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Understanding the “this” reference in object oriented programming: Misconceptions, conceptions, and teaching recommendations

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Abstract

The paper presents research that aims to expose students’ understanding of the this reference in object-oriented programming. The study was conducted with high school students (N = 86) and college engineering students (N = 77). Conceptualization of this reflects an understanding of objects in general and involves aspects of programming variants and programmers’ preferences as well. To examine students’ conceptions, perceptions, and misconceptions we developed a diagnostic tool that uses this in various contexts, such as in constructors, as a visible parameter, for calling an overloaded constructor in class, or while transiting a non-static method using this to a static one. The detailed analysis revealed difficulties, in both groups of participants, in conceptualizing the meaning of this as the current object and in its various uses in the code. The discussion presents students’ conceptions of “what is this”, nine misconceptions that we characterized, and answers to our research questions. The conclusion offers recommendations for teaching and learning processes in light of the results obtained.

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Author information

Authors and Affiliations

  1. Department of Industrial Engineering and Management, Shamoon College of Engineering, Ashdod, Israel

    Ronit Shmallo

  2. Department of Computer Science, Faculty of Education, Beit Berl College, Kfar Saba, Israel

    Noa Ragonis

Authors
  1. Ronit Shmallo
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  2. Noa Ragonis
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Correspondence to Ronit Shmallo.

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Appendices

Appendix 1: The questionnaire

Question 1: Where Is this Needed.

The following is a project that includes a simple class Date, a composed class Flight, and a main class Program.

figure afigure a
  1. (a)

    In relation to the rows marked with numbers 1–4 (// line #n) determine where this is required to be used and where it is superfluous. Explain the reason for each of your choices.

  2. (b)

    When executing instruction #5 in the main method, to what does the this appearing in the equals method in class Flight refer?

  3. (c)

    Develop a static method replacing the instance method equals in class Date.

  4. (d)

    Can this be used in the code of the main method? Explain your answer.

Question 2: Personal Preferences on Using this in Code.

Each of the following methods relates to the class Point described by two coordinates (x,y). Some of the methods use this and some do not. The methods of each clause execute the same task, and they are all syntactically correct. Please rank in each line marked by (a)–(d) your personal preferences codes by assigning numbers from 1 to 3, where 1 is your first priority. Explain your choices.

figure b

Question 3: Using this as a Parameter.

The following is a project that includes a simple class Circle and a main class Test. Some of the methods of the class Circle include only the method signature without the method’s full body. The method drawX(…) accepts a circle as a parameter and draws it, the method drawFlower(…) accepts a circle as a parameter and draws a flower consisting of circles, and the method chooseWhatToDraw(…) accepts a circle and a character and determines what to draw.

figure c

In relation to the main method and the execution of the instruction.

circle1.chooseWhatToDraw('F', circle2);

to what does the this that appears in the method chooseWhatToDraw(…) in the instruction drawFlower(this); refer?

Question 4: Using this to Invoke a Current Class Overloaded Constructor.

The following is a project that includes the classes AA and Program. Review the classes and answer the questions that follow.

figure d

Follow the execution of the main method and

  1. (a)

    Use a trace table and display the program output.

  2. (b)

    In relation to the rows marked with numbers 1–2 (// line #n), determine what the this refers to when executing the next instruction:

AA a=new AA();

Question 5: Using this—an Open Comprehension Question.

Please briefly answer the following questions:

  1. (a)

    When must this be used?

  2. (b)

    When should this be used?

  3. (c)

    When shouldn’t this be used?

  4. (d)

    What is this?

Appendix 2: Mapping the research questions and outcomes

Table 10 Mapping between Identified Misconceptions, Diagnostic Tool Questions, Research Questions, and the Different Usages of this
Full size table

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Shmallo, R., Ragonis, N. Understanding the “this” reference in object oriented programming: Misconceptions, conceptions, and teaching recommendations. Educ Inf Technol 26, 733–762 (2021). https://doi.org/10.1007/s10639-020-10265-6

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