Java題庫——Chapter8 對象和類

1)________ represents an entity（實體） in the real world that can be distinctly identified. 1) _______
A)A data field B) An object
C)A method D) A class

2)________ is a construct that defines objects of the same type.  2) _______
A)A method B) A data field
C)A class D) An object

3)An object is an instance of a ________. 3) _______
A)data B) method C) class D) program

對象是類的一個實例

4)The keyword ________ is required to declare a class.  4) _______
A)private  B) class  C)public  D) All of the above.

5)________ is invoked to create an object. 5) _______
A)A constructor
B)A method with the void return type
C)The main method
D)A method with a return type

6)Which of the following statements are true? (Choose all that apply.) 6) _______
A)Constructors must have the same name as the class itself.  
B)Constructors do not have a return type, not even void.  
C)Constructors are invoked using the new operator when an object is created. 
D)A default no-arg constructor is provided automatically if no constructors are explicitly declared in the class.
E)At least one constructor must always be defined explicitly. 必須始終顯式定義至少一個構造函數。

構造函數是在創建一個對象時由new 操作符調用的。構造函數可以不顯式定義

7) Analyze the following code: (Choose all that apply.)

public class Test {
  public static void main(String[ ] args) {
    A a = new A();
    a.print();
  }
}
class A {
  String s;
  A(String s) {
    this.s = s;
  }
  void print() {
    System.out.println(s);
  }
}

A)The program has a compilation error because class A does not have a default constructor.
B)The program has a compilation error because class A is not a public class.
C)The program would compile and run if you change A a = new A() to A a = new A("5").
D)The program compiles and runs fine and prints nothing.

當且僅當類中沒有明確定義任何構造函數的時候才會自動提供一個缺省構造函數。該類中有了構造函數，不會再提供缺省構造函數了。

8)What is wrong in the following code?

class TempClass {
  int i;
  public void TempClass(int j) {
    int i = j;
  }
}
public class C {
  public static void main(String[ ] args) {
    TempClass temp = new TempClass(2);
  }
}

A)The program has a compilation error because TempClass does not have a constructor with an int argument.
B)The program has a compilation error because TempClass does not have a default constructor.
C)The program compiles and runs fine.
D)The program compiles fine, but it does not run because class C is not public.

注意這里的TempClass的TempClass函數並不是構造函數，該函數有void返回值！！！

9)Given the declaration Circle x = new Circle(), which of the following statement is most accurate?
A)You can assign an int value to x.
B)x contains an int value.
C)x contains a reference to a Circle object.
D)x contains an object of the Circle type.

10)Analyze the following code.

public class Test {
  int x;
  public Test(String t) {
     System.out.println("Test");
  }
  public static void main(String[ ] args) {
    Test test = null;
    System.out.println(test.x);
  }
}

A)The program has a runtime NullPointerException because test is null while executing test.x.
B)The program has a compile error because you cannot create an object from the class that defines the object.
C)The program has a compile error because x has not been initialized.
D)The program has a compile error because test is not initialized.
E)The program has a compile error because Test does not have a default constructor.

11)The default value for data field of a boolean type, numeric type, object type is ________, respectively. 11) ______
A)false, 0, null  B)true, 1, null  C)true, 1, Null  D)false, 1, null  E)true, 0, null

12)Which of the following statements are true? (Choose all that apply.) 12) ______
A)You may assign an int value to a reference variable.
B)Data fields have default values.
C)Local variables do not have default values.
D)A variable of a primitive（原始的、基本的） type holds a value of the primitive type.
E)A variable of a reference type holds a reference to where an object is stored in the memory.

D基本數據類型的變量含有基本數據類型的值

13)Analyze the following code:

public class Test {
  public static void main(String[ ] args) {
    double radius;
    final double PI= 3.15169;
    double area = radius * radius * PI;
    System.out.println("Area is " + area);
  }
}

A)The program has no compile errors but will get a runtime error because radius is not initialized.
B)The program has compile errors because the variable radius is not initialized.
C)The program has a compile error because a constant PI is defined inside a method.
D)The program compiles and runs fine.

14)Analyze the following code.

public class Test {
  int x;
  public Test(String t) {
     System.out.println("Test");
  }
  public static void main(String[ ] args) {
    Test test = new Test();
    System.out.println(test.x);
  }
}

A)The program has a compile error because you cannot create an object from the class that defines the object.
B)The program has a compile error because System.out.println method cannot be invoked from the constructor.  
C)The program has a compile error because x has not been initialized.
D)The program has a compile error because Test does not have a default constructor.

將這道題變化一下

public class Test {
  int x;
  public Test(String t) {
     System.out.println("Test");
  }
  public static void main(String[] args) {
    Test test = null;
    System.out.println(test.x);
  }
}

a.    The program has a compile error because test is not initialized.
b.    The program has a compile error because x has not been initialized.
c.    The program has a compile error because you cannot create an object from the class that defines the object.
d.    The program has a compile error because Test does not have a default constructor.
e.    The program has a runtime NullPointerException because test is null while executing test.x.
Key:e

現在由於test=null,造成了空指針異常

15)Suppose TestCircle and Circle in Listing 7.1 in the textbook are in two separate files named TestCircle.java and Circle.java, respectively. What is the outcome of compiling TestCircle.java and then Circle.java? 15) ______
A)Neither compiles successfully.
B)Only TestCircle.java compiles.
C)Only Circle.java compiles.
D)Both compile fine.

16)Which of the following statement is most accurate? (Choose all that apply.) 16) ______
A)An object may contain other objects.
B)An object may contain the references of other objects.
C)A reference variable is an object.
D)A reference variable refers to an object.

一個對象中可能包含其他對象的引用，一個引用變量指向一個對象。

17)The java.util.Date class is introduced in this section. Which of the following code creates an object of the Date class? (Choose all that apply.)
A:

public class Test {
  public Test() {
    new java.util.Date();
  }
}

B:

public class Test {
  public Test() {
    java.util.Date date = new java.util.Date();
  }
}

A)A B) B C) neither

18)Which of the following statements are correct? (Choose all that apply.) 18) ______
A)The nextInt() method in the Random class returns the next random int value.
B)The nextDouble() method in the Random class returns the next random double value.
C)When creating a Random object, you have to specify（指定） the seed or use the default seed.
D)If two Random objects have the same seed, the sequence of the random numbers obtained from these two objects are identical（相同的）.

創建一個Random對象時，必須指定一個種子或使用默認的種子。種子是一個用於初始化隨機數字生成器的數字。無參構造方式使用當前已經逝去的時間作為種子，創建一個Random對象。如果這兩個Random對象有相同的種子，那么它們將產生相同的數列。

19)How many JFrame objects can you create and how many can you display? 19) ______
A)one  B) three  C)two  D) unlimited

20)Variables that are shared by every instances of a class are ________. 20) ______
A)public variables  B) class variables  C)private variables  D) instance variables

21)You should add the static keyword in the place of ? in line ________ in the following code:
1 public class Test {
2   private int age;
3
4   public ? int square(int n) {
5     return n * n;
6   }
7
8   public ? int getAge() {
9  }
10}
A)in line 4 B) in both line 4 and line 8 C)in line 8 D) none

22)A method that is associated with an individual object is called ________. 22) ______
A)an object method B) an instance method C)a static method D) a class method

調用與單個對象關聯的方法稱為實例方法，只有在實例被創建后才能使用。

23)To declare a constant MAX_LENGTH as a member of the class, you write 23) ______
A)static double MAX_LENGTH = 99.98;
B)final static float MAX_LENGTH = 99.98;
C)final static double MAX_LENGTH = 99.98;
D)final static MAX_LENGTH = 99.98;
E)final double MAX_LENGTH = 99.98;
類中的常量是被類的所有對象所共享的。注意要有關鍵詞final static和變量數據類型double

24)Analyze the following code.

public class Test {
  public static void main(String[ ] args) {
    int n = 2;
    xMethod(n);
    System.out.println("n is " + n);
  }
  void xMethod(int n) {
    n++;
  }
}

A)The code prints n is 2.
B)The code has a compile error because xMethod does not return a value.
C)The code prints n is 1.
D)The code prints n is 3.
E)The code has a compile error because xMethod is not declared static.  

該類由於沒有創建實例，必須要用靜態方法 xMethod前需要加static

25)What is the printout of the second println statement in the main method?

public class Foo {
  int i;
  static int s;
  public static void main(String[ ] args) {
    Foo f1 = new Foo();
    System.out.println("f1.i is " + f1.i + " f1.s is " + f1.s);
    Foo f2 = new Foo();
    System.out.println("f2.i is " + f2.i + " f2.s is " + f2.s);
    Foo f3 = new Foo();
    System.out.println("f3.i is " + f3.i + " f3.s is " + f3.s);
  }
  public Foo() {
    i++;
    s++;
  }
}

A)f2.i is 1 f2.s is 1 B) f2.i is 1 f2.s is 2 C)f2.i is 2 f2.s is 1  D) f2.i is 2 f2.s is 2

注意到s是靜態變量，被該類的所有對象所共享，而main方法也是一個靜態方法。
26)What is the printout of the third println statement in the main method?

public class Foo {
  int i;
  static int s;
  public static void main(String[ ] args) {
    Foo f1 = new Foo();
    System.out.println("f1.i is " + f1.i + " f1.s is " + f1.s);
    Foo f2 = new Foo();
    System.out.println("f2.i is " + f2.i + " f2.s is " + f2.s);
    Foo f3 = new Foo();
    System.out.println("f3.i is " + f3.i + " f3.s is " + f3.s);
  }
  public Foo() {
    i++;
    s++;
  }
}

A)f3.i is 3 f3.s is 3 B)f3.i is 1 f3.s is 3 C)f3.i is 1 f3.s is 1 D)f3.i is 3 f3.s is 1 E)f3.i is 1 f3.s is 2

27)What code may be filled in the blank without causing syntax or runtime errors:

public class Test {
  java.util.Date date;
  public static void main(String[ ] args) {
    Test test = new Test();
    System.out.println(________);
  }
}

A)date B) test.date C)test.date.toString() D) date.toString()

不能使用toString()的原因是沒有創建一個指向date對象的引用。

28)To prevent a class from being instantiated（實例化）, ________ 28) ______
A)use the public modifier on the constructor.
B)don't use any modifiers on the constructor.
C)use the static modifier on the constructor.
D)use the private modifier on the constructor.

為了防止類被實例化，要使用private修飾構造函數。

29)Analyze the following code:

public class Test {
  public static void main(String args[ ]) {
    NClass nc = new NClass();
    nc.t = nc.t++;
  }
}
class NClass {
  int t;
  private NClass() {
  }
}

A)The program has a compilation error because the NClass class has a private constructor.
B)The program compiles and runs fine.
C)The program does not compile because the parameter list of the main method is wrong.
D)The program compiles, but has a runtime error because t has no initial value.

接着上面的題，使用private修飾構造函數，該類就無法被實例化了！

30)Analyze the following code:

public class Test {
  private int t;
  public static void main(String[ ] args) {
    int x;
    System.out.println(t);
  }
}

A)The variable t is not initialized and therefore causes errors.
B)The variable t is private and therefore cannot be accessed in the main method.
C)t is non-static and it cannot be referenced in a static context in the main method.
D)The program compiles and runs fine.
E)The variable x is not initialized and therefore causes errors.

31)Analyze the following code and choose the best answer:

public class Foo {
  private int x;
  public static void main(String[ ] args) {
    Foo foo = new Foo();
    System.out.println(foo.x);
  }
}

A)Since x is an instance variable, it cannot be directly used inside a main method. However, it can be accessed through an object such as foo in this code.  
B)You cannot create a self-referenced object; that is, foo is created inside the class Foo.
C)Since x is defined in the class Foo, it can be accessed by any method inside the class without using an object. You can write the code to access x without creating an object such as foo in this code.
D)Since x is private, it cannot be accessed from an object foo.

不創建對象，將無法訪問x，因為x不是static類型

32)Which of the following statements are true? (Choose all that apply.) 32) ______
A)Encapsulating data fields makes the program easy to maintain.
B)Encapsulating data fields makes the program short.
C)Encapsulating data fields helps prevent programming errors.
D)Use the private modifier to encapsulate data fields.

封裝數據字段有助於防止編程錯誤

33)Suppose you wish to provide an accessor method for a boolean property finished, what signature of the method should be? 33) ______
A)public void getFinished()
B)public void isFinished()
C)public boolean isFinished()
D)public boolean getFinished()

34)Which is the advantage of encapsulation? 34) ______
A)It changes the implementation without changing a class's contract and causes no consequential changes to other code.  
B)Only public methods are needed.  
C)Making the class final causes no consequential changes to other code.  
D)It changes a class's contract without changing the implementation and causes no consequential changes to other code.  

在不更改類契約的情況下更改實現，並且不會對其他代碼造成相應的更改。

35)When invoking a method with an object argument,  ________ is passed. 35) ______
A)the contents of the object
B)the reference of the object
C)a copy of the object
D)the object is copied, then the reference of the copied object

36)What is the value of myCount.count displayed?

public class Test {
  public static void main(String[ ] args) {
    Count myCount = new Count();
    int times = 0;
    for (int i=0; i<100; i++)
      increment(myCount, times);
    System.out.println(
      "myCount.count =   " + myCount.count);
    System.out.println("times = "+ times);
  }
  public static void increment(Count c, int times) {
    c.count++;
    times++;
  }
}
class Count {
  int count;
  Count(int c) {
    count = c;
  }
  Count() {
    count = 1;
  }
}

A)101 B) 99 C) 98 D) 100

37)What is the value of times displayed?

public class Test {
  public static void main(String[ ] args) {
    Count myCount = new Count();
    int times = 0;
    for (int i=0; i<100; i++)
      increment(myCount, times);
    System.out.println(
      "myCount.count =   " + myCount.count);
    System.out.println("times = "+ times);
  }
  public static void increment(Count c, int times) {
    c.count++;
    times++;
  }
}
class Count {
  int count;
  Count(int c) {
    count = c;
  }
  Count() {
    count = 1;
  }
}

A)99 B) 100 C) 98 D) 0 E) 101

注意這里time傳遞給方法進行的是值傳遞！方法結束后不會改變原來的值。

38)What is the output of the following program?

import java.util.Date;
public class Test {
  public static void main(String[ ] args) {
    Date date = new Date(1234567);
    m1(date);
    System.out.print(date.getTime() + " ");
    m2(date);
    System.out.println(date.getTime());
  }
  public static void m1(Date date) {
    date = new Date(7654321);
  }
  public static void m2(Date date) {
    date.setTime(7654321);
  }
}

A)7654321 1234567  B) 1234567 1234567 C)7654321 7654321  D) 1234567 7654321

39)Given the declaration Circle[ ] x = new Circle[10], which of the following statement is most accurate? 39) ______
A)x contains a reference to an array and each element in the array can hold a Circle object.
B)x contains an array of ten objects of the Circle type.
C)x contains a reference to an array and each element in the array can hold a reference to a Circle object.
D)x contains an array of ten int values.

40)Assume java.util.Date[ ] dates = new java.util.Date[10], which of the following statements are true? (Choose all that apply.) 40) ______
A)dates = new Date() is fine, which creates a new Date object and assigns to dates.
B)dates is null.
C)dates = new java.util.Date[5] is fine, which assigns a new array to dates.
D)dates[0] is null.