Java quiz 25: Object references using StringBuffer

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The class StringBuffer is one of the classes that you often need by writing Java programs. This example demonstrates StringBuffer and object references. If you are not familiar with the class StringBuffer, Here is a link to the article.

Java quiz 25: Object references using StringBuffer

Level: Normal
What happens if the following code is compiled and run?

public class MC
  public StringBuffer getSb(StringBuffer sb)
    return sb;
  public static void main(String[] args)
    StringBuffer sb = new StringBuffer();
    StringBuffer sb2 = new MC().getSb(sb);
    System.out.print(sb + "" + sb2);
Author: Sar Maroof

Select the correct answer.

Answer explanation

  1. The statement StringBuffer sb2 = new MyClass().getStringBuffer(sb); invokes the method getStringBuffer, which adds the letter x to the object sb.
  2. The same previous mentioned statement refers the object sb2 to the object sb, because we passed the object sb as parameter to the method and it returns the object sb. So, the statement StringBuffer sb2 = new MyClass().getStringBuffer(sb); refers the object sb2 to the object sb.
  3. So, both objects sb and sb2 refer to the same object.
  4. The statement sb.append(“y”); adds the letter y to the object sb.
  5. The statement sb2.append(“z”); adds the letter z to the object sb2.
  6. Since both object sb and sb2 are referring to the same object, the object is now contains xyz respectively.
  7. The statement System.out.print(sb + “” + sb2); writes the content of the both objects to the standard output. The result is that the code writes xyzxyz to the standard output.

The correct answer is: d.

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Sar Maroof is graduated from HBO Amsterdam "higher professional education". He is a professional computer programming teacher, gives master classes and publishes technical articles. His experience with developing software started from 2001 and he has worked as a software developer for several big as well as small companies and later as a freelancer. He is the author of Java quizmaster and Build a Java application in 7 days.

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