AP CSA 1.12 FRQ Practice: Two References, One Object

Unit 1: Using Objects and Methods · Lesson 1.12 · FRQ Practice

Two References, One Object

A free response question in the shape the exam uses: a stated contract, four rubric parts, and no main method handed to you. Worth 4 points.

Back to the Lesson 1.12 pageAll Unit 1 lessons

Why this question is worth four points

An object is not its variable. Two names can point at the same object, and two separate objects can hold equal contents while == says they are different things. This is the single most tested idea in the whole unit and it is where == on Strings comes from.

What you are given

Three String variables are declared and assigned for you: a, b and c. a and b are built from the same literal; c is built with new.

The question

  1. (a) Print whether a and b are the same object, using ==.
  2. (b) Print whether a and c are the same object, using ==.
  3. (c) Print whether a and c hold the same characters, using equals.
  4. (d) Print the length of c.

What the reader is looking for

  1. Write a code segment that prints four lines in rubric order.
  2. Parts (a) and (b) ask about identity, which is what == compares for objects. Part (c) asks about contents, which is what equals compares.
  3. Do not use an if. A comparison is already a boolean and can be printed directly.

Worked examples

These show what a correct answer prints. There are more cases you cannot see, and they use different values, so an answer built around just these numbers will fail.

Example 1 is given
String a = "sunrise";
String b = "sunrise";
String c = new String("sunrise");
Example 1 output
true
false
true
7
Example 2 is given
String a = "north";
String b = "north";
String c = new String("south");
Example 2 output
true
false
false
5

Your answer

Main.java

Input for the Run button

How this is scored: your answer runs against every test case, and the fraction it passes becomes your score out of 4. That is not how a human AP reader marks a rubric, so treat the score as a check on whether your code works, and the rubric above as the thing you are actually practising.


  

  
Stuck?

    
Hint 1
For objects, == asks "is this the very same object in memory", not "do these look alike". Two Strings with identical characters can be two different objects.

    
Hint 2
new ALWAYS makes a new object. That is why a == c is false even though the characters match, and it is the whole reason equals exists.

    
Hint 3
You do not need an if for any part. System.out.println(a == b) prints true or false by itself.



  
Before you submit

  
1 mistake(s) that lose points on this question
    
Each of these is a real error the grader catches. Check your answer against them before you submit, not instead of trying.

    
    
      
  • part (c) uses == for contents instead of equals
    • 
    

  


  
Where to go next

  

    Rework the Lesson 1.12 page
    Try the from-scratch exercise for this lesson
    Try the debugging exercise for this lesson
  


  
  






  

















  
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