Advanced Java Study Guide

Consider a three-dimensional array

int[][][] numbers = new int[10][10][10].

If this array were to be looped through using for-each loops, what type of variable must be declared in the outermost loop?

• int

• int[]

• int[][]

• int[][][]

Answer explanation

A three-dimensional array in Java is actually an array of two-dimensional arrays. Therefore, the outermost for-each loop must iterate through these two-dimensional arrays which are of type int[][].

What keyword in Java is used to determine if an Object is a certain class?

Answer explanation

In Java, the instanceof keyword is used to test if an object is of a given type. It is also known as type comparison operator because it compares the instance of an Object with a type.

Consider two classes class ChessPiece and class King extends ChessPiece. Suppose King has a method getChecked() that is not found in the super class.

Which of the following can be used with ChessPiece cp to access the getChecked() method?

• ((King)cp).getChecked()

• (King)(cp).getChecked()

• (King)cp.getChecked()

• ((King)cp.getChecked())

Answer explanation

The ChessPiece cp must be typecast to King in order to access methods unique to King. In Java, the syntax for this is ((King)cp).getChecked().

Consider a two-dimensional array

int[][] grid = new int[8][8]. Which of these for loops will iterate only through the elements on the diagonal from grid[5][3] to grid[7][1]?

• for(int i = 5, j = 3; i <= 7 && j >= 1; i++, j--)

• for(int i = 5; i<= 7; i++)

  for(int j = 3; j >= 1; j--)

• for(int i = 5, j = 3; i <= 7 || j >= 1; i++, j--)

• for(int i = 5 && j = 3; i <= 7 && j >= 1; i++ && j--)

Answer explanation

To do a for loop in two variables, use comma-separated variable declarations, combine terminating conditions with the && operator, and comma-separate the incrementing commands. In this case, the syntax is

for(int i = 5, j = 3; i <= 7 && j >= 1; i++, j--).

What function is used to add the contents of an ArrayList to another ArrayList?

Answer explanation

An arbitrary number of elements of a Collection such as an ArrayList can be added to another ArrayList using the addAll() function.

Consider an array

int[][] grid = new int[8][8]. Given two arbitary indices x and y, which of the following for loops will iterate through the 3x3 region centered on (x,y) without ever throwing an ArrayIndexOutOfBoundsException?

• for(int i = Math.max(0,x-1); i <= Math.min(7,x+1); i++)

  for(int j = Math.max(0,y-1); j <= Math.min(7,y+1); j++)

• for(int i = Math.max(0,x-1), j = Math.max(0,y-1); i <= Math.min(7,x+1) && j <= Math.min(7,y+1); i++, j++)

• for(int i = Math.min(0,x-1); i <= Math.max(7,x+1); i++)

  for(int j = Math.min(0,y-1); j <= Math.max(7,y+1); j++)

• for(int i = Math.min(0,x-1), j = Math.min(0,y-1); i <= Math.max(7,x+1) && j <= Math.max(7,y+1); i++, j++)

Answer explanation

To prevent the throwing of an ArrayIndexOutOfBoundsException, the outer loop should iterate starting at either 0 or x-1, whichever is greater. Likewise, it should stop iterating at either 7 or x+1, whichever is smaller. The inner loop should be bounded similarly but with y. Because we want the entire 3x3 region and not just the diagonal, we should use two nested for loops. With all of that in mind, the correct code is:

for(int i = Math.max(0,x-1); i <= Math.min(7,x+1); i++)

for(int j = Math.max(0,y-1); j <= Math.min(7,y+1); j++)

Which of the following is best refactored by moving it to a separate function?

Select all that apply.

• Identical code in different for loops.

• Identical code in different classes.

• An important part of an existing function.

• Code within nested if statements that have no else statements.

Answer explanation

Repeated code such as identical code in different for loops or identical code in different classes with only small differences should be moved to its own function. Code which is an important part of an existing function is already in its own function and doesn't need to be moved. Nested if statements with no else staements can be simplified to a single if statement using the && operator, but doesn't need to go in its own function.

Consider two classes class ChessPiece and class Knight extends ChessPiece. Suppose ChessPiece and Knight have the same method getMoveset().

If Knight k = new Knight(), which of the following is executed when calling k.getMoveset()?

• Knight.getMoveset(), then ChessPiece.getMoveset()

• ChessPiece.getMoveset(), then Knight.getMoveset()

• Knight.getMoveset()

• ChessPiece.getMoveset()

Answer explanation

If an Object of a parent class is used to invoke a method, then the version of that method in the parent class will be executed, but if an Object of the subclass is used to invoke the method, then the version in the child class will be executed. Therefore, only Knight.getMoveset() is executed.

Consider two classes class ChessPiece and class Rook extends ChessPiece. Which of the following are valid declarations? Select all that apply.

• ChessPiece cp = new ChessPiece();

• ChessPiece cp = new Rook();

• Rook r = new ChessPiece();

• Rook r = new Rook();

Answer explanation

As with any Object, matching the declaring type with the constructor type is valid, so ChessPiece cp = new ChessPiece(); and Rook r = new Rook(); are both valid declarations. In addition, Objects of a superclass type can be instantiated as a subclass, so ChessPiece cp = new Rook(); is also valid.

Which of the following interfaces is used to implement mouse click functionality?

• MouseAdapter

• MouseListener

• MouseMotionListener

• MouseWheelListener

Answer explanation

Although MouseAdapter is one of the classes that can be used to handle MouseEvents, the specific interface which deals with mouse clicks is the MouseListener.