单选题1. import java.util.*;  2. class SubGen {  3. public static void main(String [] args) {  4. //insert code here  5. }  6. }  class Alpha { }  class Beta extends Alpha { }  class Gamma extends Beta { }  和四段代码片段:  s1. ArrayList〈? extends Alpha〉 list1 = ne

题目
单选题
1. import java.util.*;  2. class SubGen {  3. public static void main(String [] args) {  4. //insert code here  5. }  6. }  class Alpha { }  class Beta extends Alpha { }  class Gamma extends Beta { }  和四段代码片段:  s1. ArrayList〈? extends Alpha〉 list1 = new ArrayList〈Gamma〉();  s2. ArrayList〈Alpha〉 list2 = new ArrayList〈? extends Alpha〉();  s3. ArrayList〈? extends Alpha〉 list3 = new ArrayList〈? extends Beta〉();  s4. ArrayList〈? extends Beta〉 list4 = new ArrayList〈Gamma〉(); ArrayList〈? extends Alpha〉 list5 = list4;  哪些片段分别插入到第4行,可允许代码编译?()
A

只有s1

B

只有s3

C

只有s1和s3

D

只有s1和s4

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相似问题和答案

第1题:

现有包结构:  com |-- x | |-- Alpha.class | | | |-- y | |-- Beta.class | |-- Gamma.class  和类:  //insert code here  import com.*;  import com.x.y.*;  class Test { Alpha a; Beta b; Gamma c; }  哪两行分别插入后可允许代码编译?() 

  • A、package com.;
  • B、import com.x;
  • C、package com.x;
  • D、import com.x.Alpha;

正确答案:C,D

第2题:

现有:   1.  class HorseRadish {   2.    // insert code here   3.    protected HorseRadish(int x) {    4.      System.out.println("bok choy");  5.    }  6.  }   7.  class Wasabi extends HorseRadish {   8.    public static void main(String [] args) {   9.      Wasabi w = new Wasabi();  10.   }    11. }   分别插入到第 2 行,哪两项允许代码编译并产生"bok choy" 输出结果?() 

  • A、 // just a comment
  • B、 protected HorseRadish() { }
  • C、 protected HorseRadish() { this(42);}
  • D、 protected  HorseRadish() { new HorseRadish (42);}

正确答案:C,D

第3题:

1. import java.util.*;  2. class SubGen {  3. public static void main(String [] args) {  4. //insert code here  5. }  6. }  class Alpha { }  class Beta extends Alpha { }  class Gamma extends Beta { }  和四段代码片段:  s1. ArrayList〈? extends Alpha〉 list1 = new ArrayList〈Gamma〉();  s2. ArrayList〈Alpha〉 list2 = new ArrayList〈? extends Alpha〉();  s3. ArrayList〈? extends Alpha〉 list3 = new ArrayList〈? extends Beta〉();  s4. ArrayList〈? extends Beta〉 list4 = new ArrayList〈Gamma〉(); ArrayList〈? extends Alpha〉 list5 = list4;  哪些片段分别插入到第4行,可允许代码编译?()  

  • A、只有s1
  • B、只有s3
  • C、只有s1和s3
  • D、只有s1和s4

正确答案:D

第4题:

Given classes defined in two different files:  1. package util;  2. public class BitUtils {  3. private static void process(byte[] b) { }  4. }  1. package app;  2. public class SomeApp {  3. public static void main(String[] args) {  4. byte[] bytes = new byte[256];  5. // insert code here  6. }  7. }  What is required at line 5 in class SomeApp to use the process method of BitUtils?() 

  • A、 process(bytes);
  • B、 BitUtils.process(bytes);
  • C、 app.BitUtils.process(bytes);
  • D、 util.BitUtils.process(bytes);
  • E、 import util.BitUtils. *; process(bytes);
  • F、 SomeApp cannot use the process method in BitUtils.

正确答案:F

第5题:

1. class Pizza {  2. java.util.ArrayList toppings;  3. public final void addTopping(String topping) {  4. toppings.add(topping); 5. }  6. }  7. public class PepperoniPizza extends Pizza {  8. public void addTopping(String topping) {  9. System.out.println(”Cannot add Toppings”);  10. }  11. public static void main(String[] args) {  12. Pizza pizza = new PepperoniPizza();  13. pizza.addTopping(”Mushrooms”);  14. }  15. }  What is the result?() 

  • A、 Compilation fails.
  • B、 Cannot add Toppings
  • C、 The code runs with no output.
  • D、 A NullPointerException is thrown in Line 4.

正确答案:A

第6题:

1. public interface A {  2. public void doSomething(String thing);  3. }  1. public class AImpl implements A {  2. public void doSomething(String msg) { }  3. }  1. public class B {  2. public A doit() {  3. // more code here  4. }  5.  6. public String execute() { 7. // more code here  8. }  9. }  1. public class C extends B {  2. public AImpl doit() {  3. // more code here  4. }  5.  6. public Object execute() {  7. // more code here  8. }  9. }  Which statement is true about the classes and interfaces in the exhibit?() 

  • A、 Compilation will succeed for all classes and interfaces.
  • B、 Compilation of class C will fail because of an error in line 2.
  • C、 Compilation of class C will fail because of an error in line 6.
  • D、 Compilation of class AImpl will fail because of an error in line 2.

正确答案:C

第7题:

Which statements, when inserted at the indicated position in the following code, will cause a runtime exception when attempting to run the program?()   class A {}   class B extends A {}   class C extends A {}   public class Q3ae4 {   public static void main(String args[]) {   A x = new A();   B y = new B();   C z = new C();   // insert statement here   }   } 

  • A、x = y;
  • B、z = x;
  • C、y = (B) x;
  • D、z = (C) y;
  • E、y = (A) y;

正确答案:C

第8题:

10. class Foo {  11. static void alpha() { /* more code here */ }  12. void beta() { /* more code here */ }  13. }  Which two are true?()

  • A、 Foo.beta() is a valid invocation of beta().
  • B、 Foo.alpha() is a valid invocation of alpha().
  • C、 Method beta() can directly call method alpha().
  • D、 Method alpha() can directly call method beta().

正确答案:B,C

第9题:

1. class Alpha { void m1() {} }   2. class Beta extends Alpha { void m2() { } }   3. class Gamma extends Beta { }   4.   5. class GreekTest {   6. public static void main(String [] args) {   7. a Alpha [] a = {new Alpha(), new Beta(), new Gamma() };   8. for(Alpha a2 : a) {   9. a2.m1();   10. if (a2 instanceof Beta || a2 instanceof Gamma)   11. //insert code here   12. }   13. }   14. }   哪一行代码插入到第11行,将编译但是会在运行时产生异常?()  

  • A、 a2.m2();
  • B、 ((Beta)a2).m2();
  • C、 ((Alpha)a2).m2();
  • D、 ((Gamma)a2).m2();

正确答案:D

第10题:

Given classes defined in two different files:  1. package util;  2. public class BitUtils {  3. public static void process(byte[]) { /* more code here */ }  4. }  1. package app;  2. public class SomeApp {  3. public static void main(String[] args) {  4. byte[] bytes = new byte[256];  5. // insert code here  6. }  7. }  What is required at line 5 in class SomeApp to use the process method of BitUtils?() 

  • A、 process(bytes);
  • B、 BitUtils.process(bytes);
  • C、 util.BitUtils.process(bytes);
  • D、 SomeApp cannot use methods in BitUtils.
  • E、 import util.BitUtils.*; process(bytes);

正确答案:C

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