AP CSA 2.6 Debugging: Equal Objects, Different Boxes

Unit 2: Selection and Iteration · Lesson 2.6 · Debugging

Equal Objects, Different Boxes

Comparing Boolean Expressions. This is not a blank editor: it is someone else's attempt. Find what is wrong, fix it, and submit to be graded against hidden test cases.

Back to the Lesson 2.6 page All Unit 2 lessons

Why debugging is its own skill

Two Strings that read the same on screen are not necessarily the same object, and == asks the object question, not the reading question. This is the single most common runtime surprise in the whole course, and it never produces an error message.

What is wrong with it

Run the program with the two words typed the same. The first line prints false anyway. The comparison uses == on two Strings, which compares whether they are the same object in memory, not whether they contain the same characters. Use the method that compares contents.
The third line is supposed to print whether the two words are NOT both non-empty. The not was distributed across the and incorrectly. Fix it so it matches the sentence.
The second line, which compares the lengths, is already correct. Lengths are ints, and == is right for ints.

The program reads

Two words separated by whitespace.

The program should print

Three lines of true or false: whether the words are the same text, whether their lengths are equal, and whether it is not the case that both words have length greater than zero.

Worked examples

These show what the FIXED program should print. There are more cases you cannot see, and they use different values, so patching around just these numbers will fail.

Example 1 input

cat cat

Example 1 output

true
true
false

Example 2 input

apple pear

Example 2 output

false
false
false

Your answer

Main.java

import java.util.Scanner;

public class Main {
  public static void main(String[] args) {
    Scanner input = new Scanner(System.in);
    String first = input.next();
    String second = input.next();

    String firstCopy = new String(first);

    System.out.println(firstCopy == second);
    System.out.println(first.length() == second.length());
    System.out.println(!(first.length() > 0) && !(second.length() > 0));
  }
}

Input for the Run button

Run sends whatever is in the code box below, bugs and all, so you can see the crash or the wrong answer for yourself before you fix anything.

Stuck?

Hint 1

new String(first) deliberately builds a second object holding the same characters. == compares the two references, and they point at different objects, so it is false no matter what the words say. equals compares character by character.

Hint 2

You may have seen == appear to work on Strings in other programs. That is the string pool reusing one object for identical literals, and it is an optimization, not a guarantee. The moment a String comes from input or from new, the trick stops working. Never rely on it.

Hint 3

For the third line, !(P && Q) is false only when both P and Q are true. !P && !Q is true only when both are false. Those are different in the two mixed cases: one empty word and one not.

Where to go next

Rework the Lesson 2.6 page Try the from-scratch version of this lesson

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