OCJP(1Z0-851) 類比題分析（七）

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 201

Given:
1. public class TestString3 {
2. public static void main(String[] args) {
3. // insert code here
5. System.out.println(s);
6. }
7. }
Which two code fragments, inserted independently at line 3, generate the output 4247? (Choose two.)
A. String s = "123456789";
s = (s-"123").replace(1,3,"24") - "89";
B. StringBuffer s = new StringBuffer("123456789");
s.delete(0,3).replace(1,3,"24").delete(4,6);
C.  StringBuffer s = new StringBuffer("123456789");
s.substring(3,6).delete(1,3).insert(1, "24");
D.StringBuilder s = new StringBuilder("123456789");
s.substring(3,6).delete(1,2).insert(1, "24");
E.StringBuilder s = new StringBuilder("123456789");
s.delete(0,3).delete(1,3).delete(2,5).insert(1, "24");
Answer: BE

substring(int start, int end)函數是StringBuffer類中的一個方法，是返回特定索引（index從start到end-1）的子字串，還有一個方法是public StringBuffer delete(int start, int end)，是刪除特定索引（index從start到end-1）的字元。B和E都可以輸出4247。

QUESTION 202
Given:
11. String test = "Test A. Test B. Test C.";
12. // insert code here
13. String[] result = test.split(regex);
Which regular expression, inserted at line 12, correctly splits test into "Test A", "Test B", and "Test C"?
A. String regex = "";
B. String regex = " ";
C. String regex = ".*";
D. String regex = "\\s";
E. String regex = "\\.\\s*";
F. String regex = "\\w[ \.] +";
Answer: E

點和空格，選E，就說一個點 和 一個或者沒有空格

QUESTION 203
Which statement is true?
A. A class's finalize() method CANNOT be invoked explicitly.
B. super.finalize() is called implicitly by any overriding finalize() method.
C. The finalize() method for a given object is called no more than once by the garbage collector.
D. The order in which finalize() is called on two objects is based on the order in which the two objects
became finalizable.
Answer: C

finalize() 方法僅僅會在垃圾收集之前被調用，如果沒有垃圾收集自然就不會被調用。

finalize()方法可以被顯式調用，如super.finalize()。子類中重寫的finalize()方法不會隱式調用父類的finalize()，需要程式員自己super.finalize()。

QUESTION 204
Given:
11. public class ItemTest {
12. private final int id;
13. public ItemTest(int id) { this.id = id; }
14. public void updateId(int newId) { id = newId; }
15.
16. public static void main(String[] args) {
17. ItemTest fa = new ItemTest(42);
18. fa.updateId(69);
19. System.out.println(fa.id);
20. }
21. }
What is the result?
A. Compilation fails.
B. An exception is thrown at runtime.
C. The attribute id in the ItemTest object remains unchanged.
D. The attribute id in the ItemTest object is modified to the new value.
E. A new ItemTest object is created with the preferred value in the id attribute.
Answer: A

final變數一旦賦值，不能修改~~

QUESTION 205
Given:
11. interface DeclareStuff {
12. public static final int EASY = 3;
13. void doStuff(int t); }
14. public class TestDeclare implements DeclareStuff {
15. public static void main(String [] args) {
16. int x = 5;
17. new TestDeclare().doStuff(++x);
18. }
19. void doStuff(int s) {
20. s += EASY + ++s;
21. System.out.println("s " + s);
22. }
23. }
What is the result?
A. s 14
B. s 16
C. s 10
D. Compilation fails.
E. An exception is thrown at runtime.
Answer: D

doStuff()函數的存取修飾詞被reduce了~~interface是pubic void doStuff()，到了TestDeclare裡面就成了預設的~~

QUESTION 207
Given:
11. public class Commander {
12. public static void main(String[] args) {
13. String myProp = /* insert code here */
14. System.out.println(myProp);
15. }
16. }
and the command line:
java -Dprop.custom=gobstopper Commander Which two, placed on line 13, will produce the output
gobstopper? (Choose two.)
A. System.load("prop.custom");
B. System.getenv("prop.custom");
C. System.property("prop.custom");
D. System.getProperty("prop.custom");
E. System.getProperties().getProperty("prop.custom");
Answer: DE

QUESTION 208
Given:
1. interface DoStuff2 {
2. float getRange(int low, int high); }
3.
4. interface DoMore {
5. float getAvg(int a, int b, int c); }
6.
7. abstract class DoAbstract implements DoStuff2, DoMore { }
8.
9. class DoStuff implements DoStuff2 {
10. public float getRange(int x, int y) { return 3.14f; } }
11.
12. interface DoAll extends DoMore {
13. float getAvg(int a, int b, int c, int d); }
What is the result?
A. The file will compile without error.
B. Compilation fails. Only line 7 contains an error.
C. Compilation fails. Only line 12 contains an error.
D. Compilation fails. Only line 13 contains an error.
E. Compilation fails. Only lines 7 and 12 contain errors.
F. Compilation fails. Only lines 7 and 13 contain errors.
G. Compilation fails. Lines 7, 12, and 13 contain errors.

!!!一定要注意！這個特例，abstract類在implements介面的時候可以不實現，而是留給其子類去實現！！！！

編譯成功！！

QUESTION 211
Given:
1. package com.company.application;
2.
3. public class MainClass {
4. public static void main(String[] args) {}
5. }
And MainClass exists in the /apps/com/company/application directory. Assume the CLASSPATH
environment variable is set to "." (current directory). Which two java commands entered at the command
line will run MainClass? (Choose two.)
A. java MainClass if run from the /apps directory
B. java com.company.application.MainClass if run from the /apps directory
C. java -classpath /apps com.company.application.MainClass if run from any directory
D. java -classpath . MainClass if run from the /apps/com/company/application directory
E. java -classpath /apps/com/company/application:. MainClass if run from the /apps directory
F. java com.company.application.MainClass if run from the /apps/com/company/application directory
Answer: BC

QUESTION 212
Given:
12. import java.util.*;
13. public class Explorer2 {
14. public static void main(String[] args) {
15. TreeSet<Integer> s = new TreeSet<Integer>();
16. TreeSet<Integer> subs = new TreeSet<Integer>();
17. for(int i = 606; i < 613; i++)
18. if(i%2 == 0) s.add(i);
19. subs = (TreeSet)s.subSet(608, true, 611, true);
20. s.add(629);
21. System.out.println(s + " " + subs);
22. }
23. }
What is the result?
A. Compilation fails.
B. An exception is thrown at runtime.
C. [608, 610, 612, 629] [608, 610]
D. [608, 610, 612, 629] [608, 610, 629]
E. [606, 608, 610, 612, 629] [608, 610]
F. [606, 608, 610, 612, 629] [608, 610, 629]
Answer: E

QUESTION 213
Given that the elements of a PriorityQueue are ordered according to natural ordering, and:
2. import java.util.*;
3. public class GetInLine {
4. public static void main(String[] args) {
5. PriorityQueue<String> pq = new PriorityQueue<String>();
6. pq.add("banana");
7. pq.add("pear");
8. pq.add("apple");
9. System.out.println(pq.poll() + " " + pq.peek());
10. }
11. }
What is the result?
A. apple pear
B. banana pear
C. apple apple
D. apple banana
E. banana banana
Answer: D

pq.poll()和 pq.peek()，前者是取出第一個並返回，後者是返回第一個的副本

QUESTION 214
Given a pre-generics implementation of a method:
11. public static int sum(List list) {
12. int sum = 0;
13. for ( Iterator iter = list.iterator(); iter.hasNext(); ) {
14. int i = ((Integer)iter.next()).intValue();
15. sum += i;
16. }
17. return sum;
18. }
What three changes allow the class to be used with generics and avoid an unchecked warning? (Choose
three.)
A. Remove line 14.
B. Replace line 14 with "int i = iter.next();".
C. Replace line 13 with "for (int i : intList) {".
D. Replace line 13 with "for (Iterator iter : intList) {".
E. Replace the method declaration with "sum(List<int> intList)".
F. Replace the method declaration with "sum(List<Integer> intList)".
Answer: ACF

QUESTION 215
Given:
34. HashMap props = new HashMap();
35. props.put("key45", "some value");
36. props.put("key12", "some other value");
37. props.put("key39", "yet another value");
38. Set s = props.keySet();
39. // insert code here What, inserted at line 39, will sort the keys in the props HashMap?
A. Arrays.sort(s);
B. s = new TreeSet(s);
C. Collections.sort(s);
D. s = new SortedSet(s);
Answer: B

Arrays類中的靜態方法sort()必須實現Comparable介面的Object對象數組，即public static void sort(Object[] a)，這裡是給一個Set排序， 不符合。
Collections.sort(s)其參數必須得是實現了Comparable介面的List<T>對象，這裡是給一個實現了Set介面的對象排序， 不符合。

SortedSet是一個介面。

QUESTION 216
Given:
11. public class Person {
12. private String name;
13. public Person(String name) {
14. this.name = name;
15. }
16. public boolean equals(Object o) {
17. if ( ! ( o instanceof Person) ) return false;
18. Person p = (Person) o;
19. return p.name.equals(this.name);
20. }
21. }
Which statement is true?
A. Compilation fails because the hashCode method is not overridden.
B. A HashSet could contain multiple Person objects with the same name.
C. All Person objects will have the same hash code because the hashCode method is not overridden.
D. If a HashSet contains more than one Person object with name="Fred", then removing another Person,
also with name="Fred", will remove
them all.
Answer: B

Person類並沒有重寫hashCode()方法，所以調用Object的hashCode()方法，返回的是儲存地址，這些不同的Person對象的儲存地址當然不同。

HashSet在將對象存入的時候，首先比較的是hashCode，如果hashCode傳回值不一樣，那兩個元素肯定不同，允許添加新元素。

HashSet的add方法如下：

public boolean add(E e) {        return map.put(e, PRESENT)==null;    }

這裡PERSENT是個Object類型的Null 物件，也就是說HashSet類利用了HashMap的put方法~~兩者的原理相同。這裡的e相當於HashMap中的key~~

QUESTION 217
Given:
3. import java.util.*;
4. public class Hancock {
5. // insert code here
6. list.add("foo");
7. }
8. }
Which two code fragments, inserted independently at line 5, will compile without warnings? (Choose two.)
A. public void addStrings(List list) {
B. public void addStrings(List<String> list) {
C. public void addStrings(List<? super String> list) {
D. public void addStrings(List<? extends String> list) {
Answer: BC

PECS法則，這裡的list是消費者，用super~~

QUESTION 218
Given:
1. public class Threads4 {
2. public static void main (String[] args) {
3. new Threads4().go();
4. }
5. public void go() {
6. Runnable r = new Runnable() {
7. public void run() {
8. System.out.print("foo");
9. }
10. };
11. Thread t = new Thread(r);
12. t.start();
13. t.start();
14. }
15. }
What is the result?
A. Compilation fails.
B. An exception is thrown at runtime.
C. The code executes normally and prints "foo".
D. The code executes normally, but nothing is printed.
Answer: B

兩次start了~~illegalStateException

QUESTION 219
Given:
1. public class TestOne {
2. public static void main (String[] args) throws Exception {
3. Thread.sleep(3000);
4. System.out.println("sleep");
5. }
6. }
What is the result?
A. Compilation fails.
B. An exception is thrown at runtime.
C. The code executes normally and prints "sleep".
D. The code executes normally, but nothing is printed.
Answer: C

:

QUESTION 220
Given:
1. public class TestSeven extends Thread {
2. private static int x;
3. public synchronized void doThings() {
4. int current = x;
5. current++;
6. x = current;
7. }
8. public void run() {
9. doThings();
10. }
11.}
Which statement is true?
A. Compilation fails.
B. An exception is thrown at runtime.
C. Synchronizing the run() method would make the class thread-safe.
D. The data in variable "x" are protected from concurrent access problems.
E. Declaring the doThings() method as static would make the class thread-safe.
F. Wrapping the statements within doThings() in a synchronized(new Object()) { } block would make the
class thread-safe.

Answer: E

這裡注意x變數是靜態，和doThings()函數是非靜態。

安全執行緒的問題，如果什麼都不做，那麼這裡doThings()函數可以由不同的TestSeven對象調用來更改x，這樣就不能如願的增加x的值了，就不是安全執行緒的了。

可以public static synchronized void doThings()，把doThings()函數也變成類層級的，使用類鎖；

也可以把x改成private int x，把x弄成對象層級的，x和doThings()函數都是用對象鎖。

引用其他人的話：普通同步方法鎖定的是與 this 相關聯的監視器；而靜態同步方法則是與包含該方法的類所對應的 Class 對象相關聯的監視器；

QUESTION 221
Which two code fragments will execute the method doStuff() in a separate thread? (Choose two.)
A. new Thread() {
public void run() { doStuff(); }
};
B. new Thread() {
public void start() { doStuff(); }
};
C. new Thread() {
public void start() { doStuff(); }
}.run();
D. new Thread() {
public void run() { doStuff(); }
}.start();
E. new Thread(new Runnable() {
public void run() { doStuff(); }
}).run();
F. new Thread(new Runnable() {
public void run() { doStuff(); }
}).start();
Answer: DF

run()方法只能還是在主線程上執行，而start()方法則是建立一個子線程去執行~~

QUESTION 222
Given:
11. public static void main(String[] args) {
12. Object obj = new int[] { 1, 2, 3 };
13. int[] someArray = (int[])obj;
14. for (int i : someArray) System.out.print(i + " ");
15. }
What is the result?
A. 1 2 3
B. Compilation fails because of an error in line 12.
C. Compilation fails because of an error in line 13.
D. Compilation fails because of an error in line 14.
E. A ClassCastException is thrown at runtime.
Answer: A

QUESTION 224
Given:
11. public static void parse(String str) {
12. try {
13. float f = Float.parseFloat(str);
14. } catch (NumberFormatException nfe) {
15. f = 0;
16. } finally {
17. System.out.println(f);
18. }
19. }
20. public static void main(String[] args) {
21. parse("invalid");
22. }
What is the result?
A. 0.0
B. Compilation fails.
C. A ParseException is thrown by the parse method at runtime.
D. A NumberFormatException is thrown by the parse method at runtime.
Answer: B

第十五行的f，未聲明的變數~~13行的f範圍僅在try的{}內部~~

QUESTION 226
Given:
1. interface TestA { String toString(); }
2. public class Test {
3. public static void main(String[] args) {
4. System.out.println(new TestA() {
5. public String toString() { return "test"; }
6. });
7. }
8. }
What is the result?
A. test
B. null
C. An exception is thrown at runtime.
D. Compilation fails because of an error in line 1.
E. Compilation fails because of an error in line 4.
F. Compilation fails because of an error in line 5.
Answer: A

很簡單的局部內部類~~

QUESTION 227
Given:
11. public interface A { public void m1(); }
12.
13. class B implements A { }
14. class C implements A { public void m1() { } }
15. class D implements A { public void m1(int x) { } }
16. abstract class E implements A { }
17. abstract class F implements A { public void m1() { } }
18. abstract class G implements A { public void m1(int x) { } }
What is the result?
A. Compilation succeeds.
B. Exactly one class does NOT compile.
C. Exactly two classes do NOT compile.
D. Exactly four classes do NOT compile.
E. Exactly three classes do NOT compile.
Answer: C

B類和D類錯了；

而abstract類不用實現implementations的類~~交給它的子類實現就OK~~

QUESTION 228
Given:
21. abstract class C1 {
22. public C1() { System.out.print(1); }
23. }
24. class C2 extends C1 {
25. public C2() { System.out.print(2); }
26. }
27. class C3 extends C2 {
28. public C3() { System.out.println(3); }
29. }
30. public class Ctest {
31. public static void main(String[] a) { new C3(); }
32. }
What is the result?
A. 3
B. 23
C. 32
D. 123
E. 321
F. Compilation fails.
G. An exception is thrown at runtime.
Answer: D

子類的構造器預設要調用超類的無參數的構造器~~

QUESTION 229

Click the Exhibit button. What is the result?

A. 4321
B. 0000
C. An exception is thrown at runtime.
D. Compilation fails because of an error in line 18.
Answer: D

建構函式的問題~~Person類沒有預設建構函式

QUESTION 230
Given:
10. class One {
11. public One foo() { return this; }
12. }
13. class Two extends One {
14. public One foo() { return this; }
15. }
16. class Three extends Two {
17. // insert method here
18. }
Which two methods, inserted individually, correctly complete the Three class? (Choose two.)
A. public void foo() {}
B. public int foo() { return 3; }
C. public Two foo() { return this; }
D. public One foo() { return this; }
E. public Object foo() { return this; }
Answer: CD

重寫一個方法的時候，傳回型別可以是原來的One類，也可以使One類的子類~~