JAVA 11
What is the output of running class C? class A { public A() { System.out.println( "The default constructor of A is invoked"); } } class B extends A { public B() { System.out.println( "The default constructor of B is invoked"); } } public class C { public static void main(String[] args) { B b = new B(); } }
"The default constructor of A is invoked""The default constructor of B is invoked"
Which of the following is incorrect?
A constructor may be static.
Which of the following statements is false?
A method with no visibility modifier can be accessed by a class in a different package.
Which of the following statements are true?
A. A method can be overloaded in the same class. D. If a method overrides another method, these two methods must have the same signature. E. A method in a subclass can overload a method in the superclass.
Which of the following statements are true?
A. Override the toString method defined in the Object class whenever possible. B. Override the equals method defined in the Object class whenever possible. C. A public default no-arg constructor is assumed if no constructors are defined explicitly. D. You should follow standard Java programming style and naming conventions. Choose informative names for classes, data fields, and methods.
What is the output of the following code? public class Test { public static void main(String[] args) { new Person().printPerson(); new Student().printPerson(); } } class Student extends Person { private String getInfo() { return "Student"; } } class Person { private String getInfo() { return "Person"; } public void printPerson() { System.out.println(getInfo()); } }
A. Person Person
Which of the following statements are true?
A. To override a method, the method must be defined in the subclass using the same signature and compatible return type as in its superclass. B. Overloading a method is to provide more than one method with the same name but with different signatures to distinguish them. C. It is a compilation error if two methods differ only in return type in the same class. D. A private method cannot be overridden. If a method defined in a subclass is private in its superclass, the two methods are completely unrelated. E. A static method cannot be overridden. If a static method defined in the superclass is redefined in a subclass, the method defined in the superclass is hidden.
Which of the following statements are true?
A. You can always pass an instance of a subclass to a parameter of its superclass type. This feature is known as polymorphism. B. The compiler finds a matching method according to parameter type, number of parameters, and order of the parameters at compilation time. C. A method may be implemented in several subclasses. The Java Virtual Machine dynamically binds the implementation of the method at runtime. E. Dynamic binding can apply to instance methods. Exclude ( D. Dynamic binding can apply to static methods.)
What is the output of the following code: public class Test { public static void main(String[] args) { Object o1 = new Object(); Object o2 = new Object(); System.out.print((o1 == o2) + " " + (o1.equals(o2))); } }
A. false false
Which statements are most accurate regarding the following classes? class A { private int i; protected int j; public int getI() { return i; } public int getJ() { return j; } } class B extends A { private int k; protected int m; public int getK() { return k; } public int getM() { return m; } }
An object of B contains data fields i, j, k, m.
What is the output of the following code? public class Test1 { public static void main(String[] args) { ChildClass c = new ChildClass(); c.print(); } } class ParentClass { int id = 1; void print() { System.out.println(id); } } class ChildClass extends ParentClass { int id = 2; }
B. 1
Which of the following statements are true?
B. A subclass is usually extended to contain more functions and more detailed information than its superclass. C. "class A extends B" means A is a subclass of B.
Which of the following code is corrcet?
B. ArrayList<Double> list = new ArrayList<>(); list.add(3.4); D. ArrayList<Integer> list = new ArrayList<>(); list.add(3);
Analyze the following code: public class A extends B { } class B { public B(String s) { } }
B. The program has a compilation error because the default constructor of A invokes the default constructor of B, but B does not have a default constructor. D.The program would compile fine if you add the following constructor into A: A(String s) { super(s); }
Analyze the following code: public class Test extends A { public static void main(String[] args) { Test t = new Test(); t.print(); } } class A { String s; A(String s) { this.s = s; } public void print() { System.out.println(s); } }
B. The program has an implicit default constructor Test(), but it cannot be compiled, because its super class does not have a default constructor. The program would compile if the constructor in the class A were removed. C. The program would compile if a default constructor A(){ } is added to class A explicitly.
Suppose ArrayList x contains two strings [Beijing, Singapore]. Which of the following method will cause runtime errors?
B. x.set(2, "New York"); C. x.get(2) D. x.remove(2)
Analyze the following code: public class Test { public static void main(String[] args) { Object a1 = new A(); Object a2 = new Object(); System.out.println(a1); System.out.println(a2); } } class A { int x; public String toString() { return "A's x is " + x; } }
C. When executing System.out.println(a2), the toString() method in the Object class is invoked. D. When executing System.out.println(a1), the toString() method in the A class is invoked.
Given the following code, find the compile error? public class Test { public static void main(String[] args) { m(new GraduateStudent()); m(new Student()); m(new Person()); m(new Object()); } public static void m(Student x) { System.out.println(x.toString()); } } class GraduateStudent extends Student { } class Student extends Person { public String toString() { return "Student"; } } class Person extends Object { public String toString() { return "Person"; } }
C. m(new Person()) causes an error D. m(new Object()) causes an error
What modifier should you use on the members of a class so that they are not accessible to another class in a different package, but are accessible to any subclasses in any package?
C. protected
You can assign _________ to a variable of Object[] type.
D. new String[100] E. new java.util.Date[100]
The getValue() method is overridden in two ways. Which one is correct? I: public class Test { public static void main(String[] args) { A a = new A(); System.out.println(a.getValue()); } } class B { public String getValue() { return "Any object"; } } class A extends B { public Object getValue() { return "A string"; } } II: public class Test { public static void main(String[] args) { A a = new A(); System.out.println(a.getValue()); } } class B { public Object getValue() { return "Any object"; } } class A extends B { public String getValue() { return "A string"; } }
II
Which statements are most accurate regarding the following classes? class A { private int i; protected int j; } class B extends A { private int k; protected int m; // some methods omitted }
In the class B, an instance method can only access j, k, m.
What is the output of the following code? public class Test { public static void main(String[] args) { new Person().printPerson(); new Student().printPerson(); } } class Student extends Person { public String getInfo() { return "Student"; } } class Person { public String getInfo() { return "Person"; } public void printPerson() { System.out.println(getInfo()); } }
Person Student
Analyze the following code. // Program 1: public class Test { public static void main(String[] args) { Object a1 = new A(); Object a2 = new A(); System.out.println(a1.equals(a2)); } } class A { int x; public boolean equals(A a) { return this.x == a.x; } } // Program 2: public class Test { public static void main(String[] args) { A a1 = new A(); A a2 = new A(); System.out.println(a1.equals(a2)); } } class A { int x; public boolean equals(A a) { return this.x == a.x; } }
Program 1 displays false and Program 2 displays true
Analyze the following code. // Program 1: public class Test { public static void main(String[] args) { Object a1 = new A(); Object a2 = new A(); System.out.println(a1.equals(a2)); } } class A { int x; public boolean equals(Object a) { return this.x == ((A)a).x; } } // Program 2: public class Test { public static void main(String[] args) { Object a1 = new A(); Object a2 = new A(); System.out.println(a1.equals(a2)); } } class A { int x; public boolean equals(A a) { return this.x == a.x; } }
Program 1 displays true and Program 2 displays false
Analyze the following code. // Program 1 public class Test { public static void main(String[] args) { Object a1 = new A(); Object a2 = new A(); System.out.println(((A)a1).equals((A)a2)); } } class A { int x; public boolean equals(A a) { return this.x == a.x; } } // Program 2 public class Test { public static void main(String[] args) { A a1 = new A(); A a2 = new A(); System.out.println(a1.equals(a2)); } } class A { int x; public boolean equals(A a) { return this.x == a.x; } }
Program 1 displays true and Program 2 displays true
Analyze the following code: Circle c = new Circle (5); Cylinder c = cy;
The code has a compile error.
Analyze the following code: Cylinder cy = new Cylinder(1, 1); Circle c = cy;
The code is fine.
Analyze the following code: public class Test { public static void main(String[] args) { new B(); } } class A { int i = 7; public A() { setI(20); System.out.println("i from A is " + i); } public void setI(int i) { this.i = 2 * i; } } class B extends A { public B() { // System.out.println("i from B is " + i); } public void setI(int i) { this.i = 3 * i; } }
The constructor of class A is called and it displays "i from A is 60".
Analyze the following code: public class Test { public static void main(String[] args) { new B(); } } class A { int i = 7; public A() { System.out.println("i from A is " + i); } public void setI(int i) { this.i = 2 * i; } } class B extends A { public B() { setI(20); // System.out.println("i from B is " + i); } public void setI(int i) { this.i = 3 * i; } }
The constructor of class A is called and it displays "i from A is 7".
Analyze the following code: ArrayList<String> list = new ArrayList<String>(); list.add("Beijing"); list.add("Tokyo"); list.add("Shanghai"); list.set(3, "Hong Kong");
The last line in the code causes a runtime error because there is no element at index 3 in the array list. If you replace the last line by list.add(3, "Hong Kong"), the code will compile and run fine.
Analyze the following code: public class Test { public static void main(String[] args) { B b = new B(); b.m(5); System.out.println("i is " + b.i); } } class A { int i; public void m(int i) { this.i = i; } } class B extends A { public void m(String s) { } }
The method m is not overridden in B. B inherits the method m from A and defines an overloaded method m in B.
Suppose you create a class Square to be a subclass of GeometricObject. Analyze the following code: class Square extends GeometricObject { double length; Square(double length) { GeometricObject(length); } }
The program has a compile error because you attempted to invoke the GeometricObject class's constructor illegally.
A class design requires that a particular member variable must be accessible by any subclasses of this class, but otherwise not by classes which are not members of the same package. What should be done to achieve this?
The variable should be marked protected.
What modifier should you use on a class so that a class in the same package can access it but a class in a different package cannot access it?
Use the default modifier.
Which of the statements regarding the super keyword is incorrect?
You can use super.super.p to invoke a method in superclass's parent class.
Given the following classes and their objects: class C1 {}; class C2 extends C1 {}; class C3 extends C1 {}; C2 c2 = new C2(); C3 c3 = new C3(); Analyze the following statement: c2 = (C2)((C1)c3);
You will get a runtime error because you cannot cast objects from sibling classes.
The output from the following code is __________. java.util.ArrayList<String> list = new java.util.ArrayList<String>(); list.add("New York"); java.util.ArrayList<String> list1 = list; list.add("Atlanta"); list1.add("Dallas"); System.out.println(list1);
[New York, Atlanta, Dallas]
Given the following code: class C1 {} class C2 extends C1 { } class C3 extends C2 { } class C4 extends C1 {} C1 c1 = new C1(); C2 c2 = new C2(); C3 c3 = new C3(); C4 c4 = new C4();
c4 instanceof C2
What is output of the following code: ArrayList<String> list = new ArrayList<String>(); String s1 = new String("Java"); String s2 = new String("Java"); list.add(s1); list.add(s2); System.out.println((list.get(0) == list.get(1)) + " " + (list.get(0)).equals(list.get(1)));
false true
What is the output of the following code: public class Test { public static void main(String[] args) { String s1 = new String("Java"); String s2 = new String("Java"); System.out.print((s1 == s2) + " " + (s1.equals(s2))); } }
false true
Which of the following classes cannot be extended?
final class A { }
Object-oriented programming allows you to derive new classes from existing classes. This is called ____________.
inheritance
Which of the following are Java keywords?
instanceof
Given two reference variables t1 and t2, if t1.equals(t2) is true, t1 == t2
may be true or false
You can create an ArrayList using _________.
new ArrayList()
The visibility of these modifiers increases in this order:
private, none (if no modifier is used), protected, and public.
The equals method is defined in the Object class. Which of the following is correct to override it in the String class?
public boolean equals(Object other)
Analyze the following code: public class Test { public static void main(String[] args) { String s = new String("Welcome to Java"); Object o = s; String d = (String)o; } }
s, o, and d reference the same String object.
Inheritance means ______________.
that a class can extend another class.
Composition means ______________.
that a class contains a data field that references another object.
Polymorphism means ______________.
that a variable of supertype can refer to a subtype object.
Encapsulation means ______________.
that data fields should be declared private.
Given two reference variables t1 and t2, if t1 == t2 is true, t1.equals(t2) must be ___________.
true
What is output of the following code: ArrayList<java.util.Date> list = new ArrayList<java.util.Date>(); java.util.Date d = new java.util.Date(); list.add(d); list.add(d); System.out.println((list.get(0) == list.get(1)) + " " + (list.get(0)).equals(list.get(1)));
true true
Suppose ArrayList x contains two strings [Beijing, Singapore]. Which of the following methods will cause the list to become [Beijing, Chicago, Singapore]?
x.add(1, "Chicago")
Invoking _________ removes all elements in an ArrayList x.
x.clear()
Invoking _________ returns the first element in an ArrayList x.
x.get(0)
Suppose ArrayList x contains two strings [Beijing, Singapore]. Which of the following method will cause the list to become [Beijing]?
x.remove(1) x.remove("Singapore")
Invoking _________ returns the number of the elements in an ArrayList x.
x.size()
Suppose an ArrayList list contains {"red", "green", "red", "green"}. What is list after the following code? list.remove("red");
{"green", "red", "green"}
Suppose an ArrayList list contains {"red", "red", "green"}. What is list after the following code? String element = "red"; for (int i = 0; i < list.size(); i++) if (list.get(i).equals(element)) { list.remove(element); i--; }
{"green"}
Suppose an ArrayList list contains {"red", "red", "green"}. What is list after the following code? String element = "red"; for (int i = list.size() - 1; i >= 0; i--) if (list.get(i).equals(element)) list.remove(element);
{"green"}
Suppose an ArrayList list contains {"red", "red", "green"}. What is list after the following code? String element = "red"; for (int i = 0; i < list.size(); i++) if (list.get(i).equals(element)) list.remove(element);
{"red", "green"}