Juan Pablo Barale

In page 136, Table 3.5. the Description column for the "void push(E e)" method row should say: Adds an element to the front of the queue or throws an exception.
Is the same case as in the "boolean add(E e)" method description, in both cases a NullPointerException is thrown if the specified element is null.

Regards,

Hi @Kabi, you can do something like this:
// Complete the staircase function below.    static void staircase(int n) {      StringBuilder sb = new StringBuilder();      for(int i = 0; i < n; i++){        int k = 0;        // adds blank prefixes        for(; k < n - i; k++) {          sb.append(" ");          }          // adds # symbols        for(int j = k; j <= n; j++){                                  sb.append("#");                }        sb.append("\n");      }      System.out.println(sb.toString());    }    public static void main(String[] args) {          staircase(14);    }
You need to print n lines, all with the same length:
n-1 blanks + 1 #
n-2 blanks + 2 #
.
.
.
n-(n-1) blanks + n-1 #
0 blanks + n #

hope this helps

If a class is implementing two interfaces with the same default method (firm), the class doesn't know which implementation to use, so you are forced to override it in order to compile.
The same problem occurs in the case an interface extends two interfaces with the same default method.

84. What is the output of the following application?
package music; interface DoubleBass {  void strum();  default int getVolume() {return 5;} } interface BassGuitar {  void strum();  default int getVolume() {return 10;} } class ElectricBass implements DoubleBass, BassGuitar {  @Override public void strum() {System.out.print("A");} } public class RockBand {  public static void main(String[] strings) {    final class MyElectricBass extends ElectricBass {      public void strum() {System.out.print("E");}    }  } }
Answer:
   D. One line of code does not compile.

But the answer should be:
   E. Two lines of code do not compile.
MyElectricBass class is also inheriting the problem from the ElectricBass class.
By compiling this code we can see the 2 compilation errors showing up:
$ java -version java version "1.8.0_181" Java(TM) SE Runtime Environment (build 1.8.0_181-b13) Java HotSpot(TM) 64-Bit Server VM (build 25.181-b13, mixed mode) $ javac RockBand.java RockBand.java:10: error: class ElectricBass inherits unrelated defaults for getVolume() from types DoubleBass and BassGuitar class ElectricBass implements DoubleBass, BassGuitar { ^ RockBand.java:15: error: class MyElectricBass inherits unrelated defaults for getVolume() from types DoubleBass and BassGuitar    final class MyElectricBass extends ElectricBass {          ^ 2 errors
Regards,

Ok, got it, but I think the second option is not so "going through the branches". I mean, it's just updating the conflictive lines, and if you are trying to use a native value in a supplier (Supplier<double>) you know you should be using a DoubleSupplier.

I will take the implication of the minimum from now on.

Thanks!

There are two possible solutions for question 56 in the OCP Practice Exam.

Question:
56. How many changes do you need to make in order for this code to compile?

public class Ready {  private static double getNumber() {    return .007;  }    public static void math() {    Supplier<double> s = Ready:getNumber;    double d = s.get();    System.out.println(d);  } }
A. None
B. One
C. Two
D. Three
E. Four

Answer:
C. First, a method reference uses two colons, so it should be Ready::getNumber. Second, you can’t use generics with a primitive, so it should be Supplier<Double>. The rest of the code is correct, so Option C is correct.

The answer is correct, but there's another way to solve this by making three changes, making answer D also possible and correct:
First, a method reference uses two colons, so it should be Ready::getNumber. Second, you can’t use generics with a primitive, so it should be DoubleSupplier. Third, you need to replace the get() method by the getAsDouble() method defined in the DoubleSupplier interface.
 public static void main(String[] args) {    DoubleSupplier s = Ready::getNumber;    double d = s.getAsDouble();    System.out.println(d);  }
I think these changes should also be considered or the question should be updated to ask for the minimum changes required.

Thanks!
JP

In section Understanding Views (page 484), when introducing the methods Files.readAttributes() and Files.getFileAttributeView() the third paragraph starts with:

Both of these methods can throw a checked IOException, such as when the view class type is unsupported.

But only the Files.readAttributes() method can throw an Exception. The Files.getFileAttributeView() returns null when the view class type is not supported.