and return it. It must not change the instance variable or the count.
deposit adds the amount to the INSTANCE balance and increases the deposit count by one.
getBalance returns the instance balance and getDepositCount returns the count.
The grader calls previewDeposit and then checks that nothing changed.

Your program reads

The grader supplies the input. Your class does not read anything.

Your program prints

Your class prints nothing. The grader prints what your methods return.

The class the grader will call

Your answer must declare exactly these, spelled exactly like this. The grader creates the object and calls the methods itself, so a method under a different name is a method it cannot find.

class Vault
Vault(int balance)
int getBalance()
int previewDeposit(int amount)
void deposit(int amount)
int getDepositCount()

Do not write a class named Main and do not write a main method. The grader supplies its own.

Worked examples

These are the cases you can see. There are more you cannot, and they use different values, so an answer that prints these numbers as constants will fail.

Example 1 input

100 50 25

Example 1 output

Example 2 input

0 0 0

Example 2 output

Your answer

Main.java

class Vault {
  private int balance;
  private int depositCount;

  public Vault(int balance) {
    this.balance = balance;
  }

  public int getBalance() {
    return balance;
  }

  public int previewDeposit(int amount) {
    // Declare a local int balance here. Do not change the field.
    return 0;
  }

  public void deposit(int amount) {

  }

  public int getDepositCount() {
    return 0;
  }
}

Input for the Run button

Run uses the sample main shown below, which is not the one that grades you. Submitting runs your class against the real cases, including hidden ones.

The sample main the Run button uses

public class Main {
  public static void main(String[] args) {
    Vault v = new Vault(100);
    System.out.println(v.previewDeposit(50));
    System.out.println(v.getBalance());
    v.deposit(50);
    System.out.println(v.getBalance());
  }
}

Stuck?

Hint 1

Once a local named balance exists, the bare word balance means the local for the rest of that method. this.balance is the only way back to the field.

Hint 2

previewDeposit changes nothing, so calling it twice in a row must give the same answer twice.

Hint 3

depositCount goes up in deposit only. If your preview touches it, the third printed line will be wrong.

Where to go next

Rework the Lesson 3.8 page Pick another Unit 3 lesson

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