AP CSA 3.5 Exercise 1: Five Headers, Five Bodies

Unit 3: Class Creation · Lesson 3.5 · Exercise 1

Five Headers, Five Bodies

Methods: How to Write Them. Write real Java, run it, and submit it to be graded against hidden test cases.

Back to the Lesson 3.5 page All Unit 3 lessons

Why this one is worth doing

Return type, name, parameters, body. Four of these methods hand something back and one hands back nothing at all, and getting the return type wrong is a compile error rather than a wrong answer.

What to write

Write a class named Toolkit with the six methods listed above.
triple returns three times n. isEven returns whether n divides evenly by 2.
average returns the exact average of the two ints as a double, so 3 and 4 give 3.5 and not 3.
repeat returns s joined to itself times times. Zero times gives the empty String.
remember stores n and returns nothing. getLast returns the last value remembered, or 0 before anything has been.

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 Toolkit
int triple(int n)
double average(int a, int b)
boolean isEven(int n)
String repeat(String s, int times)
void remember(int n)
int getLast()

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

3 4 ab 3

Example 1 output

0
9
3.5
false
true
ababab
7
12

Example 2 input

10 10 x 1

Example 2 output

0
30
10.0
true
true
x
20
30

Your answer

Main.java

class Toolkit {
  // remember needs somewhere to put the value.

  public int triple(int n) {
    return 0;
  }

  public double average(int a, int b) {
    return 0.0;
  }

  public boolean isEven(int n) {
    return false;
  }

  public String repeat(String s, int times) {
    return "";
  }

  public void remember(int n) {

  }

  public int getLast() {
    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) {
    Toolkit t = new Toolkit();
    System.out.println(t.triple(5));
    System.out.println(t.average(3, 4));
    System.out.println(t.repeat("ab", 3));
  }
}

Stuck?

Hint 1

(a + b) / 2 divides two ints and loses the half. Dividing by 2.0 makes it a double division.

Hint 2

A void method has no return statement with a value after it. It changes something instead of handing something back.

Hint 3

repeat with times of 0 returns the empty String, so start your result at "" and let the loop not run.

Where to go next

Rework the Lesson 3.5 page Pick another Unit 3 lesson

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