A.4 Chapter 4: Selected classes from the Java API and arrays

Chapter 4 includes four Twist in the Tale exercises.

A.4.1 Twist in the Tale 4.1

Purpose: To remind you to be careful with the overloaded methods of the class String that accept either char or String or both, the code in this exercise passes an invalid method argument—a char—to method startsWith.

Answer: e

Explanation: When it comes to the String class, it’s easy to confuse the methods that accept char or String values as method arguments. For example, the overloaded method indexOf can accept both String and char values to search for a target value in a String. The methods startsWith and endsWith accept only arguments of type String. The method charAt accepts only method arguments of type int. Hence, this method can be passed char values, which are stored as unsigned integer values.

A.4.2 Twist in the Tale 4.2

Purpose: This exercise has multiple purposes:

• To confuse you with the use of method names, which are used in the Java API by other classes to create their objects.
• To encourage you to refer to the Java API documentation when you work with classes from the Java API. The Java API documentation is an extensive source of information and facts that are often not included in most books (because it’s practically impossible to do so).

Answer: d

Explanation: The correct way to create an object of class StringBuilder with a default capacity of 16 characters is to call StringBuilder’s no-argument constructor, as follows:

StringBuilder name = StringBuilder();

A.4.3 Twist in the Tale 4.3

Purpose: Identify the difference between an array element that isn’t initialized and an array element that doesn’t exist. A pictorial representation of a multidimensional array is quick to draw, and you can easily refer to its nonexistent or null array elements. This concept is shown in figure A.3.

Figure A.3. Array multiStrArr and its elements

Answer: b, d

Explanation: Option (a) is incorrect. Initializing a row of array multiStrArr with {"Jan","Feb",null} and {"Jan","Feb",null,null} isn’t the same. The former option defines three array elements with the last array element assigned to null. The latter option defines four array elements with the last two array elements assigned to null.

Option (b) is correct. The array element at the position exists but isn’t assigned any value. It’s assigned to null.

Option (c) is incorrect. Because multiStrArr[1] refers to null, multiStrArr[1][1] doesn’t exist.

Option (d) is correct. As shown in figure A.3, the array multiStrArr doesn’t define an equal number of elements in each row, so it’s asymmetric.

A.4.4 Twist in the Tale 4.4

Purpose: This exercise tries to trick you by using multiple objects of ArrayList, assigning the object reference of one ArrayList to another, and modifying the value of the ArrayList objects. String objects are immutable—you can’t change their values.

Answer: a

Explanation: Option (a) is correct, and options (b), (c), and (d) are incorrect. The ArrayLists myArrList and yourArrList contain String objects. The value of String objects can’t be modified once created.