OCAJP8

Question 1

If you invoke

System.out.println(exception);

will you see the stack trace on ths console?

Answer 1

No. Using System.out.println(exception) will NOT include the stack trace. It conly includes the name of the exception and the message.

Question 2

True or False:

Static initializers can call static methods in their body.

Answer 2

True.

Question 3

Is this a valid variable assignment?

int x, y, z, a; x = y = z = a = 15;

Answer 3

Yes. Variable assignments can be chained.

Question 4

What will be the output of the following?

String s = null; s += “Hello”;

System.out.println(s);

Answer 4

The output will be “nullHello”. a String that is initialized as NULL is different from “”.

Question 5

Given the following:

Integer x = new Integer(5);

x++;

Does the variable “x” still point to the same reference created in the the first line?

Answer 5

No. All the wrapper objects are immutable. So when you do x++, what actually happens is something like

x = new Integer( x.intValue() + 1);

Question 6

Is Java using pass-by-value or pass-by-reference?

Answer 6

Java uses pass by value semantics in method calls. In case of primitive variables, their values are passed, while in case of Objects, their reference values are passed. Thus, when you assign a different object to reference variable in a method, the original reference variable that was passed from the calling method still points to the same object that it pointed to before the call.

Question 7

Can the overriding method change the return type to a subclass of the original method?

Answer 7

Yes. Covariant returns are allowed since Java 1.5, which means that an overriding method can change the return type to a subclass of the return type declared in the overriden method.

Question 8

When accessing static and instance variables, which class is being considered? Is it the reference or the actual instance?

Answer 8

Access to static and instance fields and static methods depends on the class of reference variable and not the actual object to which the variable points to.

Question 9

When calling instance methods, which class is being considered? Is it the reference or the actual instance?

Answer 9

When calling instance methods, the method of the actual class of the the object is called.

Question 10

Given the following call:

int x = 5; doOperation(x++);

What value of x will be passed to “doOperation”?

Answer 10

The value 6 will be passed. When there are operands in method parameters, they are first evaluated before calling the method.

Question 11

What will the following code print?

void crazyLoop(){ int c = 0; JACK: while (c 3) break JILL; else c++; } }

Answer 11

It will not compile. Because break JILL; would be valid only when it is within the block of code under the scope of the label JILL. In this case, the scope of JILL extends only up till System.out.println(c); and break JILL; is out of the scope of the label.

Question 12

Consider the following code:

class A{ A() { print(); } void print() { System.out.println(“A”); } }

class B extends A{ int i = 4;

public static void main(String[] args){ A a = new B(); a.print(); }

void print() { System.out.println(i); } }

What will be the output when class B is run ?

Answer 12

It will print 0, 4 Note that method print() is overridden in class B. Due to polymorphism, the method to be executed is selected depending on the class of the actual object. Here, when an object of class B is created, first A’s constructor is called, which in turn calls print(). Now, since the class of actual object is B, B’s print() is selected. At this point of time, variable i has not been initialized (because we are still initializing A at this point), so its default value i.e. 0 is printed. This happens because the method print() is non-private, hence polymorphic.

Question 13

If an Exception is handled in a catch block and is not thrown, the execution continues.

Answer 13

If an Exception is handled in a catch block and is not thrown, the execution continues.

Question 14

If the array reference expression produces null instead of a reference to an array, then a NullPointerException is thrown at runtime, but only after all parts of the array reference expression have been evaluated and only if these evaluations completed normally.

Answer 14

If the array reference expression produces null instead of a reference to an array, then a NullPointerException is thrown at runtime, but only after all parts of the array reference expression have been evaluated and only if these evaluations completed normally.

Question 15

In Lambda Expressions, when is it required to put the method body inside curly braces.

Answer 15

In Lambda Expressions, you need to put the body within curly braces if you want to use the return keyword.

Question 16

What does the add method of ArrayList return?

Answer 16

The add method of ArrayList returns a boolean. Further, it returns true if the list is altered because of the call to add.

Question 17

Will an occurence of java.io.IOException be handled by a catch block that catches RuntimeException?

Answer 17

java.io.IOException extends Exception. It cannot be caught by a catch block that catches RuntimeException.

Question 18

If there is an un-reachable code in a method, will it compile?

Answer 18

Un-reachable codes will cause compilation error except if it is inside an IF block.

Question 19

Do ambiguous fields or methods cause compile time error?

Answer 19

Having ambiguous fields or methods does not cause any problem by itself but referring to such fields or methods in an ambiguous way will cause a compile time error.

Question 20

Does a lambda expression create new scope for variables?

Answer 20

A lambda expression does not create a new scope for variables. Therefore, you cannot reuse the variable names that have already been used to define new variables in your argument list.

Question 21

If an interface extends another interface with a default method, can it redeclare the default method?

Answer 21

An interface can redeclare a default method and also make it abstract.

Question 22

If an interface extends another interface with a default method, can it redeclare the same default method and provide a different implementation?

Answer 22

An interface can redeclare a default method and provide a different implementation.

Question 23

Can static methods be abstract?

Answer 23

Static methods can never be abstract (neither in an interface not in a class).

Question 24

Is an interface allowed to have a static method?

Answer 24

An interface can have a static method but the method must have a body.

Question 25

What are the default characteristis of every field in an interface?

Answer 25

Any field in an interface is implicitly public, static, and final, whether these keywords are specified or not.

Question 26

Can a final variable be hidden in a subclass.

Answer 26

A final variable can be hidden in a subclass.

Question 27

Will this compile? String s = "hello"; StringBuilder sb = new StringBuilder("hello"); booean areTheyTheSame = (s == sb);

Answer 27

No. Java does not allow you to compare String and StringBuilder using ==

Question 28

In multi-dimensional arrays, which is required to have the size initialized?

Answer 28

In multi-dimensional arrays, only the first array is required to have the size initialized.

Question 29

Are two ArrayList with the same content equal?

Answer 29

Two ArrayList with the same content are equal.

Question 30

How many times and where is it allowed to set values for static final variables?

Answer 30

static final variables must be set exactly once, and it must be in the declaration line or in a static initialization block.

Question 31

What is the result of the following expression? new Boolean("true") == Boolean.TRUE

Answer 31

The expression will result to false because they point to two different Boolean wrapper objects.

Question 32

Are two Array with the same content equal?

Answer 32

Two Array with the same content is not equal.

Question 33

Does Java allow constructors with empty bodies?

Answer 33

Constructors cannot have empty bodies.

Question 34

If a concrete class implements an interface and extends a class that implements another interface, is it required to implement all methods defined in an inherited interface?

Answer 34

A concrete class is NOT necessarily required to implement all methods defined in an inherited interface because some methods may have been already implemented by a superclass.

Question 35

Which method implementaion will be called if you cast an instance to the interface with a default method?

Answer 35

Even if you cast an instance to the interface with a default method, the instance method will be run if it overrides the default.

Question 36

When does an object become eligible for garbage collection?

Answer 36

An object can be made eligible for garbage collection by making sure there are no references pointing to that object.

Question 37

When will the implementation of a hidden method be called in a subclass?

Answer 37

With hidden methods, the specific method used depends on where it is referenced.

Question 38

Is it possible to hide protected and public methods?

Answer 38

Protected and public methods may be overridden, not hidden.

Question 39

Are private methods overriden or hidden in a subclass?

Answer 39

Private methods are always hidden in a subclass.

Question 40

Can static methods be overriden?

Answer 40

Static methods cannot be overriden, only hidden.

Question 41

Is it possible to override a variable in a subclass?

Answer 41

Variables may only be hidden, regardless of the access modifier.

Question 42

How can you invoke the garbage collector?

Answer 42

You cannot directly invoke the garbage collector. You can suggest the JVM to perform garbage collection by calling System.gc();

Question 43

Does an extra semi-colon at the end of a line cause compilation error?

Answer 43

Java compiler ignores an extra semi-colon because it is equivalent to an empty statement.

Question 44

Where can you put labels in Java code?

Answer 44

You can apply a label to any code block or a block level statement (such as a for statement) but not to declarations. For example: loopX : int i = 10;

Question 45

Can you call break and continue with label in a regular line inside a method?

Answer 45

Break and continue with labels are only valid if called from within the block of code.

Question 46

Which of the following are valid formatters? * LOCAL\_DATE\_TIME * ISO\_LOCAL\_DATE\_TIME * ISO\_DATE\_TIME

Answer 46

LOCAL\_DATE\_TIME is not a valid formatter. ISO\_LOCAL\_DATE\_TIME is valid though, which is same as ISO\_DATE\_TIME except that it does not use the Zone or Offset.

Question 47

How can you enfore immutability in a Java Bean?

Answer 47

Immutability uses private instance variables.

Question 48

If the index part in an array operation contains an expression, what the the order or execution?

Answer 48

When working with arrays, the expression for the index is always evaluated first including checkign whether the array is NULL or not.

Question 49

What is the result of the following? Integer myInt = new Integer(1); Long mylong = new Long(1); System.out.printlin(myInt == myLong);

Answer 49

The code will not compile. Using the operator == with two drifferent wrapper class types will cause compilation error.

Question 50

What is the result of the following? Integer myInt = new Integer(1); Long mylong = new Long(1); System.out.printlin(myInt.equals(myLong));

Answer 50

The equals() operator will give FALSE if you compare two different wrapper class types even with the same values.

Question 51

Which is faster when accessing an element? Array or ArrayList?

Answer 51

Accessing an element in an Array is faster than in ArrayList.

Question 52

Which takes precedence? Importing by classname or using wildcards?

Answer 52

Importing by classname takes precedence over wildcards.

Question 53

Is this a valid code? class Hello { public void Hello(){}; }

Answer 53

Yes. Methods (with return type) having the same name as the class is allowed.

Question 54

Are static variables accessible in instance initializers?

Answer 54

Both instance and static initializers are able to access static variables

Question 55

Will this compile? double amount = 0xE;

Answer 55

Yes. Hex values can be assigned to double.

Question 56

Where are underscores allowed in numeric literals?

Answer 56

Underscores are allowed in numbers as long as they are directly between two other digits.

Question 57

What is the assumption on interface methods without any access modifier?

Answer 57

Interface methods without any access modifier is always assumed to be public.

Question 58

Can a class inherit two interfaces that both define default methods with the same signature without overriding the method?

Answer 58

A class cannot inherit two interfaces that both define default methods with the same signature, unless the class implementing the interfaces overrides it iwth an abstract or concrete method.

Question 59

In a subclass constructor, if there is no explicit call to the parent constructor, what will be inserted as the first line?

Answer 59

In a subclass constructor, if there is no explicit call to the parent constructor, the default no-argument super() will be inserted as the first line.

Question 60

Does autoboxing apply in Predicates?

Answer 60

Autoboxing does not work with Predicates

Question 61

Where is synchronized keyword used?

Answer 61

Synchronized keyword is only for metods and blocks

Question 62

What will the following code produce? LocalDate myDate = LocalDate.of(2015, Calendar.January, 1);

Answer 62

The code will throw an exception. In the new Date API, months are indexed staring 1 so using the static fields from Calendar will cause trouble.

Question 63

Can you shadow instance variables in loops and methods?

Answer 63

Instance variables can be shadowed in loops and methods.

Question 64

In Encapsulation, what can be done to lists/collections in getter methods if you need to protect the contents?

Answer 64

In Encapsulation, it is good to return copies of lists/collections in getter methods if you need to protect the contents.

Question 65

Does Java allow arrays of length zero to be created?

Answer 65

Java allows arrays of length zero to be created

Question 66

Can bitwise operators (&, |, ^) have integral operands?

Answer 66

Bitwise operators (&, |, ^) can have integral as well as boolean operands

Question 67

Can an overriding method declare that it can throw a superclass of the exceptions the overridden method can throw?

Answer 67

An overriding method only needs to declare that it can throw a subset of the exceptions the overridden method can throw. Having no throws clause in the overriding method is OK.

Question 68

Describe a Functional Interface.

Answer 68

A Functional Interface must have exactly one abstract method and may have other default or static methods.

Question 69

What does the operation -1 do?

Answer 69

The operation -1 in binary would be: complement the bits for 1 and add 1

Question 70

What does the leftmost bit in a binary integer represent?

Answer 70

The leftmost bit in a binary integer is the sign bit which is 1 for negative and 0 for positive

Question 71

A class or interface type T will be initialized immediately before the first occurrence of any one of the following:

Answer 71

A class or interface type T will be initialized immediately before the first occurrence of any one of the following: * T is a class and an instance of T is created. * T is a class and a static method declared by T is invoked. * A static field declared by T is assigned. * A static field declared by T is used and the field is not a constant variable. * T is a top-level class, and an assert statement lexically nested within T is executed.

Question 72

According to the Java Language Specification, what is the rule when acessing protected members in a subclass on a different package?

Answer 72

A protected member is inherited by a subclass and it is therefore accessible in the subclass. However, In the words of Java Language Specification, protected members of a class are accessible outside the package only in subclasses of that class, and only when they are fields of objects that are being implemented by the code that is accessing them. Basically, it implies that a protected member is accessible in the subclass only using a reference whose declared type is of the same subclass (or its subclass).

Question 73

Is an empty switch block is a valid construct?

Answer 73

An empty switch block is a valid construct.

Question 74

When valueOf is called in wrapper classes, does auto-boxing apply?

Answer 74

When valueOf is called in wrapper classes, auto-boxing does not apply

Question 75

What happens when you pass a primitive type to a method?

Answer 75

When you pass a primitive type to a method, it is passed by value. Thus, a copy of the argument is made, and what occurs to the parameter that receives the argument has no effect outside the method.

Question 76

What is the effect of calling the method intern() in concatenated strings?

Answer 76

Concatenating strings results in a new String object. Calling the intern() method removes duplicates in string pool:

Question 77

If a catch block throws an exception and the finally block throws a dirrent exception, which one will be thrown?

Answer 77

If a finally block throws an exception, that is the one printed regardless of the one thrown in a catch block.

Question 78

When are you required to use a finally block in a regular try statement?

Answer 78

When there are no catch blocks in a try statement.

Question 79

Which of the following exceptions are thrown by the JVM? * ArrayIndexOutOfBoundException * ExceptionInInitializerError * java.io.IOException * NullPointerException * NumberFormatException

Answer 79

The following exceptions are thrown by the JVM: * ArrayIndexOutOfBoundException * ExceptionInInitializerError * NullPointerException

Question 80

Can a method throw Runtime exceptions without declaring it in the throws section?

Answer 80

Runtime exceptions can be thrown in any method even without the thows declaration.

Question 81

What will the following class print ? class Test { public static void main(String[] args){ int[][] a = { { 00, 01 }, { 10, 11 } }; int i = 99; try { a[val()][i = 1]++; } catch (Exception e) { System.out.println( i+", "+a[1][1]); } } static int val() throws Exception { throw new Exception("unimplemented"); } }

Answer 81

[99, 11] If evaluation of a dimension expression completes abruptly, no part of any dimension expression to its right will appear to have been evaluated. Thus, while evaluating a[val()][i=1]++, val() throws an exception and i=1 will not be executed. Therefore, i remains 99 and a[1][1] will print 11.

Question 82

Is this valid? int[10] iA;

Answer 82

No. Size of the array is NEVER specified on the Left Hand Side

Question 83

What is the default length of the args parameter in the public static void main method if the code is run without any arguments?

Answer 83

Remember that the args array is never null. If the program is run without any arguments, args points to a String array of length 0.

Question 84

class Super { static String ID = "QBANK"; } class Sub extends Super{ static { System.out.print("In Sub"); } } public class Test{ public static void main(String[] args){ System.out.println(Sub.ID); } } What will be the output when class Test is run?

Answer 84

It will print "QBANK" A reference to a static field causes initialization of only the class or interface that actually declares it, even though it might be referred to through the name of a subclass, a subinterface, or a class that implements an interface.

Question 85

What should be the return type of the following method? public RETURNTYPE methodX( byte by){ double d = 10.0; return (long) by/d\*3; }

Answer 85

double. Note that the cast (long) applies to 'by' not to the whole expression.

Question 86

Which of the following statements will evalutae to true? * "String".replace('g','G') == "String".replace('g','G') * "String".replace('g','g') == new String("String").replace('g','g') * "String".replace('g','G')=="StrinG" * "String".replace('g','g')=="String"

Answer 86

"String".replace('g','g')=="String" There are 2 points to remember: 1. replace() method creates a new String object. 2. replace() method returns the same String object if both the parameters are same, i.e. if there is no change.

Question 87

Given: byte b = 1; char c = 1; short s = 1; int i = 1; Which of the following expressions are valid? * s = b \* b; * i = b + b; * s \*= b; * c = c + b; * s += i;

Answer 87

The following are valid: * i = b + b; * s \*= b; * s += i; All compound assignment operators internally do an explicit cast.

Question 88

What are the casting rules for primitives?

Answer 88

Remember these rules for primitive types: 1. Anything bigger than an int can NEVER be assigned to an int or anything smaller than int ( byte, char, or short) without explicit cast. 2. CONSTANT values up to int can be assigned (without cast) to variables of lesser size ( for example, short to byte) if the value is representable by the variable.( that is, if it fits into the size of the variable). 3. operands of mathematical operators are ALWAYS promoted to AT LEAST int. (i.e. for byte \* byte both bytes will be first promoted to int.) and the return value will be AT LEAST int. 4. Compound assignment operators ( +=, \*= etc) have strange ways so read this carefully.

Question 89

Will the following code compile? ``` public class FunWithArgs { public static void main(String[][] args) { System.out.println(args[0][1]); } public static void main(String[] args) { FunWithArgs fwa = new FunWithArgs(); String[][] newargs = {args}; fwa.main(newargs); } } ```

Answer 89

There is no problem with the code. The main method is just overloaded. When it is run, the main method with String[] will be called. This method then calls the main with String[][] with an argument { {"a", "b", "c"} } Thus, args[0][1] refers to "b", which is what is printed.

Question 90

You want to print the date that represents upcoming tuesday from now even if the current day is a tuesday. What are the 2 ways to accomplishthis?

Answer 90

You can accomplish it by: * System.out.println(LocalDate.now().with(TemporalAdjusters.next(DayOfWeek.TUESDAY))); * System.out.println(TemporalAdjusters.next(DayOfWeek.TUESDAY).adjustInto(LocalDate.now()));

Question 91

What will be the values of a ,b, and c after the following statement? boolean bool = (a = true) || (b = true) && (c = true);

Answer 91

a = true b = false c = false Java parses the expression from left to right. Once it realizes that the left operand of a conditional "or" operator has evaluated to true, it does not even try to evaluate the right side expression.

Question 93

What is the output of the following? class Game{ public void play() throws Exception{ System.out.println("Playing..."); } } public class Soccer extends Game{ public void play(){ System.out.println("Playing Soccer..."); } public static void main(String[] args){ Game g = new Soccer(); g.play(); } }

Answer 93

``` It will not compile. Observe that play() in Game declares Exception in its throws clause. Further, class Soccer overrides the play() method without any throws clause. This is valid because a list of no exception is a valid subset of a list of exceptions thrown by the superclass method. Now, even though the actual object referred to by 'g' is of class Soccer, the class of the variable g is of class Game. Therefore, at compile time, compiler assumes that g.play() might throw an exception, because Game's play method declares it, and thus expects this call to be either wrapped in a try-catch or the main method to have a throws clause for the main() method. ```

Question 94

What is wrong with the following code? public String getDateString(LocalDateTime ldt){ return DateTimeFormatter.ISO\_ZONED\_DATE\_TIME.format(ldt); }

Answer 94

Note that LocalDateTime class does not contain Zone information but ISO\_ZONED\_DATE\_TIME requires it. Thus, it will throw the following exception: Exception in thread "main" java.time.temporal.UnsupportedTemporalTypeException: Unsupported field: OffsetSeconds UnsupportedTemporalTypeException extends DateTimeException.