OCJP(1Z0-851) 類比題分析（六）

Exam : 1Z0-851

Java Standard Edition 6 Programmer Certified Professional Exam

以下分析全都是我自己分析或者參考網上的，定有疏漏，還請大家對我的分析提出質疑。

QUESTION 167
Given:
1. import java.util.*;
2. public class WrappedString {
3. private String s;
4. public WrappedString(String s) { this.s = s; }
5. public static void main(String[] args) {
6. HashSet<Object> hs = new HashSet<Object>();
7. WrappedString ws1 = new WrappedString("aardvark");
8. WrappedString ws2 = new WrappedString("aardvark");
9. String s1 = new String("aardvark");
10. String s2 = new String("aardvark");
11. hs.add(ws1); hs.add(ws2); hs.add(s1); hs.add(s2);
12. System.out.println(hs.size()); } }
What is the result?
A. 0
B. 1
C. 2
D. 3
E. 4
F. Compilation fails.
G. An exception is thrown at runtime.
Answer: D
WrappedString 的hashCode()是繼承自Object類，而String的hashCode()時重寫過的僅與字串的內容有關。ws1 和ws2儲存地址不同，所以hashCode()的傳回值不同，而s1和s2字串的內容相同，雜湊值相同。QUESTION 168
Given a class whose instances, when found in a collection of objects, are sorted by using the compareTo()
method, which two statements are true? (Choose two.)
A. The class implements java.lang.Comparable.
B. The class implements java.util.Comparator.
C. The interface used to implement sorting allows this class to define only one sort sequence.
D. The interface used to implement sorting allows this class to define many different sort sequences.
Answer: ACComparable介面僅有一個compareTo()函數~~謹記

QUESTION 169
Given:
1. import java.util.*;
2. public class Example {
3. public static void main(String[] args) {
4. // insert code here
5. set.add(new Integer(2));
6. set.add(new Integer(1));
7. System.out.println(set);
8. }
9. }
Which code, inserted at line 4, guarantees that this program will output [1, 2]?
A. Set set = new TreeSet();
B. Set set = new HashSet();
C. Set set = new SortedSet();
D. List set = new SortedList();
E. Set set = new LinkedHashSet();
Answer: A
必須是排好序的集合，所以TreeSet合適。而HashSet應該也可以的，Integer類型的hashCode()函數返回的是相應的int類型的值。如果就只有一個答案的話TreeSet是最好的選項。SortedSet是一個interface，java裡面沒有SortedList類。

QUESTION 171
DRAG DROP
Click the Task button.

Answer:
多態性~~a和b的getName()都是A類的方法，對這個方法來說看不到B中的name變數。QUESTION 177
Given:
1. class TestException extends Exception { }
2. class A {
3. public String sayHello(String name) throws TestException {
4. if(name == null) throw new TestException();
5. return "Hello " + name;
6. }
7. }
8. public class TestA {
9. public static void main(String[] args) {
10. new A().sayHello("Aiko");
11. }
12. }
Which statement is true?
A. Compilation succeeds.
B. Class A does not compile.
C. The method declared on line 9 cannot be modified to throw TestException.
D. TestA compiles if line 10 is enclosed in a try/catch block that catches TestException.
Answer: D
第十行 new A().sayHello("Aiko");會拋出一個TestException()，所以要麼throws TestException。，要麼try{} catch(TestException e){}

QUESTION 178
Given:
11. public static void main(String[] args) {
12. for (int i = 0; i <= 10; i++) {
13. if (i > 6) break;
14. }
15. System.out.println(i);
16. }
What is the result?
A. 6
B. 7
C. 10
D. 11
E. Compilation fails.
F. An exception is thrown at runtime.
Answer: E
i的範圍QUESTION 179
Given:
3. public class Breaker {
4. static String o = "";
5. public static void main(String[] args) {
6. z:
7. o = o + 2;
8. for(int x = 3; x < 8; x++) {
9. if(x==4) break;
10. if(x==6) break z;
11. o = o + x;
12. }
13. System.out.println(o);
14. }
15. }
What is the result?
A. 23
B. 234
C. 235
D. 2345
E. 2357
F. 23457
G. Compilation fails.
Answer: G
斷點的label只能出現在for迴圈的前一句或者while迴圈的前一句~~

QUESTION 180
Given:
5. class A {
6. void foo() throws Exception { throw new Exception(); }
7. }
8. class SubB2 extends A {
9. void foo() { System.out.println("B "); }
10. }
11. class Tester {
12. public static void main(String[] args) {
13. A a = new SubB2();
14. a.foo();
15. }
16. }
What is the result?
A. B
B. B, followed by an Exception.
C. Compilation fails due to an error on line 9.
D. Compilation fails due to an error on line 14.
E. An Exception is thrown with no other output.
Answer: D
編譯器會報錯：Unhandled exception type Exception必須按照以下格式來：

 try {
a.foo();
} catch (Exception e) {
// TODO Auto-generated catch block
e.printStackTrace();
}

或者throws一個Exception。這種多態性在編譯的時候是完全按照 父類的方法來的，即在compiler的時候檢查foo()方法是否在 A類中，如果在那麼就按照父類的要求拋出異常或者捕獲異常，在runtime的時候再調用SubB2 的foo()方法。如果子類的方法允許拋出一個父類沒有的異常，那麼就沒有異常處理器來處理此異常，所以，不允許子類方法拋出父類沒有的異常。

QUESTION 182
Given:
1. public class Mule {
2. public static void main(String[] args) {
3. boolean assert = true;
4. if(assert) {
5. System.out.println("assert is true");
6. }
7. }
8. }
Which command-line invocations will compile?
A. javac Mule.java
B. javac -source 1.3 Mule.java
C. javac -source 1.4 Mule.java
D. javac -source 1.5 Mule.java
Answer: B
斷言引入是在JDK SE 1.4中，所以在之前assert不被當作保留字。

QUESTION 183
Given:
11. static void test() {
12. try {
13. String x = null;
14. System.out.print(x.toString() + " ");
15. }
16. finally { System.out.print("finally "); }
17. }
18. public static void main(String[] args) {
19. try { test(); }
20. catch (Exception ex) { System.out.print("exception "); }
21. }
What is the result?
A. null
B. finally
C. null finally
D. Compilation fails.
E. finally exception
Answer: E
x.toString() 會拋出一個NulPointerExceptionQUESTION 184
Given:
1. public class Boxer1{
2. Integer i;
3. int x;
4. public Boxer1(int y) {
5. x = i+y;
6. System.out.println(x);
7. }
8. public static void main(String[] args) {
9. new Boxer1(new Integer(4));
10. }
11. }
What is the result?
A. The value "4" is printed at the command line.
B. Compilation fails because of an error in line 5.
C. Compilation fails because of an error in line 9.
D. A NullPointerException occurs at runtime.
E. A NumberFormatException occurs at runtime.
F. An IllegalStateException occurs at runtime.
Answer: D
問題出在執行第五行的時候。

QUESTION 185
Which two code fragments are most likely to cause a StackOverflowError? (Choose two.)
A. int []x = {1,2,3,4,5};
for(int y = 0; y < 6; y++)
System.out.println(x[y]);
B. static int[] x = {7,6,5,4};
static { x[1] = 8;
x[4] = 3; }
C. for(int y = 10; y < 10; y++)
doStuff(y);
D. void doOne(int x) { doTwo(x); }
void doTwo(int y) { doThree(y); }
void doThree(int z) { doTwo(z); }
E. for(int x = 0; x < 1000000000; x++)
doStuff(x);
F. void counter(int i) { counter(++i); }
Answer: DF
不斷迭代就能StackOverflowError

QUESTION 186
Given:
11. static void test() throws RuntimeException {
12. try {
13. System.out.print("test ");
14. throw new RuntimeException();
15. }
16. catch (Exception ex) { System.out.print("exception "); }
17. }
18. public static void main(String[] args) {
19. try { test(); }
20. catch (RuntimeException ex) { System.out.print("runtime "); }
21. System.out.print("end ");
22. }
What is the result?
A. test end
B. Compilation fails.
C. test runtime end
D. test exception end
E. A Throwable is thrown by main at runtime.
Answer: D
第14行拋出的RuntimeException被16行處理了，就沒20行什麼事了。

QUESTION 187
Given:
11. public static void main(String[] args) {
12. Integer i = new Integer(1) + new Integer(2);
13. switch(i) {
14. case 3: System.out.println("three"); break;
15. default: System.out.println("other"); break;
16. }
17. }
What is the result?
A. three
B. other
C. An exception is thrown at runtime.
D. Compilation fails because of an error on line 12.
E. Compilation fails because of an error on line 13.
F. Compilation fails because of an error on line 15.
Answer: A

QUESTION 188
Given:
21. class Money {
22. private String country = "Canada";
23. public String getC() { return country; }
24. }
25. class Yen extends Money {
26. public String getC() { return super.country; }
27. }
28. public class Euro extends Money {
29. public String getC(int x) { return super.getC(); }
30. public static void main(String[] args) {
31. System.out.print(new Yen().getC() + " " + new Euro().getC());
32. }
33. }
What is the result?
A. Canada
B. null Canada
C. Canada null
D. Canada Canada
E. Compilation fails due to an error on line 26.
F. Compilation fails due to an error on line 29.
Answer: E
super加點只能調用父類的非私人方法~~

QUESTION 189
Given:
11. class ClassA {}
12. class ClassB extends ClassA {}
13. class ClassC extends ClassA {}
and:
21. ClassA p0 = new ClassA();
22. ClassB p1 = new ClassB();
23. ClassC p2 = new ClassC();
24. ClassA p3 = new ClassB();
25. ClassA p4 = new ClassC();
Which three are valid? (Choose three.)
A. p0 = p1;
B. p1 = p2;
C. p2 = p4;
D. p2 = (ClassC)p1;
E. p1 = (ClassB)p3;
F. p2 = (ClassC)p4;
Answer: AEF

QUESTION 190
Which three statements are true? (Choose three.)
A. A final method in class X can be abstract if and only if X is abstract.
B. A protected method in class X can be overridden by any subclass of X.
C. A private static method can be called only within other static methods in class X.
D. A non-static public final method in class X can be overridden in any subclass of X.
E. A public static method in class X can be called by a subclass of X without explicitly referencing the
class X.
F. A method with the same signature as a private final method in class X can be implemented in a
subclass of X.
G. A protected method in class X can be overridden by a subclass of X only if the subclass is in the same
package as X.
Answer: BEF
A：final方法不可是抽象方法。C:Class X中的非靜態方法也可以哦D:final方法不可重寫哦F:正確，因為private final方法不會被子類繼承，所以在子類中可以有一個相同簽名的方法。 注意這裡不同於重寫overrideG:proteted的話不是一個包照樣可以

QUESTION 191
Given:
10. interface A { void x(); }
11. class B implements A { public void x() {} public void y() {} }
12. class C extends B { public void x() {} }
And:
20. java.util.List<A> list = new java.util.ArrayList<A>();
21. list.add(new B());
22. list.add(new C());
23. for (A a : list) {
24. a.x();
25. a.y();
26. }
What is the result?
A. The code runs with no output.
B. An exception is thrown at runtime.
C. Compilation fails because of an error in line 20.
D. Compilation fails because of an error in line 21.
E. Compilation fails because of an error in line 23.
F. Compilation fails because of an error in line 25.
Answer: F
A介面沒有y()方法。

QUESTION 192
Given:
1. package test;
2.
3. class Target {
4. public String name = "hello";
5. }
What can directly access and change the value of the variable name?
A. any class
B. only the Target class
C. any class in the test package
D. any class that extends Target
Answer: C
Target 是預設修飾符修飾，包級可見度~~

QUESTION 193
Click the Exhibit button. What two must the programmer do to correct the compilation errors? (Choose
two.)
A. insert a call to this() in the Car constructor
B. insert a call to this() in the MeGo constructor
C. insert a call to super() in the MeGo constructor
D. insert a call to super(vin) in the MeGo constructor
E. change the wheelCount variable in Car to protected
F. change line 3 in the MeGo class to super.wheelCount = 3;
Answer: DE
MeGo會調用Car無參數建構函式~~又是這個問題wheelCount必須被MeGo類可見才是~~

QUESTION 194
A team of programmers is involved in reviewing a proposed design for a new utility class. After some
discussion, they realize that the current design allows other classes to access methods in the utility class
that should be accessible only to methods within the utility class itself. What design issue has the team
discovered?
A. Tight coupling
B. Low cohesion
C. High cohesion
D. Loose coupling
E. Weak encapsulation
F. Strong encapsulation
Answer: E
弱封裝，沒有好好把這個類封裝起來~~

QUESTION 195
Given:
5. class Thingy { Meter m = new Meter(); }
6. class Component { void go() { System.out.print("c"); } }
7. class Meter extends Component { void go() { System.out.print("m"); } }
8.
9. class DeluxeThingy extends Thingy {
10. public static void main(String[] args) {
11. DeluxeThingy dt = new DeluxeThingy();
12. dt.m.go();
13. Thingy t = new DeluxeThingy();
14. t.m.go();
15. }
16. }
Which two are true? (Choose two.)
A. The output is mm.
B. The output is mc.
C. Component is-a Meter.
D. Component has-a Meter.
E. DeluxeThingy is-a Component.
F. DeluxeThingy has-a Component.
Answer: AF
注意dt.m還有t.m都是Meter類的執行個體~~

QUESTION 196
Given:
10. interface Jumper { public void jump(); } ...
20. class Animal {} ...
30. class Dog extends Animal {
31. Tail tail; 32. } ...
40. class Beagle extends Dog implements Jumper{
41. public void jump() {}
42. } ...
50. class Cat implements Jumper{
51. public void jump() {}
52. }
Which three are true? (Choose three.)
A. Cat is-a Animal
B. Cat is-a Jumper
C. Dog is-a Animal
D. Dog is-a Jumper
E. Cat has-a Animal
F. Beagle has-a Tail
G. Beagle has-a Jumper
Answer: BCF

QUESTION 197
Click the Exhibit button. What is the result?
A. Value is: 8
B. Compilation fails.
C. Value is: 12
D. Value is: -12
E. The code runs with no output.
F. An exception is thrown at runtime.
Answer: A

:
QUESTION 198
Given a valid DateFormat object named df, and
16. Date d = new Date(0L);
17. String ds = "December 15, 2004";
18. // insert code here What updates d's value with the date represented by ds?
A. 18. d = df.parse(ds);
B. 18. d = df.getDate(ds);
C. 18. try {
19. d = df.parse(ds);
20. } catch(ParseException e) { };
D. 18. try {
19. d = df.getDate(ds);
20. } catch(ParseException e) { };
Answer: C
parse函數用來Parses text from the beginning of the given string to produce a date.沒有getDate函數~~

QUESTION 199
Which two scenarios are NOT safe to replace a StringBuffer object with a StringBuilder object? (Choose
two.)
A. When using versions of Java technology earlier than 5.0.
B. When sharing a StringBuffer among multiple threads.
C. When using the java.io class StringBufferInputStream.
D. When you plan to reuse the StringBuffer to build more than one string.
Answer: AB
StringBuffer 是安全執行緒的，而Stringbuilder不是。StringBuffer是在JDK1.0加入的，而StringBuilder是在JDK 1.5時候加入的~~